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* ABSTRACT
* I. INTRODUCTION
* II. THE ORIGIN OF MAGNETISM IN 2D MATERIALS
* III. THE 2D VDW MAGNETS DATABASE
* IV. MODIFICATIONS
* V. CONCLUSION AND OUTLOOK
* AUTHORS' CONTRIBUTIONS
* ACKNOWLEDGMENTS
* DATA AVAILABLITY
* REFERENCES
RECENT PROGRESS ON 2D MAGNETS: FUNDAMENTAL MECHANISM, STRUCTURAL DESIGN AND
MODIFICATION
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* Volume 8, Issue 3 >
* 10.1063/5.0039979
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Open Submitted: 08 December 2020 Accepted: 21 May 2021 Published Online: 23 July
2021
* RECENT PROGRESS ON 2D MAGNETS: FUNDAMENTAL MECHANISM, STRUCTURAL DESIGN AND
MODIFICATION
*
Applied Physics Reviews 8, 031305 (2021); https://doi.org/10.1063/5.0039979
Xue Jiang1, Qinxi Liu1, Jianpei Xing1, Nanshu Liu1, Yu Guo1, Zhifeng Liu2, and
Jijun Zhao1,a)
more...View Affiliations
* 1Key Laboratory of Material Modification by Laser, Ion and Electron Beams,
Dalian University of Technology, Ministry of Education, Dalian 116024, China
* 2School of Physical Science and Technology, Inner Mongolia University, Hohhot
010021, China
* a)Author to whom all correspondence should be addressed: zhaojj@dlut.edu.cn
View Contributors
* Xue Jiang
* Qinxi Liu
* Jianpei Xing
* Nanshu Liu
* Yu Guo
* Zhifeng Liu
* Jijun Zhao
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* Topics
* Topics
* Exchange interactions
* 2D materials
* Magnetic anisotropy
* Magnetic ordering
* Magnetic materials
ABSTRACT
The two-dimensional (2D) magnet, a long-standing missing member in the family of
2D functional materials, is promising for next-generation information
technology. The recent experimental discovery of 2D magnetic ordering in CrI3,
Cr2Ge2Te6, VSe2, and Fe3GeTe2 has stimulated intense research activities to
expand the scope of 2D magnets. This review covers the essential progress on 2D
magnets, with an emphasis on the current understanding of the magnetic exchange
interaction, the databases of 2D magnets, and the modification strategies for
modulation of magnetism. We will address a large number of 2D intrinsic magnetic
materials, including binary transition metal halogenides; chalogenides;
carbides; nitrides; oxides; borides; silicides; MXene; ternary transition metal
compounds CrXTe3, MPX3, Fe-Ge-Te, MBi2Te4, and MXY (M = transition metal; X = O,
S, Se, Te, N; Y = Cl, Br, I); f-state magnets; p-state magnets; and organic
magnets. Their electronic structure, magnetic moment, Curie temperature, and
magnetic anisotropy energy will be presented. According to the specific 2D
magnets, the underlying direct, superexchange, double exchange,
super-superexchange, extended superexchange, and multi-intermediate double
exchange interactions will be described. In addition, we will also highlight the
effective strategies to manipulate the interatomic exchange mechanism to improve
the Curie temperature of 2D magnets, such as chemical functionalization,
isoelectronic substitution, alloying, strain engineering, defect engineering,
applying electronic/magnetic field, interlayer coupling, carrier doping, optical
controlling, and intercalation. We hope this review will contribute to
understanding the magnetic exchange interaction of existing 2D magnets,
developing unprecedented 2D magnets with desired properties, and offering new
perspectives in this rapidly expanding field.
I. INTRODUCTION
Section:
ChooseTop of pageABSTRACTI. INTRODUCTION <<II. THE ORIGIN OF MAGNETI...III. THE
2D VDW MAGNETS D...IV. MODIFICATIONSV. CONCLUSION AND OUTLOOKAUTHORS'
CONTRIBUTIONSCITING ARTICLESChoose
Since the discovery of magnetic phenomena in ancient times, magnetism has
attracted vast research interest due to its technological importance and
theoretical complexity. From the technique point of view, permanent magnets,
which are required for high coercivity and large energy product, are widely used
in loudspeakers, earphones, electric meters, small motors, and wind power
generation. The more esoteric applications of magnetism are in magnetic
recording and storage devices of computers, as well as in audio and video
systems. Only ten years after the discovery of interlayer exchange coupling and
related giant magnetoresistance (GMR) effect in the magnetic multilayers, GMR
devices were routinely used in the hard disk drives of computers.11. M. N.
Baibich, J. M. Broto, A. Fert, F. N. Van Dau, F. Petroff, P. Etienne, G.
Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. 61(21), 2472 (1988).
https://doi.org/10.1103/PhysRevLett.61.2472 They are also crucial for the
emerging field of spintronics, which is regarded as the core of the
next-generation information technology. By using electron spin rather than
charge as the information carrier, spintronics possesses prominent advantages of
speeding up data processing, high circuit integration density, and low energy
consumption.2,32. X. Li and J. Yang, Natl. Sci. Rev. 3(3), 365 (2016).
https://doi.org/10.1093/nsr/nww0263. N. Mason and M. Stehno, Nat. Phys. 9(2), 67
(2013). https://doi.org/10.1038/nphys2529
From the theoretical point of view, two fundamental concepts have been proposed
to explain the fascinating magnetic phenomenon, namely, exchange interaction and
spin-orbit coupling (SOC). The interplay between exchange interaction,
spin-orbit coupling, and Zeeman effect is the essence of magnetism research.
Together, they explain the origin of spin arrangement, orbital moment, and
magnetocrystalline anisotropy, and the effect of external field on these
quantities.44. J. Stöhr and H. C. Siegmann, Magnetism: From Fundamentals to
Nanoscale Dynamics ( Springer, 2006). Among them, the
interatomic/interelectronic exchange interactions are at the heart of the
phenomenon of long-range magnetic ordering. Parallel and antiparallel
arrangements of spins constitute long-range ferromagnetic (FM) and
antiferromagnetic (AFM) orderings. As the dimensionality decreases, the net
magnetic moment per atom generally increases. This might be ascribed to lower
coordination number, quantum confinement effect, less quenching of orbital
magnetic moment, and so on. Meanwhile, in two-dimensional (2D) lattices,
exchange interactions would be stronger along one or two spatial directions than
others, showing large anisotropy. Depending on the type of magnetic ions and the
electronic band structures, the traditional exchange mechanisms, including
direct exchange interaction,5–75. Q. Liu, J. Xing, Z. Jiang, X. Jiang, Y. Wang,
and J. Zhao, Nanoscale 12(12), 6776 (2020). https://doi.org/10.1039/D0NR00092B6.
Z. Liu, J. Liu, and J. Zhao, Nano Res. 10(6), 1972 (2017).
https://doi.org/10.1007/s12274-016-1384-37. X. Li and J. Yang, J. Phys. Chem.
Lett. 10(10), 2439 (2019). https://doi.org/10.1021/acs.jpclett.9b00769
superexchange interaction,8–148. H. Wang, F. Fan, S. Zhu, and H. Wu, Europhys.
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following three aspects. First, the experimentally unprecedented realizations of
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of magnitude of ∼1 meV, they will have a crucial impact on the 2D materials by
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Driven by all these three aspects, the research of 2D magnets has boomed in the
last few years. In recent years, a great deal of new 2D magnetic materials have
been experimentally discovered and theoretically predicted. In this review,
however, we restricted the term to the 2D intrinsic magnets. The 2D intrinsic
nonmagnetic materials, which have already been concluded in the previous
review,8383. X. Shi, Z. Huang, M. Huttula, T. Li, S. Li, X. Wang, Y. Luo, M.
Zhang, and W. Cao, Crystals 8(1), 24 (2018).
https://doi.org/10.3390/cryst8010024 will not be covered here. In this review,
we classified these emerging 2D intrinsic magnets into binary transition metal
halogenides; chalogenides; carbides; nitrides; oxides; borides; silicides;
ternary transition metal compounds CrXTe3, MPX3, Fe-Ge-Te, MBi2Te4, and MXY (M =
transition metal; X = O, S, Se, Te, N; Y = Cl, Br, I); f-state magnets; p-state
magnets; and organic magnets such as metal organic framework (MOF) and covalent
organic framework (COF). The most common prototypes of 2D inorganic magnetic
materials are shown in Fig. 1. Their key magnetic properties, especially the
magnetic ground state, magnetic moment, TC, and MAE will be comprehensively
described in Sec. III.
FIG. 1. The most common 2D inorganic magnetic materials. The representative
crystal lattices are included, such as Binary transition metal halides (CrI3,
CrI2, Nb3Cl8, and ScCl), binary transition metal (BTM) chalcogenides (Cr3Se4 and
VSe2), MXenes (MnB and Cr2CT2), other binary transition metal compounds (Co2P,
CrB2, MnO2, and FeSi2), Ternary transition metal compounds (CrSiTe3, CrSBr,
FePS3, MnBi2Te4, Fe3GeTe2, and Fe5GeTe2), and p/f magnets (GdI2 and K2N). They
have been either experimentally realized or theoretically proposed for each type
of magnetic lattice.
* PPT
|
* High-resolution
For 2D layered materials, their magnetic properties can be readily modulated by
chemical compositions, functional groups, intercalation, and substrates.
Moreover, the spin coupling to external perturbations like strain,
electronic/magnetic fields, and carrier doping could be the other critical
factors for tailoring the strength of exchange interactions or magnetic
anisotropy. These modulation methods could be utilized on 2D intrinsic magnets
to further enhance Curie temperature. The corresponding mechanisms for
modulating the charge distributions, energy level, orbital occupation, symmetry,
and hopping paths will be discussed in Sec. IV.
In the final Sec. V, we will conclude with a discussion about future challenges
and opportunities of 2D magnets. Four potential future directions regarding the
practical applications have been proposed. In addition, this review article
mainly focuses on the underlying mechanisms of exchange interaction, which not
only determine the Curie temperature of 2D intrinsic magnets but also shed light
on the strategies to manipulate the magnetic coupling. Therefore, this
discussion is complementary to the other recent review articles on 2D magnets,
which have discussed the history,8484. D. L. Cortie, G. L. Causer, K. C. Rule,
H. Fritzsche, W. Kreuzpaintner, and F. Klose, Adv. Funct. Mater. 30(18), 1901414
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L. Duong, S. J. Yun, and Y. H. Lee, ACS Nano 11(12), 11803 (2017).
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P. Zhang, S. Xu, H. Wang, X. Zhang, and H. Zhang, Nanoscale 12(4), 2309 (2020).
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II. THE ORIGIN OF MAGNETISM IN 2D MATERIALS
Section:
ChooseTop of pageABSTRACTI. INTRODUCTIONII. THE ORIGIN OF MAGNETI... <<III. THE
2D VDW MAGNETS D...IV. MODIFICATIONSV. CONCLUSION AND OUTLOOKAUTHORS'
CONTRIBUTIONSCITING ARTICLESChoose
A. The magnetic moment of free atoms
To start discussing the magnetic properties of 2D materials, we should first
know the magnetic ground state of a multi-electron atom, which is mainly
determined by Hund's rules. Generally speaking, Hund's rules refer to a set of
rules that describe spin-spin coupling, orbital-orbital coupling, and spin-orbit
coupling, respectively. The first rule about the spin-spin coupling in a
multi-electron atom, whose strength is at the order of c.a. 2 eV is especially
important. Hence, the magnetic moment is mainly determined by the intra-atomic
exchange interaction. The spin-orbit interaction describing the coupling between
spin and orbital angular momentum further produces the energy level splitting
with the order of several to hundreds mega-electron volts. We can estimate the
moment per atom from this rule by maximizing spin (S), orbital momentum (L), and
angular moment (J).
In a solid, exchange interaction and SOC are two most important concepts for
magnetism. First of all, without interatomic exchange, there would be no
spontaneous magnetization. Interatomic exchange interaction determines the
long-range spin ordering, i.e., parallel or antiparallel spin alignment in a
magnetic material. Meanwhile, spin-orbit interaction creates orbital magnetism
and couples the spin to the lattice. Through the SOC interaction, spin and
charge can talk to each other via exchanging energy and angular momentum,
thereby establishing magnetic anisotropy. In addition, the other competition
interactions have also been investigated to determine the size of the moment,
including crystal fields, Hund's rule coupling, and onsite Coulomb
repulsion.9494. J. M. Coey, Magnetism and Magnetic Materials ( Cambridge
University Press, Cambridge, 2010). These factors will be discussed in a
following paper.
B. Ligand field theory
When a free ion is placed in a lattice and subjected to the interaction with its
surrounding atoms, another key interaction arises, termed a ligand (crystal)
field. The ligand effect on the central atom is entirely determined by symmetry
and strength of field produced by these surrounding atoms. The symmetry and
degenerate states of typical 2D magnets with 3d transition metals are summarized
in Table I. It should be noted that the amplitude of splitting energy due to
ligand field is comparable to the intra-atomic Coulomb and exchange
interactions. Thus, the final ground state depends on the relative amplitude of
them. For example, the d4-d7 configurations would show either low-spin or
high-spin states in an octahedral ligand field.
TABLE I. The types of crystal field point group symmetry, and orbital splitting
of central transition metals ions in some typical 2D magnets.
Crystal field Point group Orbitals 2D magnets Octahedral Oh t2g (dxy, dxz, dyz),
eg (dx2-y2, dz2) MX3 (X = F, Cl, Br, I);53,10453. W.-B. Zhang, Q. Qu, P. Zhu,
and C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J104. L. Webster and J.-A. Yan, Phys. Rev. B
98(14), 144411 (2018). https://doi.org/10.1103/PhysRevB.98.144411 MX2 (X= Cl,
Br, I);100100. V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A 1T-MX2 (X = S, Se, Te);105105. E. Bruyer, D.
Di Sante, P. Barone, A. Stroppa, M.-H. Whangbo, and S. Picozzi, Phys. Rev. B
94(19), 195402 (2016). https://doi.org/10.1103/PhysRevB.94.195402 MPS4;9595. Q.
Chen, Q. Ding, Y. Wang, Y. Xu, and J. Wang, J. Phys. Chem. C 124(22), 12075
(2020). https://doi.org/10.1021/acs.jpcc.0c02432 CoGaX4 (X = S, Se, Te)9696. S.
Zhang, R. Xu, W. Duan, and X. Zou, Adv. Funct. Mater. 29(14), 1808380 (2019).
https://doi.org/10.1002/adfm.201808380 MGe(Si)X3 (X = Se, Te)99,29199. W. Xing,
Y. Chen, P. M. Odenthal, X. Zhang, W. Yuan, T. Su, Q. Song, T. Wang, J. Zhong,
S. Jia, X. C. Xie, Y. Li, and W. Han, 2D Mater. 4(2), 024009 (2017).
https://doi.org/10.1088/2053-1583/aa7034291. J.-Y. You, Z. Zhang, X.-J. Dong, B.
Gu, and G. Su, Phys. Rev. Res. 2(1), 013002 (2020).
https://doi.org/10.1103/PhysRevResearch.2.013002 Distorted octahedral D4h a1
(dz2), b1 (dx2-y2), b2 (dxy), e (dxz, dyz) Mn-Pc,101101. J. Zhou and Q. Sun, J.
Am. Chem. Soc. 133(38), 15113 (2011). https://doi.org/10.1021/ja204990j
Mn-TCNB102102. M. Mabrouk and R. Hayn, Phys. Rev. B 92(18), 184424 (2015).
https://doi.org/10.1103/PhysRevB.92.184424 Trigonal prismatic D3h e1 (dxz, dyz);
e2 (dx2-y2, dxy); a1 (dz2) 2H-MX2 (X = S, Se, Te, F, Cl, Br, I,
H)10,27,103,28310. H. L. Zhuang and R. G. Hennig, Phys. Rev. B 93(5), 054429
(2016). https://doi.org/10.1103/PhysRevB.93.05442927. C. Wang, X. Zhou, L. Zhou,
Y. Pan, Z.-Y. Lu, X. Wan, X. Wang, and W. Ji, Phys. Rev. B 102(2), 020402
(2020). https://doi.org/10.1103/PhysRevB.102.020402103. X. Li, Z. Zhang, and H.
Zhang, Nanoscale Adv. 2(1), 495 (2020). https://doi.org/10.1039/C9NA00588A283.
Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J. Phys. Chem. Lett. 9(15),
4260 (2018). https://doi.org/10.1021/acs.jpclett.8b01976 Triangular prism C3v a1
(dz2); e′ (dxz, dyz) e (dx2-y2, dxy) Fe2C;9797. Y. Yue, J. Magn. Magn. Mater.
434, 164 (2017). https://doi.org/10.1016/j.jmmm.2017.03.058 M2XTx MXene1212. H.
Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano
11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578 Hexagonal C6V e1
(dxz, dyz); e2 (dx2-y2, dxy); a1 (dz2) MN (M = Cr, V, Mn)264–266264. A. V.
Kuklin, A. A. Kuzubov, E. A. Kovaleva, N. S. Mikhaleva, F. N. Tomilin, H. Lee,
and P. V. Avramov, Nanoscale 9(2), 621 (2017).
https://doi.org/10.1039/C6NR07790K265. A. V. Kuklin, S. A. Shostak, and A. A.
Kuzubov, J. Phys. Chem. Lett. 9(6), 1422 (2018).
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C 122(26), 14918 (2018). https://doi.org/10.1021/acs.jpcc.8b02323
In fact, ligand field theory provides a simple model to predict the magnetic
behavior of 2D transition metal compounds, which is strongly influenced by the
coordination environment and the number of d electrons. For example, the
configurations of M2NT2 MXene are shown in Figs. 2(a) and 2(b).1212. H. Kumar,
N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 11(8),
7648 (2017). https://doi.org/10.1021/acsnano.7b02578 For a transition metal ion
located in an octahedral crystal field, its d orbitals split into the
lower-energy t2g states and higher-energy eg orbitals. Similar to the transition
metal dichalcogenides (TMDs), these nonbonding t2g and eg states of the MXenes
are positioned between the bonding and antibonding states (σ and σ*) of M–X and
M–T bonds [Fig. 2(c)]. We assume a perfect bonding case as follows: (1) the
nonmetal elements are in their nominal oxidation state, i.e., C4–, N3–, O2–, F−,
and OH−; (2) the M–X and M–T bonding states are filled; (3) the M–X and M–T
antibonding states are empty. Therefore, only the electrons occupying the
nonbonding d orbitals will contribute to the magnetism. For example, the nominal
oxidation state of Cr ion in Cr2CF2 is +3. Based on Hund's rules, the remaining
3 electrons on Cr ion will occupy the t2g band half filled, thereby giving a
local magnetic moment of 3 μB. Similarly, the local magnetic moments of Mn2CO2,
Mn2CF2, and Mn2C(OH)2 are 3 μB, 4 μB, and 4 μB, respectively. The electron spin
arrangements on the transition metals in the nitride MXene are shown in the Fig.
2(d).1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B.
Shenoy, ACS Nano 11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578
Similar analyses based on symmetry have also been used to identify the magnetic
properties of binary transition metal halides, carbides, nitrides, oxides,
borides, phosphides, silicides, arsenides, hydrides, and ternary transition
metal compounds. The representative examples are shown in Table I. All these
results are consistent with the results from DFT
calculations.10,12,45,65,93,95–10510. H. L. Zhuang and R. G. Hennig, Phys. Rev.
B 93(5), 054429 (2016). https://doi.org/10.1103/PhysRevB.93.05442912. H. Kumar,
N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 11(8),
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Zou, Adv. Funct. Mater. 29(14), 1808380 (2019).
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Ishii, Phys. Rev. B 100(11), 115104 (2019).
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X. Zhang, W. Yuan, T. Su, Q. Song, T. Wang, J. Zhong, S. Jia, X. C. Xie, Y. Li,
and W. Han, 2D Mater. 4(2), 024009 (2017).
https://doi.org/10.1088/2053-1583/aa7034100. V. V. Kulish and W. Huang, J.
Mater. Chem. C 5(34), 8734 (2017). https://doi.org/10.1039/C7TC02664A101. J.
Zhou and Q. Sun, J. Am. Chem. Soc. 133(38), 15113 (2011).
https://doi.org/10.1021/ja204990j102. M. Mabrouk and R. Hayn, Phys. Rev. B
92(18), 184424 (2015). https://doi.org/10.1103/PhysRevB.92.184424103. X. Li, Z.
Zhang, and H. Zhang, Nanoscale Adv. 2(1), 495 (2020).
https://doi.org/10.1039/C9NA00588A104. L. Webster and J.-A. Yan, Phys. Rev. B
98(14), 144411 (2018). https://doi.org/10.1103/PhysRevB.98.144411105. E. Bruyer,
D. Di Sante, P. Barone, A. Stroppa, M.-H. Whangbo, and S. Picozzi, Phys. Rev. B
94(19), 195402 (2016). https://doi.org/10.1103/PhysRevB.94.195402
FIG. 2. Schematic diagram to explain the local magnetic moment of M2NT2 MXene (M
= Ti, V, Cr, Mn; T = F, OH, O) with different transition metal groups. (a–b) The
local coordination of transition metals, each transition-metal ion is subjected
to an octahedral crystal field. (c) The simplified density of states, the
nonbonding d-orbitals of the MXenes are positioned between bonding (σ) and
antibonding (σ*) states of M−X and M−T bonds. (d) Occupation of the electrons on
the transition metal centers. Dotted spin indicates electron occupation is
equally probable in the states corresponding to either the top (T) or the bottom
(B) layer. Each N atom gains three electrons either by accepting two electrons
from M atom in the top layer and one electron from the bottom layer M atom or
vice versa, which leads to the coexistence of two different oxidation states for
the two M atoms. Reproduced with permission from Kumar et al., ACS Nano 11, 7648
(2017). Copyright 2017 American Chemical Society.1212. H. Kumar, N. C. Frey, L.
Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b02578
* PPT
|
* High-resolution
C. The importance of exchange interaction
Heisenberg model is a simple theoretical model to describe the physical effects
of magnetic systems. Due to the strong local magnetic moment, the leading term
is symmetric exchange interaction in Heisenberg model, which has the form
𝐻̂𝑒𝑥=−∑𝑖≠𝑗𝐽𝑖𝑗𝑆̂𝑖⋅𝑆̂𝑗,Ĥex=−∑i≠jJijŜi⋅Ŝj,
(1)
where i and j denote the lattice sites bearing a localized magnetic moment, and
Ŝi or Ŝj is a quantum mechanical spin operator. Jij is the exchange constant,
which is the basis of most studies of magnetism. Evidently, Jij will decrease
rapidly with the distance between lattice i and j. The positive Jij favors
parallel spin alignment (i.e., FM), while the negative Jij favors antiparallel
spin alignment (i.e., AFM). Theoretically, the values of Jij can be obtained
from first-principles calculations. The simplest way is to calculate from the
total energy difference between different spin orderings. In experiment, Jij can
be determined by fitting the inelastic neutron scattering data to the Heisenberg
Hamiltonian.
Generally speaking, these microscopic parameters of Jij define most of
macroscopic magnetic properties, especially the Curie temperature and the
magnetic response function to an external field. For example, one can estimate
magnetic transition temperatures via mean field theory (MFT)106106. N. W.
Ashcroft and N. D. Mermin, Solid State Physics ( Holt, Rinehart and Winston,
1976). as follows:
𝑇𝐶=𝐽0𝑆(𝑆+1)3𝑘𝐵,TC=J0S(S+1)3kB,
(2)
where S is the atomic spin, J0 is the sum of exchange interactions, and kB is
the Boltzmann constant. For 2D magnets, MFT usually overestimates the transition
temperature by around 20% or even more, which is dependent on the coordination
number.106106. N. W. Ashcroft and N. D. Mermin, Solid State Physics ( Holt,
Rinehart and Winston, 1976). Monte Carlo (MC) method gives a numerical solution
to the Heisenberg model with reasonable accuracy. Therefore, it becomes the most
commonly used method to predict the critical temperature in 2D magnets. However,
MFT is still meaningful in establishing an upper limit of TC at much less
computational cost, which can be compared to MC results.
To explain the origin of spontaneous magnetization in metallic magnets with
itinerant/localized electrons, such as single layer of Fe3GeTe2, T phase of
TMDs, Cr2C, and Fe2C MXenes, another two models—Stoner model and RKKY model—have
also been proposed, which will be discussed in Secs. II F and II G,
respectively.
D. Spin orbital coupling in 2D magnets
The exchange term is isotropic in the sense that the scalar product Ŝi·Ŝj in Eq.
(1) does not change under any rotation applied to every spin in the system.
Based on Mermin-Wagner theorem,4444. N. D. Mermin and H. Wagner, Phys. Rev.
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materials can be neither FM nor AFM at nonzero temperature due to thermal
fluctuations under isotropic Heisenberg model. However, considering the
spin-orbit interaction, this symmetry is broken. Besides the symmetric exchange
term Hex discussed in Sec. II C, the spin orbital coupling gives rise to the
antisymmetric anisotropic exchange terms. Generally, the strength of
antisymmetric anisotropic exchange terms are far less than Hex. However, they
become crucial for the magnetic ground state of 2D systems.
So far, many simple anisotropic spin models have been proposed to interpret the
emerging magnetism in 2D materials. A typical 2D magnet possesses easy-plane
magnetic anisotropy, uniaxial anisotropy, and isotropic anisotropy, which can be
described in 2D XY, Ising, and Heisenberg models (XXZ), respectively.
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Y.-z. Nie, Q.-l. Xia, and G.-h. Guo, J. Magn. Magn. Mater. 508, 166878 (2020).
https://doi.org/10.1016/j.jmmm.2020.166878 and NiPS3.112112. D. Lançon, R. A.
Ewings, T. Guidi, F. Formisano, and A. R. Wildes, Phys. Rev. B 98(13), 134414
(2018). https://doi.org/10.1103/PhysRevB.98.134414
The antisymmetric anisotropic exchange terms contain various generic forms, for
example, Dzyaloshinski-Moriya (DM) interaction term and single ion
magnetocrystalline anisotropy term.113,114113. I. Dzyaloshinsky, J. Phys. Chem.
Solids 4(4), 241 (1958). https://doi.org/10.1016/0022-3697(58)90076-3114. T.
Moriya, Phys. Rev. Lett. 4(5), 228 (1960).
https://doi.org/10.1103/PhysRevLett.4.228 As stated above, both of them were
determined by spin-orbit interaction. They have the following forms:
𝐻̂𝐷𝑀=∑𝑖≠𝑗𝐷→𝑖𝑗⋅(𝑆̂𝑖×𝑆̂𝑗),ĤDM=∑i≠jD→ij⋅(Ŝi×Ŝj),
(3)
𝐻̂𝑎𝑛=∑𝑖𝐴𝑖(𝑆̂𝑖)2.Ĥan=∑iAi(Ŝi)2.
(4)
In 2D materials, the spin orbital interaction plays a similar role with
non-metal atoms in superexchange interaction. As shown in Eq. (3), the DM
interaction is characterized by the vector 𝐷→𝑖𝑗D→ij, which is proportional to
spin orbit coupling constant. It is also dependent on the position of the
non-metal atom between the two magnetic atoms. Clearly, DM interaction favors an
orthogonal alignment between spins.
In Eq. (4), Ai is the easy-axis single ion anisotropy factor. The magnetic
anisotropy energy (MAE) is defined as the largest possible energy difference
between two different magnetization directions. Within DFT framework, it can be
easily obtained by performing total energy calculations including SOC.5151. D.
Hobbs, G. Kresse, and J. Hafner, Phys. Rev. B 62(17), 11556 (2000).
https://doi.org/10.1103/PhysRevB.62.11556 To clarify the origin of MAEs, the
torque method115115. X. D. Wang, R. Q. Wu, D. S. Wang, and A. J. Freeman, Phys.
Rev. B 54(1), 61 (1996). https://doi.org/10.1103/PhysRevB.54.61 was implemented
in either all-electron full potential linearized augmented plane wave (FPLAPW)
or Vienna Ab-initio Simulation Package (VASP) with plane wave basis
sets.116,117116. D.-s. Wang, R. Wu, and A. J. Freeman, Phys. Rev. B 47(22),
14932 (1993). https://doi.org/10.1103/PhysRevB.47.14932117. M. Weinert, E.
Wimmer, and A. J. Freeman, Phys. Rev. B 26(8), 4571 (1982).
https://doi.org/10.1103/PhysRevB.26.4571 To evaluate the contribution of SOC to
the magnetic anisotropy, second-order perturbation theory has also been
introduced.116116. D.-s. Wang, R. Wu, and A. J. Freeman, Phys. Rev. B 47(22),
14932 (1993). https://doi.org/10.1103/PhysRevB.47.14932
The DM interaction and single ion magnetocrystalline anisotropy term are
typically a few percent of the isotropic term, producing a modest canting to
symmetric exchange interactions. Nevertheless, the DM and single ion
magnetocrystalline anisotropy terms are the good supplement to the 2D materials.
A spontaneous rearrangement of atoms to favor the DM interaction can produce a
large electric polarization in magnetoelectric materials. Meanwhile, the
competition of spin-spin directions would be helpful to understand complicated
magnetic behavior, such as magnetic skyrmions, and quantum spin liquid. The
magnetic anisotropy is a prerequisite to realize FM or AFM states at the 2D
limit.
A much-studied spin model (Kitaev model) is developed to describe the
anisotropic spin exchange coupling for honeycomb spin lattice.118118. A. Kitaev,
Ann. Phys. 321(1), 2 (2006). https://doi.org/10.1016/j.aop.2005.10.005 It has
the form
𝐻𝐾=∑⟨𝑖,𝑗⟩𝛾𝐾𝛾𝑆𝑖𝛾𝑆𝑗𝛾.HK=∑⟨i,j⟩γKγSiγSjγ.
(5)
Neighboring spins couple depending on the direction of their bond γ with SxSx,
SySy, or SzSz. This exchange term and the symmetric Heisenberg term
(Kitaev-Heisenberg model) together serve as a putative minima model for several
materials, including α-RuCl3119119. K. W. Plumb, J. P. Clancy, L. J. Sandilands,
V. V. Shankar, Y. F. Hu, K. S. Burch, H.-Y. Kee, and Y.-J. Kim, Phys. Rev. B
90(4), 041112 (2014). https://doi.org/10.1103/PhysRevB.90.041112 and
Li(Na)2IrO3.120120. J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev.
Lett. 110(9), 097204 (2013). https://doi.org/10.1103/PhysRevLett.110.097204
Beyond these terms, one should also note that the real materials may have
additional exchange couplings other than these couplings, including dipolar
term, biquadratic interaction, and Zeeman coupling to the external magnetic
field.121121. S. Baierl, M. Hohenleutner, T. Kampfrath, A. K. Zvezdin, A. V.
Kimel, R. Huber, and R. V. Mikhaylovskiy, Nat. Photon. 10(11), 715 (2016).
https://doi.org/10.1038/nphoton.2016.181 The strength of these exchange
interactions is also much lower than that of symmetric exchange interaction.
Therefore, to simplify, only the dominated spin models are investigated and
discussed in this review, which are roughly enough to describe the critical
parameters of 2D magnets.
E. The known exchange interaction
In this section, we discuss the identified exchange mechanisms in the real 2D
magnets based on the abovementioned three models, including direct exchange,
superexchange, double exchange, super-superexchange, extended superexchange,
multi-intermediate double exchange interaction, itinerant electrons, and RKKY
(Fig. 3). Their regimes of applicability depend on the electronic band
structures and the types of magnetic ions, i.e., metal or insulator, and
localized or delocalized. However, it is really difficult to distinguish them
completely. For example, 3d electrons are partially localized on the atomic
sites and also partially delocalized in the crystal. In each exchange
interaction, the magnetic ground state and Curie temperature will be further
discussed, which are determined from the competition between the kinetic
exchange energy and the Coulomb repulsion. It is worth noting that magnetic
exchange interactions in the 2D magnets are rather complicated; thus, they are
unable to be generalized under a one-theory umbrella. Although these types of
exchange mechanism have been utilized to explain different materials
appropriately, there are no clear borderlines between them. Moreover, several
exchange interactions may possibly coexist in one real material. To understand
the magnetic ordering and magnetic coupling strength, we always need to ask
which one is dominant.
FIG. 3. Schematic diagram of the representative exchange interaction mechanisms.
Several kinds of magnetic interaction between localized moments have been
included, such as the conventional direct exchange, superexchange, double
exchange, indirect exchange, and RKKY. The exchange interaction between
itinerant electrons have also been covered. In addition, the small blue circle
in superexchange region is super-superexchange interaction (SSE). The small grey
circle in double exchange region is multi-intermediate double exchange
interaction (MDE). The overlap region of double exchange and superexchange—the
blue part—is the extended superexchange (ESE) interaction.
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1. Direct exchange interaction
Direct exchange interaction is based on the overlap of electronic wavefunctions
and is therefore very short ranged.122122. A. J. Freeman and R. E. Watson, Phys.
Rev. 124(5), 1439 (1961). https://doi.org/10.1103/PhysRev.124.1439 It is always
confined to electrons in the orbitals from the nearest neighboring atoms.
Considering the distance of the magnetic atoms, the strength of direct exchange
interaction is always very weak. As a consequence, the direct exchange
interaction is neither the main source of magnetism nor can it appropriately
describe the magnetic behavior in most of the reported 2D magnets. Even so,
direct exchange interaction between the neighboring sites is still dominant in
the magnetic materials with peculiar d-d, d-p, and p-p hybridizations. With
sufficiently large overlap, the exchange integral Jij tends to be
antiferromagnetic, ferromagnetic, or ferrimagnetic, depending on symmetry
relationship and occupation number of the orbitals.
As a specific example, Liu et al.55. Q. Liu, J. Xing, Z. Jiang, X. Jiang, Y.
Wang, and J. Zhao, Nanoscale 12(12), 6776 (2020).
https://doi.org/10.1039/D0NR00092B proposed a novel class of 2D magnetic
metal-shrouded materials, namely tetragonal transition metal phosphides (TM2P),
which showed peculiar coexistence of in-plane TM–P covalent bonds and interlayer
TM–TM metallic bonds. For Co2P, the spin-up dz2 orbital contributes to the total
on-site moment of ∼1 μB. The electrons can hop from the occupied dz2 orbital to
the empty dz2 orbital via Co–Co metallic bonds; thus, the d-d exchange between
the neighboring sites is ferromagnetic. However, the more active dxz/dyz
orbitals dominate the AFM ground states in Fe2P. There is overlap between
dxz/dyz orbitals at two neighboring metal sites, and electrons would hop between
these two active orbitals of Fe atoms. The resulting exchange interaction is
antiferromagnetic and strong. Consequently, Fe2P behaves as an antiferromagnetic
material with TN = 23 K, while Co2P is a ferromagnetic material with TC = 580 K.
The corresponding AFM and FM direct interaction mechanisms are shown in Fig.
4(a).
FIG. 4. (a) Direct FM (AFM) exchange interactions between dz2-dz2
(dyz/dxz-dyz/dxz) orbitals. (b) Schematic diagram of the possible paths for
magnetic exchange interaction in MPS3 (M = Ni, Mn, Fe) monolayer. (c) Schematic
diagram of the FM coupling mechanism: p-p direct exchange interaction in TaN2
monolayer. (d) Schematic diagram of the FM coupling mechanism: d-p direct
exchange interaction in 2D organometallic lattices. Panel (a) reproduced with
permission from Liu et al., Nanoscale 12, 6776 (2020). Copyright 2020, Royal
Society of Chemistry.55. Q. Liu, J. Xing, Z. Jiang, X. Jiang, Y. Wang, and J.
Zhao, Nanoscale 12(12), 6776 (2020). https://doi.org/10.1039/D0NR00092B Panel
(b) reproduced with permission from Lançon et al., Phys. Rev. B 98, 134414
(2018). Copyright 2018 American Physical Society.112112. D. Lançon, R. A.
Ewings, T. Guidi, F. Formisano, and A. R. Wildes, Phys. Rev. B 98(13), 134414
(2018). https://doi.org/10.1103/PhysRevB.98.134414 Panel (c) reproduced with
permission from Liu et al., J. Mater. Chem. C 5, 727 (2017). Copyright 2017
Royal Society of Chemistry.124124. J. Liu, Z. Liu, T. Song, and X. Cui, J.
Mater. Chem. C 5(3), 727 (2017). https://doi.org/10.1039/C6TC04490E Panel (d)
reproduced with permission from Li et al., J. Phys. Chem. Lett. 10, 2439 (2019).
Copyright 2019 American Chemical Society.77. X. Li and J. Yang, J. Phys. Chem.
Lett. 10(10), 2439 (2019). https://doi.org/10.1021/acs.jpclett.9b00769
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To clarify the novel AFM ground state of ternary MPS3 monolayer, the dominant
electron hopping paths were analyzed. As a representative case, the first
neighboring-to-neighboring interaction for MnPSe3 monolayer is shown in Fig.
4(b).123123. Q. Pei, X.-C. Wang, J.-J. Zou, and W.-B. Mi, Front. Phys. 13(4),
137105 (2018). https://doi.org/10.1007/s11467-018-0796-9 Electrons hopping from
two paths has been discussed—one is short-range direct interaction between the
two neighboring Mn ions, and the other one is long-range Mn–Se–Mn superexchange
with an angle of 84.1°. Owing to the strong interaction between neighboring Mn2+
cations and the large electron excitation energy from Se p orbital to Mn d
orbital, direct AFM exchange interaction prevails over FM superexchange
interaction.
Moreover, robust FM coupling with high Curie temperature was also observed in
1T-TaN2124124. J. Liu, Z. Liu, T. Song, and X. Cui, J. Mater. Chem. C 5(3), 727
(2017). https://doi.org/10.1039/C6TC04490E and 1T-YN2 monolayers.66. Z. Liu, J.
Liu, and J. Zhao, Nano Res. 10(6), 1972 (2017).
https://doi.org/10.1007/s12274-016-1384-3 In these systems, the N–N distances
(∼1.74 Å) are short enough to generate the strong direct exchange interaction.
In TaN2, the magnetic moment arises mainly from the fully filled spin-up pz
orbitals and nearly unfilled spin-down pz orbitals. Benefiting from the
delocalized feature of p orbitals of N atoms, p-p direct exchange interaction
[see Fig.4(c)] leads to strong long-range FM coupling. In 1T-YN2, the J1
parameter is 11.3 meV, confirming again that the direct interaction is FM
coupling.
Robust ferrimagnetic ordering was proposed in 2D metal organic frameworks with
conjugated electron acceptors diketopyrrolopyrrole (DPP) as organic linkers and
transition metal Cr as nodes, namely, Cr-DPP [Fig. 4(d)].77. X. Li and J. Yang,
J. Phys. Chem. Lett. 10(10), 2439 (2019).
https://doi.org/10.1021/acs.jpclett.9b00769 In 2D Cr-DPP, each Cr atom possesses
a spin magnetic moment of around 4 μB, and each DPP unit has a spin magnetic
moment of about 1 μB. Considering the symmetry matching rule, the majority of
magnetic coupling between Cr and DPP can be ascribed to direct exchange
interaction between dxy↑ orbital of Cr and p↓ orbital (px or py) of the adjacent
N atoms. Meanwhile, direct exchange between dxz/dyz↑ orbital of Cr and pz↓
orbital of N contributes to the minority part due to the comparatively big
energy gap between the two orbitals. The strong d-p direct exchange coupling of
2D Cr-DPP yields a Curie temperature of 316 K. Robust d-p direct exchange
coupling was also found in 2D ferrimagnetic V-DPP77. X. Li and J. Yang, J. Phys.
Chem. Lett. 10(10), 2439 (2019). https://doi.org/10.1021/acs.jpclett.9b00769 and
Cr-pentalene MOF,125125. X. Li and J. Yang, J. Am. Chem. Soc. 141(1), 109
(2019). https://doi.org/10.1021/jacs.8b11346 and the corresponding TC was 406
and 560 K, respectively.
2. Superexchange interaction
Among the reported 2D magnets, there is a large number of transition metal
compounds, such as binary/ternary transition metal halides, chalcogenides,
borides, carbides, nitrides, oxides, hydrides, and silicides. Their detailed
magnetic properties will be discussed in Sec. III. As we stated above, the
direct overlap between d orbitals in these transition metal compounds is
generally too small due to the large distance. Thus, d electrons can only move
through hybridization with the ligand atoms between them, like 2p orbitals of B,
C, N, O, H, and F. Such p-d hybridization provides a common type of exchange
mechanism, known as superexchange interaction. That is to say, superexchange
interaction arises from the non-neighboring magnetic ions mediated by the
neighboring non-magnetic ions. Empirically, the magnetic ground state is
determined by Goodenough-Kanamori-Anderson (GKA) rules,126–128126. J. B.
Goodenough, Phys. Rev. 100(2), 564 (1955).
https://doi.org/10.1103/PhysRev.100.564127. J. Kanamori, J. Appl. Phys. 31(5),
S14 (1960). https://doi.org/10.1063/1.1984590128. P. W. Anderson, Phys. Rev.
115(1), 2 (1959). https://doi.org/10.1103/PhysRev.115.2 which are based on the
symmetry relationships and electron occupancy of the overlapping atomic
orbitals. According to these rules: (1) A 180° superexchange interaction of two
magnetic ions with partially filled d shells is AFM if virtual electron transfer
occurs between the overlapping orbitals that are each half filled. (2) A 180°
superexchange interaction of two magnetic ions with partially filled d shells is
FM if virtual electron transfer occurs from a half-filled to an empty orbital or
from a filled to a half-filled orbital. (3) A 90° superexchange interaction
where the occupied d orbitals of metal atom overlap with different orthogonal p
orbitals of the ligands results in weak ferromagnetism. In addition, the
strength of superexchange coupling is also sensitive to two factors, i.e., (1)
the degree of p-d hopping process and (2) the strength of SOC.
Based on the superexchange mechanism and GKA rules, the magnetic behavior of a
variety of 2D magnets has been successfully explained, including the recently
highlighted CrI3,88. H. Wang, F. Fan, S. Zhu, and H. Wu, Europhys. Lett. 114(4),
47001 (2016). https://doi.org/10.1209/0295-5075/114/47001 VI3,99. K. Yang, F.
Fan, H. Wang, D. I. Khomskii, and H. Wu, Phys. Rev. B 101(10), 100402 (2020).
https://doi.org/10.1103/PhysRevB.101.100402 VS2,1010. H. L. Zhuang and R. G.
Hennig, Phys. Rev. B 93(5), 054429 (2016).
https://doi.org/10.1103/PhysRevB.93.054429 Cr2Ge2Te3,1111. Y. Sun, R. C. Xiao,
G. T. Lin, R. R. Zhang, L. S. Ling, Z. W. Ma, X. Luo, W. J. Lu, Y. P. Sun, and
Z. G. Sheng, Appl. Phys. Lett. 112(7), 072409 (2018).
https://doi.org/10.1063/1.5016568 MXenes,12,1312. H. Kumar, N. C. Frey, L. Dong,
B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b0257813. J. He, P. Lyu, and P. Nachtigall, J.
Mater. Chem. C 4(47), 11143 (2016). https://doi.org/10.1039/C6TC03917K
MnBi2Te4,1414. J. Li, Y. Li, S. Du, Z. Wang, B.-L. Gu, S.-C. Zhang, K. He, W.
Duan, and Y. Xu, Sci. Adv. 5(6), eaaw5685 (2019).
https://doi.org/10.1126/sciadv.aaw5685 GdI2,129129. B. Wang, X. Zhang, Y. Zhang,
S. Yuan, Y. Guo, S. Dong, and J. Wang, Mater. Horiz. 7, 1623 (2020).
https://doi.org/10.1039/D0MH00183J and so on. For instance, 2D CrI3 is an
insulator [Fig. 5(a)] with local spin of S = 3/2.88. H. Wang, F. Fan, S. Zhu,
and H. Wu, Europhys. Lett. 114(4), 47001 (2016).
https://doi.org/10.1209/0295-5075/114/47001 Considering the local octahedral
coordination field [Fig. 5(b)], the valence bands and the conduction bands
consist of t2g states and eg states, respectively. The closed t2g3 configuration
indicates the formal Cr3+ charge state. The exchange splitting in 2D CrI3 is
about 3 eV, which is larger than the t2g-eg crystal field splitting. For the
corner sharing I atoms, their in-plane px/py orbitals couple to the Cr dx2-y2
orbital, and the out-of-plane I pz to Cr dxz/yz [Fig. 5(c)]. Owing to the closed
t2g3 subshell, the direct exchange interaction between two neighboring Cr3+ ions
is AFM, which is associated with Pauli exclusion principle in the virtually
excited t2g2-t2g4 state. However, the Cr–Cr distance is as large as 3.95 Å; thus
the AFM exchange should be weak. As a consequence, the two nearly 90° Cr–I–Cr
superexchange mechanisms dominate in 2D CrI3. One originates from the stronger
p-d hybridization via the orthogonal orbitals [Figs. 5(d) and 5(f)], and the
other involves the relatively weak p-d hybridization via the same px orbital
[Figs. 5(d) and 5(e)]. Both of them give an effective Cr–Cr FM coupling.
According to the weak 90° d-p-d superexchange interaction, the Curie temperature
of CrI3 monolayer is 45 K.2828. B. Huang, G. Clark, E. Navarro-Moratalla, D. R.
Klein, R. Cheng, K. L. Seyler, D. Zhong, E. Schmidgall, M. A. McGuire, D. H.
Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero, and X. Xu, Nature 546(7657), 270
(2017). https://doi.org/10.1038/nature22391 In fact, the weak superexchange
interaction also results in low Curie temperatures for the experimentally
reported 2D magnets, such as 30 K for Cr2Ge2Te63737. C. Gong, L. Li, Z. Li, H.
Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang, Z. Q. Qiu, R. J. Cava,
S. G. Louie, J. Xia, and X. Zhang, Nature 546(7657), 265 (2017).
https://doi.org/10.1038/nature22060 and 34 K for CrBr3.3131. Z. Zhang, J. Shang,
C. Jiang, A. Rasmita, W. Gao, and T. Yu, Nano Lett. 19(5), 3138 (2019).
https://doi.org/10.1021/acs.nanolett.9b00553
FIG. 5. (a) Honeycomb lattice of CrI3 monolayer. Three magnetic pair
interactions are marked with J1, J2, and J3. (b) Edge-sharing CrI6 octahedra.
(c) Partial density of states (DOS) for Cr 3d and I 5p orbitals, and the Fermi
level is set to zero. (d)–(f) Schematic structures of FM superexchange
interactions in CrI3 monolayer. (g) Illustrations of the Cr–Cr direct exchange,
Cr–X–Cr superexchange, and Cr–X–Cr double exchange interactions in Cr3X4 (X = S,
Se, Te) monolayers. Panels (a)–(f) reproduced with permission from Wang et al.,
Europhys. Lett. 114, 47001 (2016). Copyright 2016 IOP.88. H. Wang, F. Fan, S.
Zhu, and H. Wu, Europhys. Lett. 114(4), 47001 (2016).
https://doi.org/10.1209/0295-5075/114/47001 Panel (g) reproduced with permission
from Zhang et al., Nanoscale Horiz. 4, 859 (2019). Copyright 2019 Royal Society
of Chemistry.1515. X. Zhang, B. Wang, Y. Guo, Y. Zhang, Y. Chen, and J. Wang,
Nanoscale Horiz. 4(4), 859 (2019). https://doi.org/10.1039/C9NH00038K
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3. Double exchange interaction
Besides direct exchange and superexchange, double exchange interaction always
exists in the 2D magnets with high-spin states. It arises between the ions in
different oxidation states. In double exchange pictures, the interaction occurs
when one atom has an extra electron compared to the other one. The electron
transfer from the neighboring sites should have the same direction of spin.
Therefore, the magnetic coupling is ferromagnetic.130130. P. G. De Gennes, Phys.
Rev. 118(1), 141 (1960). https://doi.org/10.1103/PhysRev.118.141 For example,
Zhang et al.1515. X. Zhang, B. Wang, Y. Guo, Y. Zhang, Y. Chen, and J. Wang,
Nanoscale Horiz. 4(4), 859 (2019). https://doi.org/10.1039/C9NH00038K provided a
double exchange model [Fig. 5(g)] in Cr3X4 monolayers (X = S, Se, Te), where
seven hexagonal atomic layers are stacked in the sequence of
X1–Cr1–X2–Cr2–X2–Cr1–X1 along the z direction. The Cr1 and Cr2 atoms have
different local coordination environments and show different valence states as
Cr13+ and Cr22+, respectively. The X atom connects the nearest-neighboring Cr13+
and Cr22+ ions and gives up its spin-up or spin-down electron to Cr13+. Then its
vacant orbital could be filled by an electron from Cr22+. Mediated by the X
atom, double exchange process is revealed by the electron hopping from one Cr
ion to the other neighboring Cr ion of different oxidation state. Such mechanism
dominates in Cr3Se4 and Cr3Te4 monolayers, and strengthens the FM coupling.
Consequently, high Curie temperatures of 370 and 460 K were reported for Cr3Se4
and Cr3Te4 monolayers, respectively.
4. Extended superexchange theory
The above three fundamental magnetic interactions have already explained the
origin of most 2D magnetic insulators. However, the magnetic ground state of
some complicated 2D materials are still too difficult to be determined from
these theories. In such situations, a few new theories, i.e., extended
superexchange interaction, super-superexchange interaction, and
multi-intermediate double exchange interactions, have been proposed recently.
For the 2D materials containing anions with different valence states, an
extended superexchange theory was further proposed.2525. F. Zhang, Y.-C. Kong,
R. Pang, L. Hu, P.-L. Gong, X.-Q. Shi, and Z.-K. Tang, New J. Phys. 21(5),
053033 (2019). https://doi.org/10.1088/1367-2630/ab1ee4 It has been validated in
the representative CrOCl and FeOCl monolayers. Four possible superexchange paths
(P1–P4) in CrOCl are displayed in Fig. 6(a). Similar to 2D CrI3, the Cr atoms in
2D CrOCl are still located in a distorted octahedral crystal field. The d3 state
(Cr3+) in CrI3 can be written as t2g3eg,0 while the d3 state (Cr3+) and d4 state
(Cr2+) in CrOCl is t2g3eg0 and t2g3eg1, respectively. The extended superexchange
theory indicates that (1) a 180° bond angle in the interaction path Cr–O–Cr (P4)
favors strongly FM configuration through the dominated pσ-pσ bond; (2) a 180°
bond angle in the P3 path corresponds to strong AFM, where the interaction is
conducted through pσ-pσ bond; (3) For Cr–O–Cr path (P2) with 90° bond angle, the
d3 state has a crucial unfilled eg orbital; thus, the interaction is FM with
weak coupling strength, owing to the competition of the two pσ-pπ bonds; (4) For
a 90° bond angle in the interaction path of Cr-Cl-Cr (P1), the d4 states
interact through two pσ-pπ bonds with moderate strength of AFM coupling. Based
on first-principles calculations, they further clarified that monolayer CrOCl
exhibited antiferromagnetic ordering.2525. F. Zhang, Y.-C. Kong, R. Pang, L. Hu,
P.-L. Gong, X.-Q. Shi, and Z.-K. Tang, New J. Phys. 21(5), 053033 (2019).
https://doi.org/10.1088/1367-2630/ab1ee4 On all accounts, this extended
superexchange theory supports the strongly FM/AFM configurations, which are
highly anticipated to design robust 2D magnetic materials with polyvalent
anions. Based on this theory, the high Curie/Néel temperatures in 2D MXY (M =
metal; X = S, Se, Te; Y = F, Cl, Br, I) compounds could be
explained.26,65,70,131–13326. C. Wang, X. Zhou, L. Zhou, N.-H. Tong, Z.-Y. Lu,
and W. Ji, Sci. Bull. 64(5), 293 (2019).
https://doi.org/10.1016/j.scib.2019.02.01165. Z. Jiang, P. Wang, J. Xing, X.
Jiang, and J. Zhao, ACS Appl. Mater. Inter. 10(45), 39032 (2018).
https://doi.org/10.1021/acsami.8b1403770. R. Han, Z. Jiang, and Y. Yan, J. Phys.
Chem. C 124(14), 7956 (2020). https://doi.org/10.1021/acs.jpcc.0c01307131. Y.
Guo, Y. Zhang, S. Yuan, B. Wang, and J. Wang, Nanoscale 10(37), 18036 (2018).
https://doi.org/10.1039/C8NR06368K132. J. Pan, J. Yu, Y.-F. Zhang, S. Du, A.
Janotti, C.-X. Liu, and Q. Yan, npj Computat. Mater. 6, 152 (2020).
https://doi.org/10.1038/s41524-020-00419-y133. S. Wang, J. Wang, and M. Khazaei,
Phys. Chem. Chem. Phys. 22(20), 11731 (2020). https://doi.org/10.1039/D0CP01767A
According to above discussions, the extended superexchange theory may be
regarded as a combination of superexchange and double exchange. Similar
mechanism has also been proposed by Wang et al.2626. C. Wang, X. Zhou, L. Zhou,
N.-H. Tong, Z.-Y. Lu, and W. Ji, Sci. Bull. 64(5), 293 (2019).
https://doi.org/10.1016/j.scib.2019.02.011
FIG. 6. (a) Four possible superexchange paths in CrOCl monolayer. Electrons can
only hop between orbitals that are connected by dashed blue and red lines. (b)
Hopping of t2g−t2g and t2g-eg orbitals in the FM/AFM alignment. (c) Interlayer
Cr nearest-neighboring J1, the second-nearest-neighboring J2 in AB-stacking, and
the nearest-neighboring J′1 in AB′-stacking CrI3. (d) Schematic diagrams for the
AFM/FM super-superexchange interaction involving different orbital
hybridizations. (e) Electronic structure, FM-AFM, and FM-FM interlayer double
superexchange mechanism of bilayer CrSe2. The up and down solid arrows represent
the electron with different spin components and the hollow arrows display
different magnetic moments of the Cr atoms. The interlayer sharing electrons are
surrounded by dashed green circles. The length of the arrow qualitatively shows
the amounts of electrons with given spin component. Panel (a) reproduced with
permission from Zhang et al., New J. Phys. 21, 053033 (2019). Licensed under a
Creative Commons Attribution (CC-BY-3.0).2525. F. Zhang, Y.-C. Kong, R. Pang, L.
Hu, P.-L. Gong, X.-Q. Shi, and Z.-K. Tang, New J. Phys. 21(5), 053033 (2019).
https://doi.org/10.1088/1367-2630/ab1ee4 Panels (b)–(d) reproduced with
permission from Sivadas et al., Nano Lett. 18, 7658 (2018). Copyright 2018
American Chemical Society.2323. N. Sivadas, S. Okamoto, X. Xu, C. J. Fennie, and
D. Xiao, Nano Lett. 18(12), 7658 (2018).
https://doi.org/10.1021/acs.nanolett.8b03321 Panel (e) reproduced with
permission from Wang et al., Phys. Rev. B 102, 020402 (2020). Copyright 2020
American Physical Society.2727. C. Wang, X. Zhou, L. Zhou, Y. Pan, Z.-Y. Lu, X.
Wan, X. Wang, and W. Ji, Phys. Rev. B 102(2), 020402 (2020).
https://doi.org/10.1103/PhysRevB.102.020402
* PPT
|
* High-resolution
5. Super-superexchange interaction
The super-superexchange interaction is different from the typical superexchange
interactions by the mediated ligands.23,2423. N. Sivadas, S. Okamoto, X. Xu, C.
J. Fennie, and D. Xiao, Nano Lett. 18(12), 7658 (2018).
https://doi.org/10.1021/acs.nanolett.8b0332124. J. Xiao and B. Yan, 2D Mater.
7(4), 045010 (2020). https://doi.org/10.1088/2053-1583/ab9ea4 As we stated
above, single anion serves as an intermediate to bridge two magnetic cations in
the superexchange interaction (M–X–M). In contrast, the super-superexchange
interaction involves longer M–X⋯X–M hopping paths. According to the distance
between magnetic ions, the strength of super-superexchange interaction is
generally much weaker than those of direct interaction and superexchange
interaction. Taking single layer CrAsS4 as a representative, the exchange
interaction parameter contributed by both short-range Cr–Cr direct exchange
interaction and Cr–S–Cr superexchange interaction is 3.03 meV, while the value
for long-range Cr–S–As–S–Cr super-superexchange interaction is –0.49 meV.
Therefore, the super-superexchange interaction is usually neglected in most
investigations. However, it could become stronger than superexchange interaction
in some few-layer magnetic systems with non-covalent van der Waals (vdW) gaps.
The importance of super-superexchange interaction is highlighted in the
well-known bilayer CrI3.23,28,13423. N. Sivadas, S. Okamoto, X. Xu, C. J.
Fennie, and D. Xiao, Nano Lett. 18(12), 7658 (2018).
https://doi.org/10.1021/acs.nanolett.8b0332128. B. Huang, G. Clark, E.
Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E. Schmidgall,
M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero, and X. Xu,
Nature 546(7657), 270 (2017). https://doi.org/10.1038/nature22391134. P. Jiang,
C. Wang, D. Chen, Z. Zhong, Z. Yuan, Z.-Y. Lu, and W. Ji, Phys. Rev. B 99(14),
144401 (2019). https://doi.org/10.1103/PhysRevB.99.144401 For bilayer CrI3, the
local magnetic moments form intralayer FM ordering below 45 K.2828. B. Huang, G.
Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E.
Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero,
and X. Xu, Nature 546(7657), 270 (2017). https://doi.org/10.1038/nature22391
However, its interlayer magnetic coupling varies between FM and AFM, depending
on local stacking geometry.2323. N. Sivadas, S. Okamoto, X. Xu, C. J. Fennie,
and D. Xiao, Nano Lett. 18(12), 7658 (2018).
https://doi.org/10.1021/acs.nanolett.8b03321 These significant results reveal
that even weak overlap has a large impact on the interlayer magnetic coupling
through exchange between two adjacent interlayer I atoms, which signifies a new
exchange interaction mechanism, i.e., super-superexchange. It also means that
the super-superexchange is basically the coupling between next-next nearest
neighbor.
Sivadas et al.2323. N. Sivadas, S. Okamoto, X. Xu, C. J. Fennie, and D. Xiao,
Nano Lett. 18(12), 7658 (2018). https://doi.org/10.1021/acs.nanolett.8b03321
carefully investigated the details of stacking-dependent interlayer exchange
interaction for bilayer CrI3 systems. The AB stacking (S6 point group) from the
low-temperature bulk structure and the AB′ stacking (C2h point group) from the
high-temperature bulk structure were considered. Figure 6(b) schematically shows
different exchange interactions between Cr atoms in different layers. One can
see that hopping from a t2g to t2g orbital is prohibited for FM alignment, while
this is allowed for AFM alignment. Therefore, t2g to t2g hybridization leads to
AFM ordering. On the other hand, hopping from t2g-eg leads to an exchange
coupling that is predominantly FM because of the local Hund coupling. All these
interlayer Cr−Cr exchange interactions are mediated by the hybridization between
I pz orbitals in different layers, which is the nature of super-superexchange
interaction.
The stacking-dependent magnetism originates from a competition between different
interlayer orbital hybridizations. Interlayer Cr−Cr nearest neighboring
interaction J1 and the second neighboring one J2 in AB-stacking are shown in
Fig. 6(c). One can see that J1 is dominated by the virtual excitations between
the half-filled t2g orbitals of Cr and induces an AFM coupling, while J2 is
dominated by a virtual excitation from the half-filled t2g orbitals of Cr to the
empty eg orbitals, resulting in a FM coupling [Fig. 6(c)]. Clearly, the second
neighboring FM interlayer super-superexchange prevails as the AFM nearest
neighboring interlayer exchange, making AB-stacking system ferromagnetic. A
lateral shift of one layer to AB′-stacking of the bilayer system breaks the
interlayer hybridization between I p states and generates a new type of
hybridization, which in turn would reduce the strength of FM exchange
interactions and result in an AFM ground state for AB′-stacking [Fig. 6(d)].
6. Multi-intermediate double exchange interaction
Compared to bilayer CrI3 with the same Cr3+ ions, a mixture of Cr4+ and Cr3+ was
found in bilayer CrS2 due to the charge transfer from eg to t2g orbitals. This
mixed valance state, together with delocalized S p orbitals and their resulting
strongly interlayered S-S hopping, favor the double exchange interaction
mechanism.135135. C. Wang, X. Zhou, Y. Pan, J. Qiao, X. Kong, C.-C. Kaun, and W.
Ji, Phys. Rev. B 97(24), 245409 (2018).
https://doi.org/10.1103/PhysRevB.97.245409 With further increase of the strength
of interlayer coupling, a novel multi-intermediate double exchange interaction
has been revealed by extensively investigating nine TMD bilayers MX2 (M = V, Cr,
Mn; X = S, Se, Te).2727. C. Wang, X. Zhou, L. Zhou, Y. Pan, Z.-Y. Lu, X. Wan, X.
Wang, and W. Ji, Phys. Rev. B 102(2), 020402 (2020).
https://doi.org/10.1103/PhysRevB.102.020402 Taking 2D CrSe2 as a prototype, one
can see a distinct overlapped region (OR) at the interlayer area, as displayed
in Fig. 6(e). In other words, OR could be effectively considered as an area
accumulating an appreciable shared charge from the two adjacent interfacial Se
sublayers. The OR can be regarded as a real atomic site and plays an important
role in determining the interlayer magnetic coupling. In the interlayer FM
configuration, the transferred spin-up charge of Se pz to Cr leaves the
spin-down component predominated at the OR [Fig. 6(e)]. Hence, the spin-up
electrons of the bottom Cr atom could hop into the top Cr atom through 4pz
orbital of Se_ib atom [defined in Fig. 6(e)], and then through OR upon
excitation, and further through Se_it 4pz, as denoted by the wave-like
red-dotted arrow [Fig. 6(e)]. Such electron hopping process largely reduces the
kinetic energy of spin-up electron across the bilayer. The process in CrSe2 is
similar to double exchange interaction of CrS2135135. C. Wang, X. Zhou, Y. Pan,
J. Qiao, X. Kong, C.-C. Kaun, and W. Ji, Phys. Rev. B 97(24), 245409 (2018).
https://doi.org/10.1103/PhysRevB.97.245409 but is mediated by multiple sites,
which is termed as a new multi-intermediate double exchange interaction.
Similarly, the interlayer AFM bilayer also has stacking-induced charge transfer
and the interfacial Se pz overlapping area. As displayed in Fig. 6(e), the
spin-up electron of bottom Cr could still hop into the Se_ib 4pz orbital and
reach the OR. However, the next hopping step from OR to the Se_it 4pz orbital is
forbidden since the spin-up component is fully occupied. This appreciably lifts
up the kinetic energy. Based on the modified interlayer Hubbard model, the
competition between the interlayer hopping across the bilayer and the Pauli and
Coulomb repulsions at OR will determine the MX2 bilayers to have FM or AFM
magnetic ground state.
F. Stoner model
In the above discussions, we mainly focus on 2D FM/AFM insulators. Their
magnetism originates from local magnetic moments with exchange interactions that
can be interpreted by the Heisenberg exchange mode. For 2D magnetic metals, a
simplified model called Stoner model,1616. H. L. Zhuang, P. R. C. Kent, and R.
G. Hennig, Phys. Rev. B 93(13), 134407 (2016).
https://doi.org/10.1103/PhysRevB.93.134407 can be formulated in terms of
dispersion relations for the spin-up and spin-down electrons. In the Stoner
model, there is a competition between kinetic energy and exchange energy due to
Coulomb repulsion. In ferromagnetic metals, the exchange interaction will split
the energy of states with different spins. This restructuring of the spins leads
to a change in the energy of the system. The up spins occupying higher energy
states would cost the kinetic energy. At the same time, the potential energy
would decrease due to spin-spin exchange interaction. These changes in total
energy provide the Stoner criterion for itinerant ferromagnetism. The Stoner
criterion for itinerant ferromagnetic ordering is D(EF)×I > 1, where D(EF) is
the total density of states at the Fermi level (EF), and the Stoner parameter I
can be estimated from dividing the exchange splitting of spin-up and spin-down
bands by the corresponding magnetic moments. These two parameters reflect the
competition between the exchange energy and kinetic energy. The former parameter
D(EF) is inversely proportional to the kinetic energy of electrons, whereas the
latter one, I, describes the strength of electron exchange. Owing to the nature
of itinerant electrons, Stoner model is applicable to illustrate the origin of
the spontaneous magnetization in plenty of 2D metallic magnets, such as
Fe3GeTe2,1616. H. L. Zhuang, P. R. C. Kent, and R. G. Hennig, Phys. Rev. B
93(13), 134407 (2016). https://doi.org/10.1103/PhysRevB.93.134407
TMDs,17,18,20,2117. Y. Ma, Y. Dai, M. Guo, C. Niu, Y. Zhu, and B. Huang, ACS
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Mazurenko, and D. W. Boukhvalov, Phys. Chem. Chem. Phys. 21(40), 22647 (2019).
https://doi.org/10.1039/C9CP03726H21. Y. Wen, Z. Liu, Y. Zhang, C. Xia, B. Zhai,
X. Zhang, G. Zhai, C. Shen, P. He, R. Cheng, L. Yin, Y. Yao, M. Getaye Sendeku,
Z. Wang, X. Ye, C. Liu, C. Jiang, C. Shan, Y. Long, and J. He, Nano Lett. 20(5),
3130 (2020). https://doi.org/10.1021/acs.nanolett.9b05128 MXene,19,97,13619.
J.-J. Zhang, L. Lin, Y. Zhang, M. Wu, B. I. Yakobson, and S. Dong, J. Am. Chem.
Soc. 140(30), 9768 (2018). https://doi.org/10.1021/jacs.8b0647597. Y. Yue, J.
Magn. Magn. Mater. 434, 164 (2017).
https://doi.org/10.1016/j.jmmm.2017.03.058136. C. Si, J. Zhou, and Z. Sun, ACS
Appl. Mater. Inter. 7(31), 17510 (2015). https://doi.org/10.1021/acsami.5b05401
as well as a variety of charge doped 2D materials.137–142137. T. Cao, Z. Li, and
S. G. Louie, Phys. Rev. Lett. 114(23), 236602 (2015).
https://doi.org/10.1103/PhysRevLett.114.236602138. S.-H. Zhang and B.-G. Liu, J.
Mater. Chem. C 6(25), 6792 (2018). https://doi.org/10.1039/C8TC01450G139. S.
Gong, W. Wan, S. Guan, B. Tai, C. Liu, B. Fu, S. A. Yang, and Y. Yao, J. Mater.
Chem. C 5(33), 8424 (2017). https://doi.org/10.1039/C7TC01399J140. H. Xiang, B.
Xu, Y. Xia, J. Yin, and Z. Liu, Sci. Rep. 6(1), 39218 (2016).
https://doi.org/10.1038/srep39218141. Y. Nie, M. Rahman, P. Liu, A. Sidike, Q.
Xia, and G.-h. Guo, Phys. Rev. B 96(7), 075401 (2017).
https://doi.org/10.1103/PhysRevB.96.075401142. H. Y. Lv, W. J. Lu, X. Luo, X. B.
Zhu, and Y. P. Sun, Phys. Rev. B 99(13), 134416 (2019).
https://doi.org/10.1103/PhysRevB.99.134416 Most of these reported 2D magnetic
metals have excitingly high Curie temperatures.
The electronic and magnetic properties of 2D Fe3GeTe2 have been investigated by
Zhuang et al.1616. H. L. Zhuang, P. R. C. Kent, and R. G. Hennig, Phys. Rev. B
93(13), 134407 (2016). https://doi.org/10.1103/PhysRevB.93.134407 The spin
orbital projected band structures [Fig. 7(a)] show its metallic behavior.
Several partially occupied d bands crossing the Fermi level contribute to the
noninteger magnetic moment of Fe (1.484 μB). Both characters indicate the
itinerant ferromagnetism in Fe3GeTe2. The two important parameters of Stoner
model, i.e., D(EF) and I, were determined from DOS as 1.56 states/eV per Fe atom
and 0.71 eV, respectively. Therefore, Stoner's criterion of I×D(EF) > 1 is
satisfied, giving rise to the itinerant ferromagnetic ordering in monolayer
Fe3GeTe2. The validity of Stoner model was also extended to the successively
reported 2D Fe5GeTe2,143143. M. Joe, U. Yang, and C. Lee, Nano Mater. Sci. 1(4),
299 (2019). https://doi.org/10.1016/j.nanoms.2019.09.009 which has a Curie
temperature of 270 K.4242. A. F. May, D. Ovchinnikov, Q. Zheng, R. Hermann, S.
Calder, B. Huang, Z. Fei, Y. Liu, X. Xu, and M. A. McGuire, ACS Nano 13(4), 4436
(2019). https://doi.org/10.1021/acsnano.8b09660
FIG. 7. (a) Orbital-resolved spin-up and spin-down band structures of
single-layer Fe3GeTe2. (b) A summary of four types of known room-temperature vdW
ferromagnets and their unit cell. (c) Hopping paths of the nearest,
next-nearest, and next-next-nearest interactions in a MnSiTe3 monolayer. (d)
RKKY interaction strength as a function of Mn–Mn distance and first-principles
results for (JMnSiTe3–JCrSiTe3). The yellow triangle, blue circle, and green
square denote the first-principles results for the difference between JMnSiTe3
and JCrSiTe3 for the N, NN, and NNN neighbors, respectively. The red solid line
denotes the interaction strength calculated within RKKY model with kF = 0.47
Å−1. Panel (a) reproduced with permission from Zhuang et al., Phys. Rev. B 93,
134407 (2016). Copyright 2016 American Physical Society.1616. H. L. Zhuang, P.
R. C. Kent, and R. G. Hennig, Phys. Rev. B 93(13), 134407 (2016).
https://doi.org/10.1103/PhysRevB.93.134407 Panel (b) reproduced with permission
from Sun et al., Nano Res. 13, 3358 (2020). Copyright 2020 Springer
Nature.147147. X. Sun, W. Li, X. Wang, Q. Sui, T. Zhang, Z. Wang, L. Liu, D. L.
Li, S. Feng, S. Zhong, H. Wang, V. Bouchiat, M. N. Regueiro, N. Rougemaille, J.
Coraux, Z. Wang, B. Dong, X. Wu, T. Yang, G. Yu, B. Wang, Z. V. Han, X. Han, and
Z. Zhang, Nano Res. 13, 3358 (2020). https://doi.org/10.1007/s12274-020-3021-4
Panels (c) and (d) reproduced with permission from Zhang et al., Phys. Rev. B
101, 205119 (2020). Copyright 2020 American Physical Society.2222. D. Zhang, A.
Rahman, W. Qin, X. Li, P. Cui, Z. Zhang, and Z. Zhang, Phys. Rev. B 101(20),
205119 (2020). https://doi.org/10.1103/PhysRevB.101.205119
* PPT
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* High-resolution
Intrinsic vdW ferromagnets with TC above 300 K have been experimentally reported
in some binary transitional metal dichalcogenides (MX2) with structural phases
containing the octahedral units [Fig. 7(b)], including 1T-MnSex,144144. I. Eren,
F. Iyikanat, and H. Sahin, Phys. Chem. Chem. Phys. 21(30), 16718 (2019).
https://doi.org/10.1039/C9CP03112J 1T-VSe2,34,14534. M. Bonilla, S. Kolekar, Y.
Ma, H. C. Diaz, V. Kalappattil, R. Das, T. Eggers, H. R. Gutierrez, M.-H. Phan,
and M. Batzill, Nature Nanotechnol. 13(4), 289 (2018).
https://doi.org/10.1038/s41565-018-0063-9145. P. K. J. Wong, W. Zhang, F.
Bussolotti, X. Yin, T. S. Herng, L. Zhang, Y. L. Huang, G. Vinai, S.
Krishnamurthi, D. W. Bukhvalov, Y. J. Zheng, R. Chua, A. T. N'Diaye, S. A.
Morton, C.-Y. Yang, K.-H. O. Yang, P. Torelli, W. Chen, K. E. J. Goh, J. Ding,
M.-T. Lin, G. Brocks, M. P. de Jong, A. H. C. Neto, and A. T. S. Wee, Adv.
Mater. 31(23), 1901185 (2019). https://doi.org/10.1002/adma.201901185
1T-VTe2,146146. E. Vatansever, S. Sarikurt, and R. F. L. Evans, Mater. Res.
Express 5(4), 046108 (2018). https://doi.org/10.1088/2053-1591/aabca6 and
1T-CrTe2.147147. X. Sun, W. Li, X. Wang, Q. Sui, T. Zhang, Z. Wang, L. Liu, D.
L. Li, S. Feng, S. Zhong, H. Wang, V. Bouchiat, M. N. Regueiro, N. Rougemaille,
J. Coraux, Z. Wang, B. Dong, X. Wu, T. Yang, G. Yu, B. Wang, Z. V. Han, X. Han,
and Z. Zhang, Nano Res. 13, 3358 (2020).
https://doi.org/10.1007/s12274-020-3021-4 Both theoretical and experimental
results suggested that exchange coupling due to the enhancement of itinerant
type is responsible for their room-temperature ferromagnetism. The above four 2D
room-temperature ferromagnets are compared to the corresponding elemental metals
and some ferromagnetic elemental metals like Fe, Co, and Ni. As expected, only
Fe, Co, and Ni among all the considered elemental metals meet the Stoner
criterion to exhibit band ferromagnetism. In contrast, 2D VSe2, CrTe2, MnTe2,
and MnSe2 sheets have much higher I×D(EF) values than those of the corresponding
elementary metals; thus the Stoner criterion is met for a band ferromagnetism in
these 2D materials.147147. X. Sun, W. Li, X. Wang, Q. Sui, T. Zhang, Z. Wang, L.
Liu, D. L. Li, S. Feng, S. Zhong, H. Wang, V. Bouchiat, M. N. Regueiro, N.
Rougemaille, J. Coraux, Z. Wang, B. Dong, X. Wu, T. Yang, G. Yu, B. Wang, Z. V.
Han, X. Han, and Z. Zhang, Nano Res. 13, 3358 (2020).
https://doi.org/10.1007/s12274-020-3021-4
G. RKKY mechanism
RKKY is a particular form of magnetic interaction that occurs in metals with
localized magnetic moments. In the RKKY picture, the magnetic moments interact
effectively through an indirect exchange process mediated by the conduction
electrons. A localized magnetic moment induces spin polarization to the
surrounding conduction electrons, and such polarization in turn couples to
another neighboring localized moment. The coupling strength takes the form of
distance-dependent exchange interaction given by
𝐽𝑖𝑗(𝑟)∝ sin [(𝑘→↑𝐹+𝑘→↓𝐹)·𝑅→𝑖𝑗𝑅3𝑖𝑗,Jij(r)∝ sin [(k→F↑+k→F↓)·R→ijRij3],
(6)
where kF is the Fermi wave vector for two spin channels, and Rij is the distance
between the two magnetic atoms i and j. First, this interaction is of the
long-range type. Second, FM or AFM ground states depend on the interatomic
distance. Third, its strength oscillates with the distance. Similar to the
Heisenberg model, the strength of exchange coupling is related to the magnetic
transition temperature, but is not rooted in the symmetry. RKKY interaction is
well understood in conventional 3D intermetallic compounds, which is the
dominant coupling mechanism between rare earth ions. In 2D materials, however,
only a few theoretical studies have directly discussed the existence of
RKKY-type interaction.2222. D. Zhang, A. Rahman, W. Qin, X. Li, P. Cui, Z.
Zhang, and Z. Zhang, Phys. Rev. B 101(20), 205119 (2020).
https://doi.org/10.1103/PhysRevB.101.205119 In this regard, Zhang et al.2222. D.
Zhang, A. Rahman, W. Qin, X. Li, P. Cui, Z. Zhang, and Z. Zhang, Phys. Rev. B
101(20), 205119 (2020). https://doi.org/10.1103/PhysRevB.101.205119 inferred
that ferromagnetic interaction between Mn ions in MnSiTe3 monolayer possibly
originated from the RKKY coupling. Figure 7(c) displays the possible electron
hopping paths between Mn ions of the nearest (N), next-nearest (NN), and
next-next-nearest (NNN) interactions (JN, JNN, JNNN). Unlike CrSiTe3 and
CrGeTe3, the theoretical results showed that MnSiTe3 monolayer is a half metal
with the largest positive JNNN, which is inconsistent with superexchange model.
Benefiting from the half metallic nature, the mediation carriers between Mn ions
are 100% spin-polarized free carriers. Figure 7(d) presents the RKKY interaction
as a function of Mn–Mn distance. The RKKY contributions to the interaction for
the N, NN, NNN neighbors in the model show good agreement with the results from
first-principles calculations.2222. D. Zhang, A. Rahman, W. Qin, X. Li, P. Cui,
Z. Zhang, and Z. Zhang, Phys. Rev. B 101(20), 205119 (2020).
https://doi.org/10.1103/PhysRevB.101.205119 Nevertheless, further theoretical
and experimental efforts are still needed to exploit the RKKY mechanism in other
2D magnets.
III. THE 2D VDW MAGNETS DATABASE
Section:
ChooseTop of pageABSTRACTI. INTRODUCTIONII. THE ORIGIN OF MAGNETI...III. THE 2D
VDW MAGNETS D... <<IV. MODIFICATIONSV. CONCLUSION AND OUTLOOKAUTHORS'
CONTRIBUTIONSCITING ARTICLESChoose
Two-dimensional vdW magnets are defined as the layered materials that adopt
ferromagnetic/antiferromagnetic ground states at finite temperature. The
theoretical concept was first established 150 years ago.148148. L. Onsager,
Phys. Rev. 65(3–4), 117 (1944). https://doi.org/10.1103/PhysRev.65.117 Since
2004, many efforts have been devoted to searching for new 2D magnetic materials
with spontaneous magnetization.8484. D. L. Cortie, G. L. Causer, K. C. Rule, H.
Fritzsche, W. Kreuzpaintner, and F. Klose, Adv. Funct. Mater. 30(18), 1901414
(2020). https://doi.org/10.1002/adfm.201901414 In 2017, for the first time, the
intrinsic 2D ferromagnetism in atomically thin CrI3 and Cr2Ge2Te6 was
demonstrated in an experiment.28,3728. B. Huang, G. Clark, E. Navarro-Moratalla,
D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E. Schmidgall, M. A. McGuire, D.
H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero, and X. Xu, Nature 546(7657), 270
(2017). https://doi.org/10.1038/nature2239137. C. Gong, L. Li, Z. Li, H. Ji, A.
Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang, Z. Q. Qiu, R. J. Cava, S. G.
Louie, J. Xia, and X. Zhang, Nature 546(7657), 265 (2017).
https://doi.org/10.1038/nature22060 Nowadays, finding 2D ferromagnets with high
Curie temperatures remains an important issue for spintronics, and many
promising 2D materials have been attained theoretically and experimentally over
the past five years.
A. Binary transition metal halides
1. MX3
The first class of widely investigated 2D vdW magnets is binary transition metal
halides. During the past 150 years, many bulk crystals of transition metal
halides have been synthesized in the laboratory.149149. A. M. McGuire, Crystals
7(5), 121 (2017). https://doi.org/10.3390/cryst7050121 These bulk materials
adopted simple AA or ABC layered stacking geometry with weak interlayer vdW
interaction; thus, they can be mechanically exfoliated to fabricate 2D monolayer
or few-layered sheets.54,14954. J. Liu, Q. Sun, Y. Kawazoe, and P. Jena, Phys.
Chem. Chem. Phys. 18(13), 8777 (2016). https://doi.org/10.1039/C5CP04835D149. A.
M. McGuire, Crystals 7(5), 121 (2017). https://doi.org/10.3390/cryst7050121 The
compounds composed of transition metals with partially filled d shell are
commonly observed, since they are easy to host local magnetic moments and
produce ordered magnetism. Most of transition metal trihalides (MX3), transition
metal dihalides (MX2), transition metal monohalides (MX), and other
stoichiometric M-X monolayers reported to date are listed in Table II and
categorized by their compositions as well as magnetic properties for discussion.
TABLE II. A list of 2D magnets in binary transition metal halides family with
their compositions and key electronic and magnetic properties, including
magnetic ground state (GS), values of Hubbard U term, energy gap (Eg), magnetic
moment on per transition metal atom (Ms), Curie temperature (TC), and magnetic
anisotropy energy per unit cell (MAE). The experimental result is indicated by
the superscript i. Positive/negative MAE value corresponds to the
out-of-plane/in-plane easy magnetization direction.
Compositions GS U (eV) Eg (eV) Ms (μB) TC (K) MAE (meV) Ref. MX3 CrI3 FMi – – –
45i – 2828. B. Huang, G. Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K.
L. Seyler, D. Zhong, E. Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D.
Xiao, P. Jarillo-Herrero, and X. Xu, Nature 546(7657), 270 (2017).
https://doi.org/10.1038/nature22391 CrI3 FM 2.65 1.09 3.44 107 – 5454. J. Liu,
Q. Sun, Y. Kawazoe, and P. Jena, Phys. Chem. Chem. Phys. 18(13), 8777 (2016).
https://doi.org/10.1039/C5CP04835D CrI3 FM – 1.53 3 95 0.686 5353. W.-B. Zhang,
Q. Qu, P. Zhu, and C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J CrI3 FM 2.7 ∼1.0 3 33 0.65 151151. J. L. Lado
and J. Fernández-Rossier, 2D Mater. 4(3), 035002 (2017).
https://doi.org/10.1088/2053-1583/aa75ed CrI3 FM – 1.1 – 75 – 88. H. Wang, F.
Fan, S. Zhu, and H. Wu, Europhys. Lett. 114(4), 47001 (2016).
https://doi.org/10.1209/0295-5075/114/47001 CrI3 FM – 0.89 – 46 0.804 104104. L.
Webster and J.-A. Yan, Phys. Rev. B 98(14), 144411 (2018).
https://doi.org/10.1103/PhysRevB.98.144411 CrI3 FM 2.65 1.1 3.44 45 1.674
155155. M. Moaied, J. Lee, and J. Hong, Phys. Chem. Chem. Phys. 20(33), 21755
(2018). https://doi.org/10.1039/C8CP03489C CrI3 FM – 1.85 3.15 60 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 CrBr3 FM 2.68 1.76 3.25 86 – 5454. J.
Liu, Q. Sun, Y. Kawazoe, and P. Jena, Phys. Chem. Chem. Phys. 18(13), 8777
(2016). https://doi.org/10.1039/C5CP04835D CrBr3 FM – 2.54 3 73 0.186 5353.
W.-B. Zhang, Q. Qu, P. Zhu, and C.-H. Lam, J. Mater. Chem. C 3(48), 12457
(2015). https://doi.org/10.1039/C5TC02840J CrBr3 FM i – – ∼3 34 – 3131. Z.
Zhang, J. Shang, C. Jiang, A. Rasmita, W. Gao, and T. Yu, Nano Lett. 19(5), 3138
(2019). https://doi.org/10.1021/acs.nanolett.9b00553 CrBr3 FM – 1.38 – 41 0.16
104104. L. Webster and J.-A. Yan, Phys. Rev. B 98(14), 144411 (2018).
https://doi.org/10.1103/PhysRevB.98.144411 CrBr3 FM 2.68 1.82 3.25 33 0.327
155155. M. Moaied, J. Lee, and J. Hong, Phys. Chem. Chem. Phys. 20(33), 21755
(2018). https://doi.org/10.1039/C8CP03489C CrBr3 FM – 2.87 3.03 105 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 CrCl3 FM 2.63 2.28 3.12 66 – 5454. J.
Liu, Q. Sun, Y. Kawazoe, and P. Jena, Phys. Chem. Chem. Phys. 18(13), 8777
(2016). https://doi.org/10.1039/C5CP04835D CrCl3 FM – 3.44 3 49 0.031 5353.
W.-B. Zhang, Q. Qu, P. Zhu, and C.-H. Lam, J. Mater. Chem. C 3(48), 12457
(2015). https://doi.org/10.1039/C5TC02840J CrCl3 FM – 1.58 – 30 0.025 104104. L.
Webster and J.-A. Yan, Phys. Rev. B 98(14), 144411 (2018).
https://doi.org/10.1103/PhysRevB.98.144411 CrCl3 FM 2.63 2.3 2.12 23 0.038
155155. M. Moaied, J. Lee, and J. Hong, Phys. Chem. Chem. Phys. 20(33), 21755
(2018). https://doi.org/10.1039/C8CP03489C CrCl3 FM – 2.93 3.84 35 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 CrF3 FM – 4.68 3 41 0.119 5353. W.-B.
Zhang, Q. Qu, P. Zhu, and C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J CrF3 FM – 5.09 2.86 40 – 164164. S. Tomar, B.
Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A. Agarwal, and
S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 VF3 FM – 3.23 1.89 760 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 VCl3 FM 3.35 DHM 4 80 – 161161. J.
He, S. Ma, P. Lyu, and P. Nachtigall, J. Mater. Chem. C 4(13), 2518 (2016).
https://doi.org/10.1039/C6TC00409A VCl3 FM – 2.51 1.96 500 – 164164. S. Tomar,
B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A. Agarwal,
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https://doi.org/10.1016/j.jmmm.2019.165384 VI3 FM 3.68 DHM 4 98 – 161161. J. He,
S. Ma, P. Lyu, and P. Nachtigall, J. Mater. Chem. C 4(13), 2518 (2016).
https://doi.org/10.1039/C6TC00409A VI3 FM 3 0.89 2.01 27 – 160160. M. An, Y.
Zhang, J. Chen, H.-M. Zhang, Y. Guo, and S. Dong, J. Phys. Chem. C 123(50),
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164164. S. Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S.
Chauhan, A. Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 VBr3 FM – HM 1.97 190 –0.15 162162.
J. Sun, X. Zhong, W. Cui, J. Shi, J. Hao, M. Xu, and Y. Li, Phys. Chem. Chem.
Phys. 22(4), 2429 (2020). https://doi.org/10.1039/C9CP05084A VBr3 FM – 2.12 2.03
430 – 164164. S. Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S.
Chauhan, A. Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 NiF3 FM – 0 0.86 720 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 NiCl3 FM – DHM 2 400 – 110110. J. He,
X. Li, P. Lyu, and P. Nachtigall, Nanoscale 9(6), 2246 (2017).
https://doi.org/10.1039/C6NR08522A NiCl3 FM – 0 0.94 400 – 164164. S. Tomar, B.
Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A. Agarwal, and
S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 NiBr3 FM – HM – 100 –0.05 162162. J.
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22(4), 2429 (2020). https://doi.org/10.1039/C9CP05084A NiBr3 FM – 0 1.02 460 2
164164. S. Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S.
Chauhan, A. Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 NiI3 FM – 0 1.09 440 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 PdBr3 FM – HM – 110 –0.196 162162. J.
Sun, X. Zhong, W. Cui, J. Shi, J. Hao, M. Xu, and Y. Li, Phys. Chem. Chem. Phys.
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Chauhan, A. Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 FeCl3 AFM – 3.03 4.02 100 – 164164.
S. Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 FeBr3 AFM – HM – 70 – 162162. J. Sun,
X. Zhong, W. Cui, J. Shi, J. Hao, M. Xu, and Y. Li, Phys. Chem. Chem. Phys.
22(4), 2429 (2020). https://doi.org/10.1039/C9CP05084A FeBr3 AFM – 2.57 3.94 160
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Chauhan, A. Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 FeI3 AFM – 1.83 3.82 70 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 ReI3 FM 1 HM 2 165 – 170170. Q. Sun
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https://doi.org/10.1039/C9NR00315K ReBr3 FM 1 HM 2 390 – 170170. Q. Sun and N.
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α-RuCl3 FM – 0.003 1 14 – 5555. S. Sarikurt, Y. Kadioglu, F. Ersan, E.
Vatansever, O. Ü. Aktürk, Y. Yüksel, Ü. Akıncı, and E. Aktürk, Phys. Chem. Chem.
Phys. 20(2), 997 (2018). https://doi.org/10.1039/C7CP07953B α-RuCl3 AFM 2 0.69
0.9 – 0.95 156156. F. Iyikanat, M. Yagmurcukardes, R. T. Senger, and H. Sahin,
J. Mater. Chem. C 6(8), 2019 (2018). https://doi.org/10.1039/C7TC05266A RuBr3 FM
1.5 0.70 1 13 5.26 171171. F. Ersan, E. Vatansever, S. Sarikurt, Y. Yüksel, Y.
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α-MoCl3 AFMi – – 3 780 – 167167. M. A. McGuire, J. Yan, P. Lampen-Kelley, A. F.
May, V. R. Cooper, L. Lindsay, A. Puretzky, L. Liang, S. Kc, E. Cakmak, S.
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https://doi.org/10.1103/PhysRevMaterials.1.064001 MnF3 FM 3.9 DHM 3.92 450
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https://doi.org/10.1103/PhysRevB.97.094408 MnF3 FM – 0 3.87 600 – 164164. S.
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https://doi.org/10.1016/j.jmmm.2019.165384 MnCl3 FM 3.9 DHM 4.08 750 –0.46
163163. Q. Sun and N. Kioussis, Phys. Rev. B 97(9), 094408 (2018).
https://doi.org/10.1103/PhysRevB.97.094408 MnCl3 FM – 0 3.95 100 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 MnBr3 FM 3.9 DHM 4.18 810 –8.71
163163. Q. Sun and N. Kioussis, Phys. Rev. B 97(9), 094408 (2018).
https://doi.org/10.1103/PhysRevB.97.094408 MnBr3 FM – 0 3.03 120 – 164164. S.
Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A.
Agarwal, and S. Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 MnI3 FM 3.9 DHM 4.27 820 –11.86
163163. Q. Sun and N. Kioussis, Phys. Rev. B 97(9), 094408 (2018).
https://doi.org/10.1103/PhysRevB.97.094408 MnI3 FM – 0 3.15 140 – 164164. S.
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https://doi.org/10.1103/PhysRevB.95.201402 H-CoBr3 FM 1.2 0.0087 – 264 7.7
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https://doi.org/10.1039/C9NA00588A VCl2 AFM 4 I 2.67 – 0.53 174174. A. S. Botana
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https://doi.org/10.1103/PhysRevMaterials.3.044001 VBr2 AFM 3.1 – 3 – – 100100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
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https://doi.org/10.1039/C9NA00588A VI2 AFM 4 I 2.67 – 0.25 174174. A. S. Botana
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https://doi.org/10.1103/PhysRevMaterials.3.044001 CrCl2 AFM 3.5 – 4 – – 100100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A CrBr2 AFM 3.5 – 4 – – 100100. V. V. Kulish
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https://doi.org/10.1039/C7TC02664A CrI2 AFM 3.5 – 4 – – 100100. V. V. Kulish and
W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A MnCl2 AFM 4 – 5 – – 100100. V. V. Kulish and
W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A MnCl2 AFM 2 ∼2.5 4 – <0.01 103103. X. Li, Z.
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https://doi.org/10.1039/C9NA00588A MnCl2 AFM 4 I 4.54 0.2 174174. A. S. Botana
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https://doi.org/10.1103/PhysRevMaterials.3.044001 MnBr2 AFM 4 – 5 – – 100100. V.
V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A MnBr2 AFM 4 I 4.52 0.25 174174. A. S. Botana
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https://doi.org/10.1103/PhysRevMaterials.3.044001 MnI2 AFM 4 – 5 – – 100100. V.
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https://doi.org/10.1039/C9NA00588A MnI2 AFM 4 I 4.46 0.18 174174. A. S. Botana
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https://doi.org/10.1103/PhysRevMaterials.3.044001 FeCl2 FM 4 – 4 109 – 100100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A FeCl2 FM 4 HM 3.57 160 0.89 174174. A. S.
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https://doi.org/10.1103/PhysRevMaterials.3.044001 FeCl2 FM 4 HM 4 – 0.07 177177.
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https://doi.org/10.1039/C7TC02664A FeBr2 FM 4 HM – 210 ∼0.06 178178. M. Ashton,
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42 – 100100. V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A FeI2 FM 4 HM – 122 ∼0.06 178178. M. Ashton,
D. Gluhovic, S. B. Sinnott, J. Guo, D. A. Stewart, and R. G. Hennig, Nano Lett.
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3.45 47 0.59 174174. A. S. Botana and M. R. Norman, Phys. Rev. Mater. 3(4),
044001 (2019). https://doi.org/10.1103/PhysRevMaterials.3.044001 CoCl2 FM 3.3 –
3 85 – 100100. V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A CoCl2 FM 4 I 2.54 135 0.69 174174. A. S.
Botana and M. R. Norman, Phys. Rev. Mater. 3(4), 044001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.044001 CoBr2 FM 3.3 – 3 23 – 100100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A CoBr2 FM 4 I 2.49 24 0.68 174174. A. S.
Botana and M. R. Norman, Phys. Rev. Mater. 3(4), 044001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.044001 CoBr2 FM 3.67 2.35 2.67 2 0.52
142142. H. Y. Lv, W. J. Lu, X. Luo, X. B. Zhu, and Y. P. Sun, Phys. Rev. B
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3 – – 100100. V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A CoI2 AFM 2 ∼0.2 3 – <0.01 103103. X. Li, Z.
Zhang, and H. Zhang, Nanoscale Adv. 2(1), 495 (2020).
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and M. R. Norman, Phys. Rev. Mater. 3(4), 044001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.044001 NiCl2 FM 6.4 – 2 138 – 100100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
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Botana and M. R. Norman, Phys. Rev. Mater. 3(4), 044001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.044001 NiBr2 FM 6.4 – 2 132 – 100100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A NiBr2 FM 4 I 1.63 173 0.02 174174. A. S.
Botana and M. R. Norman, Phys. Rev. Mater. 3(4), 044001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.044001 NiI2 FM 6.4 – 2 129 – 100100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A NiI2 FM 4 I 1.53 178 0.18 174174. A. S.
Botana and M. R. Norman, Phys. Rev. Mater. 3(4), 044001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.044001 PtCl2 FM 2 ∼0.8 1 – <0.01
103103. X. Li, Z. Zhang, and H. Zhang, Nanoscale Adv. 2(1), 495 (2020).
https://doi.org/10.1039/C9NA00588A AgCl2 FM 2 HM 1 – – 103103. X. Li, Z. Zhang,
and H. Zhang, Nanoscale Adv. 2(1), 495 (2020).
https://doi.org/10.1039/C9NA00588A AgBr2 FM 2 HM 1 – – 103103. X. Li, Z. Zhang,
and H. Zhang, Nanoscale Adv. 2(1), 495 (2020).
https://doi.org/10.1039/C9NA00588A AgI2 FM 2 HM 1 – – 103103. X. Li, Z. Zhang,
and H. Zhang, Nanoscale Adv. 2(1), 495 (2020).
https://doi.org/10.1039/C9NA00588A LaBr2 FM – – 1 235 – 179179. J. Zhou, Y. P.
Feng, and L. Shen, arXiv:1904.04952 (2019). GdI2 FM 8 0.62 8 241 0.553 129129.
B. Wang, X. Zhang, Y. Zhang, S. Yuan, Y. Guo, S. Dong, and J. Wang, Mater.
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1.56 185 – 180180. B. Wang, Q. Wu, Y. Zhang, Y. Guo, X. Zhang, Q. Zhou, S. Dong,
and J. Wang, Nanoscale Horiz. 3(5), 551 (2018).
https://doi.org/10.1039/C8NH00101D ScCl FM 4 Metal 0.364 355 – 6565. Z. Jiang,
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(2018). https://doi.org/10.1021/acsami.8b14037 LaCl FM 4 Metal 0.333 460 – 6565.
Z. Jiang, P. Wang, J. Xing, X. Jiang, and J. Zhao, ACS Appl. Mater. Inter.
10(45), 39032 (2018). https://doi.org/10.1021/acsami.8b14037 YCl FM 5 Metal
0.317 260 – 6565. Z. Jiang, P. Wang, J. Xing, X. Jiang, and J. Zhao, ACS Appl.
Mater. Inter. 10(45), 39032 (2018). https://doi.org/10.1021/acsami.8b14037 M3X8
Nb3Cl8 FM 2 1.164 – 31 – 181181. J. Jiang, Q. Liang, R. Meng, Q. Yang, C. Tan,
X. Sun, and X. Chen, Nanoscale 9(9), 2992 (2017).
https://doi.org/10.1039/C6NR07231C Nb3Br8 FM 2 1.125 – 56 – 181181. J. Jiang, Q.
Liang, R. Meng, Q. Yang, C. Tan, X. Sun, and X. Chen, Nanoscale 9(9), 2992
(2017). https://doi.org/10.1039/C6NR07231C Nb3I8 FM 2 0.758 – 87 – 181181. J.
Jiang, Q. Liang, R. Meng, Q. Yang, C. Tan, X. Sun, and X. Chen, Nanoscale 9(9),
2992 (2017). https://doi.org/10.1039/C6NR07231C Nb3F8 FM – HM 1.167 77 – 7474.
H. Xiao, X. Wang, R. Wang, L. Xu, S. Liang, and C. Yang, Phys. Chem. Chem. Phys.
21(22), 11731 (2019). https://doi.org/10.1039/C9CP00850K Nb3Cl8 AFM – 0.086
1.167 – – 7474. H. Xiao, X. Wang, R. Wang, L. Xu, S. Liang, and C. Yang, Phys.
Chem. Chem. Phys. 21(22), 11731 (2019). https://doi.org/10.1039/C9CP00850K
Nb3Br8 FM – HM 1.167 – – 7474. H. Xiao, X. Wang, R. Wang, L. Xu, S. Liang, and
C. Yang, Phys. Chem. Chem. Phys. 21(22), 11731 (2019).
https://doi.org/10.1039/C9CP00850K Nb3I8 FM – HM 1.167 103 – 7474. H. Xiao, X.
Wang, R. Wang, L. Xu, S. Liang, and C. Yang, Phys. Chem. Chem. Phys. 21(22),
11731 (2019). https://doi.org/10.1039/C9CP00850K
The successful exfoliation of bulk CrI3 crystal into atomic monolayers opened a
new era of 2D magnetism.2828. B. Huang, G. Clark, E. Navarro-Moratalla, D. R.
Klein, R. Cheng, K. L. Seyler, D. Zhong, E. Schmidgall, M. A. McGuire, D. H.
Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero, and X. Xu, Nature 546(7657), 270
(2017). https://doi.org/10.1038/nature22391 Using scanning magneto-optic Kerr
microscopy, neutron scattering, and NMR spectroscopy, pristine monolayer CrI3
has been proven to be an Ising ferromagnetic semiconductor with a bandgap of 1.2
eV,150150. J. F. Dillon, Jr. and C. E. Olson, J. Appl. Phys. 36(3), 1259 (1965).
https://doi.org/10.1063/1.1714194 a Curie temperature of 45 K,2828. B. Huang, G.
Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E.
Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero,
and X. Xu, Nature 546(7657), 270 (2017). https://doi.org/10.1038/nature22391 and
an out-plane MAE of 0.69 meV [Figs. 8(a)–(e)].5353. W.-B. Zhang, Q. Qu, P. Zhu,
and C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J In fact, the study of magnetism of layered
CrI3 began even before that. In prior experiments, several theoretical groups
already predicted robust long-range ferromagnetic ordering in the monolayer
limit of CrI3.8,53,548. H. Wang, F. Fan, S. Zhu, and H. Wu, Europhys. Lett.
114(4), 47001 (2016). https://doi.org/10.1209/0295-5075/114/4700153. W.-B.
Zhang, Q. Qu, P. Zhu, and C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J54. J. Liu, Q. Sun, Y. Kawazoe, and P. Jena,
Phys. Chem. Chem. Phys. 18(13), 8777 (2016). https://doi.org/10.1039/C5CP04835D
The reported magnetic moment, magnetic anisotropy energy, and Curie temperature
of 2D CrI3 were 3∼3.44 μB per formula unit, 0.65∼0.686 meV, and 61∼107 K,
respectively.8,53,548. H. Wang, F. Fan, S. Zhu, and H. Wu, Europhys. Lett.
114(4), 47001 (2016). https://doi.org/10.1209/0295-5075/114/4700153. W.-B.
Zhang, Q. Qu, P. Zhu, and C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J54. J. Liu, Q. Sun, Y. Kawazoe, and P. Jena,
Phys. Chem. Chem. Phys. 18(13), 8777 (2016). https://doi.org/10.1039/C5CP04835D
The magnetic moment in monolayer CrI3 with honeycomb lattice is contributed by
the Cr3+ ions with electronic configuration of 3s03d3 under edge sharing
octahedral crystal field coordinated by six nonmagnetic I¯ ions. In the
octahedral crystal field, Cr3+ ions prefer S = 3/2 with three d electrons
occupying the lower-energy t2g triplet state via splitting. The long-range FM
ordering is driven by the competition between direct antiferromagnetic exchange
interaction of Cr–Cr sites and the superexchange interaction in the near 90°
Cr–I–Cr bonds.88. H. Wang, F. Fan, S. Zhu, and H. Wu, Europhys. Lett. 114(4),
47001 (2016). https://doi.org/10.1209/0295-5075/114/47001 The large magnetic
anisotropy is attributed to the spin-orbit coupling of the intermediate I atom
along the superexchange path.151151. J. L. Lado and J. Fernández-Rossier, 2D
Mater. 4(3), 035002 (2017). https://doi.org/10.1088/2053-1583/aa75ed Using
magneto-Raman spectroscopy, Xu et al.152152. J. Cenker, B. Huang, N. Suri, P.
Thijssen, A. Miller, T. Song, T. Taniguchi, K. Watanabe, M. McGuire, X. Di, and
X. Xu, arXiv:2001.07025 (2020). directly observed 2D magnons with acoustic
magnon mode of 0.3 meV in monolayer CrI3. Moreover, the magnetism in CrI3 is
layer dependent, that is, monolayer, trilayer, bulk CrI3 systems are
ferromagnetic, whereas the weak magnetic coupling between the individual FM
monolayer leads to the AFM ground state in bilayer CrI3 [Figs. 8(f)–8(h)].2828.
B. Huang, G. Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler,
D. Zhong, E. Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P.
Jarillo-Herrero, and X. Xu, Nature 546(7657), 270 (2017).
https://doi.org/10.1038/nature22391 An exfoliated thin film of CrI3 sandwiched
between graphene contacts acts as a spin-filter tunnel barrier, showing a record
high tunneling magnetoresistance of 190 000%.153,154153. Z. Wang, I.
Gutierrez-Lezama, N. Ubrig, M. Kroner, M. Gibertini, T. Taniguchi, K. Watanabe,
A. Imamoglu, E. Giannini, and A. F. Morpurgo, Nat. Commun. 9(1), 2516 (2018).
https://doi.org/10.1038/s41467-018-04953-8154. T. Song, X. Cai, M. W.-Y. Tu, X.
Zhang, B. Huang, N. P. Wilson, K. L. Seyler, L. Zhu, T. Taniguchi, K. Watanabe,
M. A. McGuire, D. H. Cobden, D. Xiao, W. Yao, and X. Xu, Science 360(6394), 1214
(2018). https://doi.org/10.1126/science.aar4851
FIG. 8. Magneto-optical Kerr effect (MOKE) measurements of monolayer CrI3. (a)
Polar MOKE signal for a CrI3 monolayer. (b) Power dependence of the MOKE signal
taken at three different incident powers (3, 10, and 30 μW). (c) MOKE maps at
μoH = 0 T, 0.15 T, and 0.3 T. (d) Temperature dependence of MOKE signal. (e) The
relationship between θK abd μ0H marked by dots in (c). (f)–(h) Layer-dependent
magnetic ordering in atomically thin CrI3. MOKE signal on a monolayer, bilayer,
and trilayer flakes. They show ferromagnetic, antiferromagnetic, and
ferromagnetic behavior, respectively. Reproduced with permission from Huang et
al., Nature 546, 270 (2017). Copyright 2017 Springer Nature.2828. B. Huang, G.
Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E.
Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero,
and X. Xu, Nature 546(7657), 270 (2017). https://doi.org/10.1038/nature22391
* PPT
|
* High-resolution
Motivated by the discovery of CrI3, its sister transition metal compounds, i.e.,
chromium trihalides CrX3 (X= F, Cl, Br) monolayers have been explored as
potential 2D intrinsic magnetic semiconductors by many
groups.30,53,54,104,155,15630. X. Cai, T. Song, N. P. Wilson, G. Clark, M. He,
X. Zhang, T. Taniguchi, K. Watanabe, W. Yao, D. Xiao, M. A. McGuire, D. H.
Cobden, and X. Xu, Nano Lett. 19(6), 3993 (2019).
https://doi.org/10.1021/acs.nanolett.9b0131753. W.-B. Zhang, Q. Qu, P. Zhu, and
C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J54. J. Liu, Q. Sun, Y. Kawazoe, and P. Jena,
Phys. Chem. Chem. Phys. 18(13), 8777 (2016).
https://doi.org/10.1039/C5CP04835D104. L. Webster and J.-A. Yan, Phys. Rev. B
98(14), 144411 (2018). https://doi.org/10.1103/PhysRevB.98.144411155. M. Moaied,
J. Lee, and J. Hong, Phys. Chem. Chem. Phys. 20(33), 21755 (2018).
https://doi.org/10.1039/C8CP03489C156. F. Iyikanat, M. Yagmurcukardes, R. T.
Senger, and H. Sahin, J. Mater. Chem. C 6(8), 2019 (2018).
https://doi.org/10.1039/C7TC05266A Owing to the structural similarity with CrI3,
FM is also the ground state for monolayer CrX3 (X= F, Cl, and Br), and the
magnetic moment of each Cr3+ ion in these systems is about 3 μB. However, with
the increase in the atomic radius of halogen, the theoretical bandgap reduces
from 4.68 eV for CrF3, to 3.44 eV for CrCl3, to 2.54 eV for CrBr3, and finally
to 1.53 eV for CrI3 at HSE06 level of theory.5353. W.-B. Zhang, Q. Qu, P. Zhu,
and C.-H. Lam, J. Mater. Chem. C 3(48), 12457 (2015).
https://doi.org/10.1039/C5TC02840J Considering the microscopic origin of
long-range FM ordering, a strongly increased hybridization between X-p and Cr-3d
states will strengthen the Cr–X–Cr FM exchange interactions as X goes from F to
I. However, direct AFM exchange interaction is expected to be weakened with
increasing Cr–Cr distance along this series. Indeed, MC simulations based on
classical Heisenberg model predicted Curie temperatures of CrF3, CrCl3, CrBr3,
and CrI3 monolayers to be 41, 49, 73, and 95 K, respectively. Moving from Cl to
I would increase the strength of spin-orbit coupling of halogen, which accounts
for the enhancement of MAE from 0.031 meV for CrCl3, to 0.186 meV for CrBr3, and
then to 0.686 meV for CrI3.5353. W.-B. Zhang, Q. Qu, P. Zhu, and C.-H. Lam, J.
Mater. Chem. C 3(48), 12457 (2015). https://doi.org/10.1039/C5TC02840J Similar
theoretical results were also reported in the other four
papers.54,104,155,15654. J. Liu, Q. Sun, Y. Kawazoe, and P. Jena, Phys. Chem.
Chem. Phys. 18(13), 8777 (2016). https://doi.org/10.1039/C5CP04835D104. L.
Webster and J.-A. Yan, Phys. Rev. B 98(14), 144411 (2018).
https://doi.org/10.1103/PhysRevB.98.144411155. M. Moaied, J. Lee, and J. Hong,
Phys. Chem. Chem. Phys. 20(33), 21755 (2018).
https://doi.org/10.1039/C8CP03489C156. F. Iyikanat, M. Yagmurcukardes, R. T.
Senger, and H. Sahin, J. Mater. Chem. C 6(8), 2019 (2018).
https://doi.org/10.1039/C7TC05266A Experimentally, Gao et al.3131. Z. Zhang, J.
Shang, C. Jiang, A. Rasmita, W. Gao, and T. Yu, Nano Lett. 19(5), 3138 (2019).
https://doi.org/10.1021/acs.nanolett.9b00553 demonstrated ferromagnetism in 2D
vdW CrBr3 using direct d-d transition induced photoluminescence probing. They
argued that spontaneous magnetization persists in monolayer CrBr3 with Curie
temperature of 34 K. The magnetic moment of each Cr3+ ion in monolayer CrBr3
aligns in the out-of-plane direction, and the corresponding magnetic moment is
about 3 μB. Although the Curie temperature and MAE are slightly smaller than
those of monolayer CrI3, monolayers of other chromium trihalides CrX3 (X= F, Cl,
Br) could be more stable in air.28,15728. B. Huang, G. Clark, E.
Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E. Schmidgall,
M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero, and X. Xu,
Nature 546(7657), 270 (2017). https://doi.org/10.1038/nature22391157. W. Jin, H.
H. Kim, Z. Ye, S. Li, P. Rezaie, F. Diaz, S. Siddiq, E. Wauer, B. Yang, C. Li,
S. Tian, K. Sun, H. Lei, A. W. Tsen, L. Zhao, and R. He, Nat. Commun. 9(1), 5122
(2018). https://doi.org/10.1038/s41467-018-07547-6
Beyond chromium halides, the magnetic properties of vanadium trihalides (e.g.,
VI3) monolayer were also intensively investigated as potential 2D magnetic
materials. In the periodic table, vanadium locates as the left neighbor of
chromium. Thus, V atom in VI3 tends to adopt high-spin state of V3+ with 3d2
electronic configuration, that is, two valence electrons occupy the triply
degenerate t2g state. Such partial occupancy could raise the possibility for
weak Jahn-Teller distortion of the octahedra and the orbital ordering in
vanadium trihalides, which would modify the superexchange interactions in its
planar honeycomb lattice. Recently, two experimental groups confirmed that bulk
VI3 is a correlated Mott insulator with a bandgap of ∼1 eV.29,15829. S. Tian,
J.-F. Zhang, C. Li, T. Ying, S. Li, X. Zhang, K. Liu, and H. Lei, J. Am. Chem.
Soc. 141(13), 5326 (2019). https://doi.org/10.1021/jacs.8b13584158. T. Kong, K.
Stolze, E. I. Timmons, J. Tao, D. Ni, S. Guo, Z. Yang, R. Prozorov, and R. J.
Cava, Adv. Mater. 31(17), 1808074 (2019). https://doi.org/10.1002/adma.201808074
In addition, the exfoliation energy from first-principles calculation revealed
that VI3 is more easily decoupled than CrI3.159159. J. Yan, X. Luo, F. C. Chen,
J. J. Gao, Z. Z. Jiang, G. C. Zhao, Y. Sun, H. Y. Lv, S. J. Tian, Q. W. Yin, H.
C. Lei, W. J. Lu, P. Tong, W. H. Song, X. B. Zhu, and Y. P. Sun, Phys. Rev. B
100(9), 094402 (2019). https://doi.org/10.1103/PhysRevB.100.094402 Hence,
vanadium trihalides provide a new platform to investigate 2D magnets with S = 1.
Using CVD method, Tian et al.2929. S. Tian, J.-F. Zhang, C. Li, T. Ying, S. Li,
X. Zhang, K. Liu, and H. Lei, J. Am. Chem. Soc. 141(13), 5326 (2019).
https://doi.org/10.1021/jacs.8b13584 firstly confirmed layered VI3 is a FM
semiconductor with a Curie temperature of 50 K. It was further demonstrated to
be a 2D Ising ferromagnet by DFT calculations, crystal field level diagrams,
superexchange model analyses, and MC simulations.99. K. Yang, F. Fan, H. Wang,
D. I. Khomskii, and H. Wu, Phys. Rev. B 101(10), 100402 (2020).
https://doi.org/10.1103/PhysRevB.101.100402 The monolayer limit of VI3 has also
been explored by first-principles calculations. The intrinsic ferromagnetism can
persist and the Curie temperature is reduced to 17 K from the bulk value of
60 K.160160. M. An, Y. Zhang, J. Chen, H.-M. Zhang, Y. Guo, and S. Dong, J.
Phys. Chem. C 123(50), 30545 (2019). https://doi.org/10.1021/acs.jpcc.9b08706
Based on DFT calculations combined with self consistently determined Hubbard U
approach, He et al.161161. J. He, S. Ma, P. Lyu, and P. Nachtigall, J. Mater.
Chem. C 4(13), 2518 (2016). https://doi.org/10.1039/C6TC00409A reported that
monolayer VI3 possesses not only intrinsic ferromagnetism but also exciting
Dirac half-metallicity, and VCl3 shows similar magnetic behavior. Their
corresponding Curie temperatures were 80 and 98 K, respectively. For monolayer
VBr3, first-principles calculations predicted that it has intrinsic
half-metallicity and high Curie temperature of 190 K. Moreover, topological
nontrivial states, which were identified by calculations of Berry curvature and
the corresponding edge states, surprisingly emerge at the Fermi level.162162. J.
Sun, X. Zhong, W. Cui, J. Shi, J. Hao, M. Xu, and Y. Li, Phys. Chem. Chem. Phys.
22(4), 2429 (2020). https://doi.org/10.1039/C9CP05084A
Manganese atom is the right neighbor of chromium in periodic table. In the
manganese trihalides MnX3 (X = F, Cl, Br, I), low crystal field splitting is
caused by the octahedral coordination of Mn ions. Thus, Mn ion has +3 oxide
state with spin configuration of S = 2t2g3eg1, exhibiting a magnetic moment of
c.a. 4 μB. Sun and Kioussis predicted that 2D MnX3 sheets are intrinsic Dirac
half metals (DHMs).163163. Q. Sun and N. Kioussis, Phys. Rev. B 97(9), 094408
(2018). https://doi.org/10.1103/PhysRevB.97.094408 The bandgaps of the minority
spin channel from PBE+U calculations were 6.3, 4.33, 3.85, and 3.10 eV for MnF3,
MnCl3, MnBr3, and MnI3, respectively. In-plane magnetization orientation was
found in all MnX3 systems and the magnetocrystalline anisotropy increases with
increasing atomic size of halogen. Based on DFT derived exchange interaction
parameters, the estimated Curie temperatures were greater than 450 K.
Spin-polarized Dirac half metallic states in MnF3, MnCl3, and MnBr3 were also
confirmed by Tomar et al.164164. S. Tomar, B. Ghosh, S. Mardanya, P. Rastogi, B.
S. Bhadoria, Y. S. Chauhan, A. Agarwal, and S. Bhowmick, J. Magn. Magn. Mater.
489, 165384 (2019). https://doi.org/10.1016/j.jmmm.2019.165384 using DFT
calculations with both PBE and HSE06 functionals. Based on Curie-Weiss mean
field theory, MnF3 was demonstrated as a room-temperature ferromagnet.
Two-dimensional intrinsic magnets are also predicted for many other trihalides
of open-shell 3d transition metals (M = Ti, Fe, Co, Ni). For instance, TiCl3
monolayer possesses weak interlayer interaction of 0.33 J/m2, half metallicity
with a bandgap of 0.6 eV in the majority spin channel, long-range FM ordering
contributed by 3d valence states, and a high Curie temperature of 376 K.165165.
Y. Zhou, H. Lu, X. Zu, and F. Gao, Sci. Rep. 6(1), 19407 (2016).
https://doi.org/10.1038/srep19407 FeX3 monolayers were found to have
antiferromagnetic ground state with Néel temperature of 70∼180 K.162,164162. J.
Sun, X. Zhong, W. Cui, J. Shi, J. Hao, M. Xu, and Y. Li, Phys. Chem. Chem. Phys.
22(4), 2429 (2020). https://doi.org/10.1039/C9CP05084A164. S. Tomar, B. Ghosh,
S. Mardanya, P. Rastogi, B. S. Bhadoria, Y. S. Chauhan, A. Agarwal, and S.
Bhowmick, J. Magn. Magn. Mater. 489, 165384 (2019).
https://doi.org/10.1016/j.jmmm.2019.165384 Using DFT and DFT+U calculations, Sun
et al.162162. J. Sun, X. Zhong, W. Cui, J. Shi, J. Hao, M. Xu, and Y. Li, Phys.
Chem. Chem. Phys. 22(4), 2429 (2020). https://doi.org/10.1039/C9CP05084A found
Dirac spin-gapless half-metallic features in NiBr3 monolayer, and the
corresponding Curie temperature was 100 K. The NiCl3 monolayer was also shown to
have Dirac spin-gapless semiconducting characteristics and high-temperature
ferromagnetism.110110. J. He, X. Li, P. Lyu, and P. Nachtigall, Nanoscale 9(6),
2246 (2017). https://doi.org/10.1039/C6NR08522A The MC simulations based on
Ising model demonstrated that the Curie temperature of NiCl3 monolayer is as
high as ∼400 K. The calculated Fermi velocity of Dirac fermions was about
4 × 105 ms−1. Among them, Ti, Fe, and Ni ions are still octahedrally coordinated
to the halogen atoms, thus they can exist in either high-spin or low-spin state
due to crystal field splitting. According to the calculated magnetic moment
listed in Table II, low-spin state is observed in the cases of Ti3+ and Ni3+
with magnetic moment of about 1 μB. The magnetic moment of iron ion is found to
be close to 4 μB, which corresponds to neither high-spin (5 μB) nor low-spin (1
μB) state of Fe3+. It could explain why AFM coupling is only found in FeX3
series among the transition metal trihalides with CrI3 type structure. For Co
ion located in the octahedral coordination environments of bromides,
non-magnetic behavior was observed in CoBr3 with P-31m phase.162162. J. Sun, X.
Zhong, W. Cui, J. Shi, J. Hao, M. Xu, and Y. Li, Phys. Chem. Chem. Phys. 22(4),
2429 (2020). https://doi.org/10.1039/C9CP05084A However, DFT calculations166166.
W.-x. Zhang, Y. Li, H. Jin, and Y.-c. She, Phys. Chem. Chem. Phys. 21(32), 17740
(2019). https://doi.org/10.1039/C9CP03337H demonstrated that the P6/mmm phase of
CoBr3 monolayer hosts 2D intrinsic ferromagnetism with metallic behavior, Dirac
cone, and quantum anomalous Hall effect simultaneously. Its Curie temperature
was 264 K and Chern number was C = 2.
Another important group of transition metal trihalides contains the heavier 4d
and 5d open-shell transition metal elements like Mo, Ru, Rh, Tc, Pd, Ir, Pt, and
Os. Moving from 3d to 4d and 5d series increases the spin-orbit coupling effect,
which is beneficial for designing 2D spintronic devices with large MAE,
topological phenomena, and spin controlling. Antiferromagnetic coupling between
Mo atoms was confirmed in the high-temperature phase of α-MoCl3 by an combined
experimental and theoretical study.167167. M. A. McGuire, J. Yan, P.
Lampen-Kelley, A. F. May, V. R. Cooper, L. Lindsay, A. Puretzky, L. Liang, S.
Kc, E. Cakmak, S. Calder, and B. C. Sales, Phys. Rev. Mater. 1(6), 064001
(2017). https://doi.org/10.1103/PhysRevMaterials.1.064001 Unlike CrCl3, α-MoCl3
adopts the monoclinic AlCl3 structure with space group of C2/m at room
temperature. Originated from the magneto-structural phase transition, magnetic
interactions in the high-temperature phase of α-MoCl3 are stronger by at least
one order of magnitude than those in the analogous CrCl3 and CrBr3. Similar
coupled structural and magnetic transition is also expected in TcCl3 and TiCl3.
Experimentally, the fractional Majorana fermion excitations of a Kitaev quantum
spin liquid have been observed in α-RuCl3,3232. A. Banerjee, J. Yan, J. Knolle,
C. A. Bridges, M. B. Stone, M. D. Lumsden, D. G. Mandrus, D. A. Tennant, R.
Moessner, and S. E. Nagler, Science 356(6342), 1055 (2017).
https://doi.org/10.1126/science.aah6015 which leads to enormous amounts of
research on the magnetic properties of layered α-RuCl3. Motivated by the
exfoliation of α-RuCl3 monolayer from its 3D crystal,168168. D. Weber, L. M.
Schoop, V. Duppel, J. M. Lippmann, J. Nuss, and B. V. Lotsch, Nano Lett. 16(6),
3578 (2016). https://doi.org/10.1021/acs.nanolett.6b00701 the magnetic property
has been further analyzed by DFT calculations and MC simulations.55,15655. S.
Sarikurt, Y. Kadioglu, F. Ersan, E. Vatansever, O. Ü. Aktürk, Y. Yüksel, Ü.
Akıncı, and E. Aktürk, Phys. Chem. Chem. Phys. 20(2), 997 (2018).
https://doi.org/10.1039/C7CP07953B156. F. Iyikanat, M. Yagmurcukardes, R. T.
Senger, and H. Sahin, J. Mater. Chem. C 6(8), 2019 (2018).
https://doi.org/10.1039/C7TC05266A It was demonstrated to be a stable 2D
intrinsic ferromagnetic semiconductor. The obtained Curie temperature and MAE
were 14.21 K and 0.95 meV, respectively. Using first-principles calculations,
Kan et al.169169. C. Huang, J. Zhou, H. Wu, K. Deng, P. Jena, and E. Kan, Phys.
Rev. B 95(4), 045113 (2017). https://doi.org/10.1103/PhysRevB.95.045113
predicted RuI3 monolayer to be an intrinsic ferromagnetic quantum anomalous Hall
(QAH) insulator with topologically nontrivial global bandgap of 11 meV. The
Curie temperature and nearest-neighboring exchange coupling parameter were
estimated to be 360 K and 82 meV, respectively. It was also found that FM RuCl3
and RuBr3 monolayers show similar electronic behavior as RuI3 monolayer.
However, their exchange energies are very small and sensitive to the choice of
effective U value. For RuBr3 and RuI3 monolayers,170170. Q. Sun and N. Kioussis,
Nanoscale 11(13), 6101 (2019). https://doi.org/10.1039/C9NR00315K the bandgap,
possible magnetic ground state, Curie temperature, and magnetic anisotropy
energy have been reexamined by DFT calculations with PBE+U and inclusion of SOC.
It was shown that they are FM semiconductors with indirect bandgaps of 0.7 and
0.32 eV, respectively. According to MC simulations, their magnetic transition
temperatures from FM to PM were 13.0 and 2.1 K, respectively. The magnetic
anisotropy energies obtained for RuBr3 and RuI3 were 5.26 and 12.88 meV,
respectively. Robust intrinsic ferromagnetism has also been realized in 2D
rhenium trihalides.171171. F. Ersan, E. Vatansever, S. Sarikurt, Y. Yüksel, Y.
Kadioglu, H. D. Ozaydin, O. Ü. Aktürk, Ü. Akıncı, and E. Aktürk, J. Magn. Magn.
Mater. 476, 111 (2019). https://doi.org/10.1016/j.jmmm.2018.12.032 The dynamic
and thermodynamic stabilities were found in the heavier halides (Br and I), in
contrast to the lighter halides (F and Cl). ReBr3 and ReI3 are half metals with
large bandgap in the spin-up channel. Moreover, high Curie temperatures (390 and
165 K) and Chern number (C = –4) were obtained from DFT calculations with PBE
functional. Both Weyl half semimetal and tunable QAH effects were simultaneously
realized in monolayer PtCl3,172172. J.-Y. You, C. Chen, Z. Zhang, X.-L. Sheng,
S. A. Yang, and G. Su, Phys. Rev. B 100(6), 064408 (2019).
https://doi.org/10.1103/PhysRevB.100.064408 as signified by the in-plane
magnetization, high Curie temperature, and mirror symmetry protected two 2D Weyl
points. A room-temperature intrinsic QAH insulator was predicted in the
ferromagnetic insulating OsCl3 monolayer, which is characterized by an energy
gap of 67 meV, a Chern number of C = 1, and a Curie temperature of 350 K.173173.
X.-L. Sheng and B. K. Nikolić, Phys. Rev. B 95(20), 201402 (2017).
https://doi.org/10.1103/PhysRevB.95.201402
2. MX2
Similar to transition metal trihalides, layered transition metal dihalides have
also drawn significant attentions for exhibiting ferromagnetic,
antiferromagnetic, and half-metallic characteristics. The structure of monolayer
MX2 is analogous to transition metal dichalcogenides, which contains a
triangular lattice of transition metal cations. In these compounds, the metal
ions are in the formal oxidation state of +2. Divalence and octahedral
coordination renders V, Cr, Mn, Fe, Co, and Ni cations partially filled 3d3,
3d4, 3d5, 3d6, 3d7, and 3d8 electronic configurations, with S = 3/2, 2, 5/2, 2,
3/2, and 1, respectively. Considering their structural similarity, the sign of
superexchange interaction is mainly determined by the orbital occupations, and
thus a variety of magnetic ground states are anticipated.
The evolution of electronic and magnetic properties of these first-row
transition metal dihalides MX2 (M = V, Cr, Mn, Fe, Co, Ni; X = Cl, Br, I) has
been systemically examined by first-principles calculations.100,111,142,174100.
V. V. Kulish and W. Huang, J. Mater. Chem. C 5(34), 8734 (2017).
https://doi.org/10.1039/C7TC02664A111. E. A. Kovaleva, I. Melchakova, N. S.
Mikhaleva, F. N. Tomilin, S. G. Ovchinnikov, W. Baek, V. A. Pomogaev, P.
Avramov, and A. A. Kuzubov, J. Phys. Chem. Solids 134, 324 (2019).
https://doi.org/10.1016/j.jpcs.2019.05.036142. H. Y. Lv, W. J. Lu, X. Luo, X. B.
Zhu, and Y. P. Sun, Phys. Rev. B 99(13), 134416 (2019).
https://doi.org/10.1103/PhysRevB.99.134416174. A. S. Botana and M. R. Norman,
Phys. Rev. Mater. 3(4), 044001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.044001 Using PBE functional, T
configuration (P-3m1) is energetically favorable for the monolayers of all
considered cases, while correction by a Hubbard U term leads to inversion of the
favorable monolayer configuration to H phase (P-6m2), as demonstrated for FeBr2
and FeI2.111111. E. A. Kovaleva, I. Melchakova, N. S. Mikhaleva, F. N. Tomilin,
S. G. Ovchinnikov, W. Baek, V. A. Pomogaev, P. Avramov, and A. A. Kuzubov, J.
Phys. Chem. Solids 134, 324 (2019). https://doi.org/10.1016/j.jpcs.2019.05.036
Among them, FeCl2, FeBr2, and FeI2 monolayers are ferromagnetic half metals,
while CoCl2, CoBr2, NiCl2, NiBr2, and NiI2 monolayers are ferromagnetic
insulators. For VCl2, VBr2, VI2, MnCl2, MnBr2, MnI2, CrI2, and CoI2 monolayers,
they were found to be antiferromagnetic semiconductors with bandgap in range of
0.2∼2.5 eV. Introducing U correction will largely enlarge their bandgap.111111.
E. A. Kovaleva, I. Melchakova, N. S. Mikhaleva, F. N. Tomilin, S. G.
Ovchinnikov, W. Baek, V. A. Pomogaev, P. Avramov, and A. A. Kuzubov, J. Phys.
Chem. Solids 134, 324 (2019). https://doi.org/10.1016/j.jpcs.2019.05.036 For
example, the theoretical bandgaps of VBr2 monolayer are 1.1 and 3.1 eV at PBE
and PBE+U level, respectively. The easy axis of all eight abovementioned MX2
monolayers in FM state is perpendicular to the basal plane with MAE in range of
0.02 to 0.89 meV. The highest TC is observed in NiCl2, which is 205 K predicted
by Ising model.
Among the first-row transition metal dihalides, FeX2 series have attracted more
attentions, mainly due to the following two reasons: (1) largest out-of-plane
MAE is found in FeCl2, which is beneficial for the presence of 2D long-range
magnetic ordering; (2) monolayer 1T-FeCl2 films on Au(111) and graphite have
been successfully synthesized using MBE technique.175175. X. Zhou, B.
Brzostowski, A. Durajski, M. Liu, J. Xiang, T. Jiang, Z. Wang, S. Chen, P. Li,
Z. Zhong, A. Drzewiński, M. Jarosik, R. Szczęśniak, T. Lai, D. Guo, and D.
Zhong, J. Phys. Chem. C 124(17), 9416 (2020).
https://doi.org/10.1021/acs.jpcc.0c03050 Torun et al.176176. E. Torun, H. Sahin,
S. K. Singh, and F. M. Peeters, Appl. Phys. Lett. 106(19), 192404 (2015).
https://doi.org/10.1063/1.4921096 investigated the structural and magnetic
properties of FeCl2 monolayer using first-principles calculations. They found
that 1T-FeCl2 is more favorable than the 1H phase. Both PBE and HSE06
calculations demonstrated that 1T-FeCl2 is an intrinsic half-metallic
ferromagnet with Curie temperature of 17 K.177177. E. Torun, H. Sahin, C.
Bacaksiz, R. T. Senger, and F. M. Peeters, Phys. Rev. B 92(10), 104407 (2015).
https://doi.org/10.1103/PhysRevB.92.104407 Hennig et al. have found that the
Fe2+ ions in FeCl2, FeBr2, and FeI2 are in a high-spin octahedral d6
configuration, resulting in a large magnetic moment of 4 μB.178178. M. Ashton,
D. Gluhovic, S. B. Sinnott, J. Guo, D. A. Stewart, and R. G. Hennig, Nano Lett.
17(9), 5251 (2017). https://doi.org/10.1021/acs.nanolett.7b01367 A classical XY
model with nearest neighboring coupling was used to estimate their critical
temperatures, which range from 122 K for FeI2 to 210 K for FeBr2. Moreover, all
three 2D FeX2 materials as half metals were predicted to have appreciable
electron densities of state at the Fermi level comparable to those of typical
metals, suggesting good on/off ratios in spintronic devices.178178. M. Ashton,
D. Gluhovic, S. B. Sinnott, J. Guo, D. A. Stewart, and R. G. Hennig, Nano Lett.
17(9), 5251 (2017). https://doi.org/10.1021/acs.nanolett.7b01367
In addition to first-row transition metals, the magnetic ground state of 4d/5d
MX2 monolayers in both 1T and 2H phases have been investigated by
high-throughput first-principles calculations.103103. X. Li, Z. Zhang, and H.
Zhang, Nanoscale Adv. 2(1), 495 (2020). https://doi.org/10.1039/C9NA00588A Among
them, 23 out of 90 MX2 monolayers exhibit robust magnetic ground states that are
retained even after introducing the U terms. Besides the previously reported
NiCl2, VCl2, MnCl2, VBr2, VI2, MnI2, and CoI2, PtCl2 is predicted to be a new
noncollinear antiferromagnetic insulator. Meanwhile, AgCl2, AgBr2, and AuI2 are
found to be half metallic ferromagnets with spin splitting of 0.2∼0.5 eV. To
find more 2D intrinsic magnets in MH2 family, Shen et al.179179. J. Zhou, Y. P.
Feng, and L. Shen, arXiv:1904.04952 (2019). also focused on 5d transition metal
based MX2 monolayers. They screened more than 6000 kinds of 2D electrenes (i.e.,
materials with excess electrons acting as anions) and found that LaBr2 is one of
most intriguing FM semiconductors with unusual long-range ferromagnetism induced
by anions. From DFT calculations, its on-site moment, Curie temperature, and
coercive field are 1 μB, 235 K, and 0.53 T, respectively.179179. J. Zhou, Y. P.
Feng, and L. Shen, arXiv:1904.04952 (2019). For LaBr2 monolayer, delocalized
spin density in the intermediate region between La atoms was also unveiled in
another theoretical paper by Jiang et al.6565. Z. Jiang, P. Wang, J. Xing, X.
Jiang, and J. Zhao, ACS Appl. Mater. Inter. 10(45), 39032 (2018).
https://doi.org/10.1021/acsami.8b14037
3. MX
Among transition metal halides, MX compounds have the simplest stoichiometric
ratio of 1:1. Without octahedral type crystal field, MX monolayer structures
mainly consist of double hexagonal layers of metal atoms sandwiched by two
hexagonal layers of halogen atoms. It is interesting to ask whether such MX
monolayers still possess long-range magnetic ordering. By comprehensive
first-principles calculations and MC simulations, Jiang et al.6565. Z. Jiang, P.
Wang, J. Xing, X. Jiang, and J. Zhao, ACS Appl. Mater. Inter. 10(45), 39032
(2018). https://doi.org/10.1021/acsami.8b14037 have identified ScCl, YCl, and
LaCl monolayers as ferromagnetic metals with appreciable Curie temperatures of
280, 240, and 260 K, respectively, while PBE+U calculations revised those values
to 355, 460, and 260 K, respectively. Band structure analysis has shown that the
spin density is relatively delocalized in the intermediate region between metal
atoms, resulting in a small magnetic moment of 0.364, 0.333, and 0.317 μB per
Sc, Y, and La atom, respectively. Wang et al.180180. B. Wang, Q. Wu, Y. Zhang,
Y. Guo, X. Zhang, Q. Zhou, S. Dong, and J. Wang, Nanoscale Horiz. 3(5), 551
(2018). https://doi.org/10.1039/C8NH00101D also found that 2D ScCl monolayer is
an intrinsic ferromagnet with large spin polarization. Their predicted Curie
temperature was 185 K. Even so, both studies indicated that introduction of more
transition metal in M-X systems would strongly quench its magnetic moment, while
retaining Curie temperature at rather high value.
4. M3X8
Niobium halides form another kind of potential 2D vdW magnets with an unusual
composition—Nb3X8. In Nb3X8 monolayer,181181. J. Jiang, Q. Liang, R. Meng, Q.
Yang, C. Tan, X. Sun, and X. Chen, Nanoscale 9(9), 2992 (2017).
https://doi.org/10.1039/C6NR07231C triangular Nb clusters are formed by Nb
atoms; and consequently, every Nb atom is still arranged in a distorted
octahedral environment. Therefore, both ferromagnetic ground state and
semiconducting behavior were found in 2D Nb3X8 (X = Cl, Br, I) monolayers from
GGA+U calculations. The Curie temperatures estimated by mean field approximation
based on Heisenberg model were 31, 56, and 87 K for Nb3Cl8, Nb3Br8, and Nb3I8,
respectively. Among them, the Nb3I8 monolayer has been successfully cleaved from
its bulk phase.182182. B. J. Kim, B. J. Jeong, S. Oh, S. Chae, K. H. Choi, S. S.
Nanda, T. Nasir, S. H. Lee, K.-W. Kim, H. K. Lim, L. Chi, I. J. Choi, M.-K.
Hong, D. K. Yi, H. K. Yu, J.-H. Lee, and J.-Y. Choi, Phys. Status Solidi R
13(3), 1800448 (2019). https://doi.org/10.1002/pssr.201800448 Without
considering U term, Xiao et al.7474. H. Xiao, X. Wang, R. Wang, L. Xu, S. Liang,
and C. Yang, Phys. Chem. Chem. Phys. 21(22), 11731 (2019).
https://doi.org/10.1039/C9CP00850K investigated the magnetic properties of the
family 2D V3X8 (X = F, Cl, Br, I) in the framework of DFT. They found that V3Cl8
monolayer is an intrinsic AFM semiconductor, while the other three systems are
FM half metals. The estimated Curie temperatures from MC simulations were 77 and
103 K for 2D V3F8 and V3I8, respectively.
B. Binary transition metal chalcogenides
Two-dimensional binary transition metal chalcogenides, including transition
metal dichalcogenides, transition metal monochalcogenides, and other
stoichiometries in a general form of MmXn (M refers to transition metal, and X
represents S, Se, and Te), have provided a gorgeous platform for exploring
interesting electronic and magnetic properties, such as valley polarization and
2D magnetism.
1. Transition metal dichalcogenides
Among 2D binary transition metal chalcogenides, the most widely studied ones are
TMDs. Generally speaking, 2D TMDs form sandwich type structures in the X-M-X
sequence, where transition metal atoms are sandwiched in between two layers of
chalcogen atoms. There are four reported structural phases for TMDs, i.e.,
trigonal prismatic H-phase, octahedral T-phase, distorted octahedral 1T'-type,
and Td-type lattices.183183. D. Puotinen and R. E. Newnham, Acta Crystallogr.
14(6), 691 (1961). https://doi.org/10.1107/S0365110X61002084 In all these
phases, each transition metal atom is surrounded by six chalcogen atoms. The
five formerly degenerate d orbitals of 3d transition metal ion would split in
energy as it is bonded to the chalcogen ligands. Under the crystal field with
D3h symmetry in H phase, the five degenerate 3d orbitals split into a single
state a1 (dz2) and two twofold degenerate states e1 (dx2-y2/dxy) and e2
(dxz/dyz). While in T phase, the triangle sublattice of transition metal atoms
gives rise to first-neighboring coordination number of 6, forming octahedral
crystal field; thus, the d states split into t2g and eg manifolds. Due to the
trigonal distortion, t2g degeneracy is further lifted to form higher-lying a1g
level and twofold degenerate eg states in T′ and Td phases. Intuitively, the
magnetic properties of 2D TMDs should be determined by splitting and filling
behavior of d orbitals of the transition metal ions under various crystal
fields. For the same transition metal ion, the electronegativity of a chalcogen
atom also plays some role, such that the lighter chalcogen atom draws more
electrons from the metal ions and affects their on-site magnetic moments.
It was computationally confirmed that intrinsic long-range magnetic ordering can
be realized on the transition metal sites with respect to delocalized p states
of S/Se/Te atoms in a variety of transition metal dichalcogenides. Early in
2002, extensive analyses of the stability of TMD monolayers based on DFT
calculations predicted that, out of 88 combinations of TMDs compounds, 52 H or T
structures can occur as the freestanding phase. Among them, H-phase TMDs with M
= Cr, Mo, V, Mn, Co, and W, and X = S, Se, and Te were predicted to be
ferromagnetic metals with net magnetic moment ranging from 0.2 to 3.0 μB per
formula.5656. C. Ataca, H. Şahin, and S. Ciraci, J. Phys. Chem. C 116(16), 8983
(2012). https://doi.org/10.1021/jp212558p Similarly, Chen et al.5757. W. Chen,
J.-m. Zhang, Y.-z. Nie, Q.-l. Xia, and G.-h. Guo, J. Magn. Magn. Mater. 508,
166878 (2020). https://doi.org/10.1016/j.jmmm.2020.166878 also systematically
explored the magnetic properties of MTe2 (M = Ti, V, Cr, Mn, Fe, Co, Ni)
monolayers in both H and T phases. Their results indicated that H-VTe2, T-MnTe2
and H-FeTe2 are ferromagnetic metals with magnetic moments of 0.78, 2.80, and
1.48 μB per formula unit (f.u.), respectively, while T-VTe2 is an indirect
bandgap semiconductor with a magnetic moment of 1.0 μB per formula. Using 2D
Ising model and MFT, the estimated Curie temperatures were 301, 33, 88, and
229 K for H-VTe2, T-VTe2, T-MnTe2, and H-FeTe2 monolayers, respectively.
Moreover, non-collinear DFT calculations revealed that H-VTe2, T-VTe2, and
H-FeTe2 monolayers have in-plane easy magnetization direction with MAE values of
0.51, 1.74, and 2.57 meV/f.u., respectively, while T-MnTe2 has a perpendicular
easy magnetization axis with MAE of 0.54 meV/f.u. DFT calculations within LDA+U
approximation demonstrated that single-layer VS2 of 1T phase is a strongly
correlated material, where 2H structure of monolayer VS2 is a ferromagnetic
semiconductor.1010. H. L. Zhuang and R. G. Hennig, Phys. Rev. B 93(5), 054429
(2016). https://doi.org/10.1103/PhysRevB.93.054429 The magnetic moments are
localized on the V atoms and couple ferromagnetically via superexchange
interactions mediated by the S atoms. Calculations of magnetic anisotropy showed
an easy plane for the magnetic moment in 2H VS2. The magnetic properties of 2D
NbS2 and ReS2 nanosheets have been investigated using the HSE06 hybrid
functional.184,185184. Z. Sun, H. Lv, Z. Zhuo, A. Jalil, W. Zhang, X. Wu, and J.
Yang, J. Mater. Chem. C 6(5), 1248 (2018).
https://doi.org/10.1039/C7TC05303G185. Y. Sun, Z. Zhuo, and X. Wu, J. Mater.
Chem. C 6(42), 11401 (2018). https://doi.org/10.1039/C8TC04188A Both of them are
bipolar magnetic semiconductors with spin gaps of 0.27 and 1.63 eV. Furthermore,
MC simulations predicted the Curie temperatures to be 141 and 157 K for 2D NbS2
and ReS2 systems, respectively.
Among the large family of potential 2D magnetic TMDs, VSe2, VTe2, MnSex, NbTe2,
NbSe2, and CrTe2 have attracted more and more attentions from both experimental
and theoretical aspects, mainly because of the reported room-temperature TC. In
the following content, we will discuss the major developments that have paved
the way to this point. First, all these six materials belong to the family of T
phase [Fig. 9(a)] and exhibit metallic nature. The magnetic coupling mechanism
stems from the competition between indirect superexchange and itinerant exchange
interactions. Second, 2D metallic TMDs are well-known charge density wave (CDW)
systems and the CDW phase transition steers the change of electronic structures
in them. Therefore, the correlation effect and the possible competition between
CDW ordering and magnetic ordering also needs to be clarified.
FIG. 9. (a) Top and side views of 1T TMD lattice. (b) M–H hysteresis loop of
bare VSe2 flakes on SiO2 substrate under an in‐plane magnetic field at 300 and
10 K. (c) Temperature‐dependent saturated magnetization (Ms) of bare VSe2 from
10 to 500 K. Inset: M–H curve for the sample at 470 K indicating the loose of
magnetization. (d), (e) Magnetic hysteresis loops for VTe2 at 10 and 300 K,
respectively. (f) Magnetic hysteresis loop of ∼1 ML MnSex on the GaSe base layer
showing ferromagnetic ordering. Inset: the unprocessed SQUID data prior to
background subtraction. Panels (a)–(c) reproduced with permission from Yu et
al., Adv. Mater. 31, 1903779 (2019). Copyright 2019 John Wiley and Sons.186186.
W. Yu, J. Li, T. S. Herng, Z. Wang, X. Zhao, X. Chi, W. Fu, I. Abdelwahab, J.
Zhou, J. Dan, Z. Chen, Z. Chen, Z. Li, J. Lu, S. J. Pennycook, Y. P. Feng, J.
Ding, and K. P. Loh, Adv. Mater. 31(40), e1903779 (2019).
https://doi.org/10.1002/adma.201903779 Panels (d) and (e) reproduced with
permission from Li et al., Adv. Mater. 30, 1801043 (2018). Copyright 2018 John
Wiley and Sons.3535. J. Li, B. Zhao, P. Chen, R. Wu, B. Li, Q. Xia, G. Guo, J.
Luo, K. Zang, Z. Zhang, H. Ma, G. Sun, X. Duan, and X. Duan, Adv. Mater. 30(36),
1801043 (2018). https://doi.org/10.1002/adma.201801043 Panel (f) reproduced with
permission from O'Hara et al., Nano Lett. 18, 3125 (2018). Copyright 2018
American Chemical Society.3636. D. J. O'Hara, T. Zhu, A. H. Trout, A. S. Ahmed,
Y. K. Luo, C. H. Lee, M. R. Brenner, S. Rajan, J. A. Gupta, D. W. McComb, and R.
K. Kawakami, Nano Lett. 18(5), 3125 (2018).
https://doi.org/10.1021/acs.nanolett.8b00683
* PPT
|
* High-resolution
Monolayer VSe2 material has been reported as one of the first room-temperature
2D ferromagnets.3434. M. Bonilla, S. Kolekar, Y. Ma, H. C. Diaz, V. Kalappattil,
R. Das, T. Eggers, H. R. Gutierrez, M.-H. Phan, and M. Batzill, Nature
Nanotechnol. 13(4), 289 (2018). https://doi.org/10.1038/s41565-018-0063-9 From
the electronic structure point of view, VSe2 exhibits a 3d1 configuration, which
invokes both metallic and magnetic properties. Bonilla et al.3434. M. Bonilla,
S. Kolekar, Y. Ma, H. C. Diaz, V. Kalappattil, R. Das, T. Eggers, H. R.
Gutierrez, M.-H. Phan, and M. Batzill, Nature Nanotechnol. 13(4), 289 (2018).
https://doi.org/10.1038/s41565-018-0063-9 synthesized single- and few-layer VSe2
sheets on HOPG and MoS2 substrates using MBE and performed magnetic
characterization by protecting the films with a Se capping layer. Their studies
revealed a significant enhancement of magnetic moment in single-layer samples
compared with multi-layer ones. Surprisingly, the ferromagnetic ordering is very
robust and persists above room temperature. The room-temperature ferromagnetism
was also observed on the exfoliated 2D VSe2 flakes using superconducting quantum
interference device (SQUID), XMCD [Figs. 9(b) and 9(c)], and magnetic force
microscopy (MFM), where the monolayer flake displayed the strongest
ferromagnetic properties.186186. W. Yu, J. Li, T. S. Herng, Z. Wang, X. Zhao, X.
Chi, W. Fu, I. Abdelwahab, J. Zhou, J. Dan, Z. Chen, Z. Chen, Z. Li, J. Lu, S.
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(2019). https://doi.org/10.1002/adma.201903779 First-principles calculations
using both GGA and high-level LDA+DMFT (dynamical mean-field theory) approaches
explained that it might be a ferromagnet with both itinerant and localized
characters.17,18,2017. Y. Ma, Y. Dai, M. Guo, C. Niu, Y. Zhu, and B. Huang, ACS
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https://doi.org/10.1039/C9CP03726H For example, LDA+DMFT calculations predicted
ferromagnetic ordering in VSe2 monolayer without CDW below 250 K. However, the
origin of ferromagnetism in VSe2 has spurred great controversies. First, the
reported experimental magnetic moments (5∼15 μB)34,18734. M. Bonilla, S.
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signal was detected in another XMCD experiment and the angle resolved
photoemission spectrum showed no exchange splitting.188,189188. P. Chen, W. W.
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Yin, T. S. Herng, L. Zhang, Y. L. Huang, G. Vinai, S. Krishnamurthi, D. W.
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W. Wan, C. González-Orellana, M. Peña-Díaz, C. Rogero, J. Herrero-Martín, P.
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transition distortion can suppress the intrinsic ferromagnetic ground states in
VSe2. Wong et al.145145. P. K. J. Wong, W. Zhang, F. Bussolotti, X. Yin, T. S.
Herng, L. Zhang, Y. L. Huang, G. Vinai, S. Krishnamurthi, D. W. Bukhvalov, Y. J.
Zheng, R. Chua, A. T. N'Diaye, S. A. Morton, C.-Y. Yang, K.-H. O. Yang, P.
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H. C. Neto, and A. T. S. Wee, Adv. Mater. 31(23), 1901185 (2019).
https://doi.org/10.1002/adma.201901185 observed traits of spin frustration in
monolayer VSe2 with long-range intrinsic ferromagnetism from complementary
temperature- and field-dependent susceptibility measurements. They have also
reported that the frustrated intrinsic magnetism in 2D VSe2 can be lifted by the
introduction of the Se-deficient defects.192192. R. Chua, J. Yang, X. He, X. Yu,
W. Yu, F. Bussolotti, P. K. J. Wong, K. P. Loh, M. B. H. Breese, K. E. J. Goh,
Y. L. Huang, and A. T. S. Wee, Adv. Mater. 32(24), 2000693 (2020).
https://doi.org/10.1002/adma.202000693 Yu et al. inferred that a defect-free
sample is the key to verify the intrinsic ferromagnetism of VSe2.186186. W. Yu,
J. Li, T. S. Herng, Z. Wang, X. Zhao, X. Chi, W. Fu, I. Abdelwahab, J. Zhou, J.
Dan, Z. Chen, Z. Chen, Z. Li, J. Lu, S. J. Pennycook, Y. P. Feng, J. Ding, and
K. P. Loh, Adv. Mater. 31(40), e1903779 (2019).
https://doi.org/10.1002/adma.201903779 Nakano et al.193193. M. Nakano, Y. Wang,
S. Yoshida, H. Matsuoka, Y. Majima, K. Ikeda, Y. Hirata, Y. Takeda, H. Wadati,
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8806 (2019). https://doi.org/10.1021/acs.nanolett.9b03614 demonstrated the
emergence of intrinsic ferromagnetism in V5Se8 (V0.25VSe2) epitaxial thin films
grown by MBE, which can be classified as itinerant 2D Heisenberg ferromagnets
with weak magnetic anisotropy.
Due to strong 3d1 electron coupling in the neighboring M4+–M4+ pairs (M = V, Nb,
Ta) of 2D TMDs, metallic VTe2, NbTe2, NbSe2, and TaTe2 systems composed of group
VB elements have been regarded as the potential intrinsic magnets. In some
theoretical investigations, 2D VTe2, NbTe2, and TaTe2 have been predicted to
exhibit intrinsic magnetic ordering.58,59,194,19558. H.-R. Fuh, C.-R. Chang,
Y.-K. Wang, R. F. L. Evans, R. W. Chantrell, and H.-T. Jeng, Sci. Rep. 6(1),
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In particular, VTe2 monolayer was found to be a room-temperature ferromagnet
with highest TC value of 553∼618 K. Meanwhile, MC simulation of the hysteresis
features of VTe2 monolayer illustrated that it is possible to observe finite
remanence and coercivity treatments nearly or well beyond room
temperature.146146. E. Vatansever, S. Sarikurt, and R. F. L. Evans, Mater. Res.
Express 5(4), 046108 (2018). https://doi.org/10.1088/2053-1591/aabca6 Similar to
the discussions on VSe2, Wong et al.196196. P. K. J. Wong, W. Zhang, J. Zhou, F.
Bussolotti, X. Yin, L. Zhang, A. T. N'Diaye, S. A. Morton, W. Chen, J. Goh, M.
P. de Jong, Y. P. Feng, and A. T. S. Wee, ACS Nano 13(11), 12894 (2019).
https://doi.org/10.1021/acsnano.9b05349 also found that CDW order would rule out
the ferromagnetic behavior in VTe2 monolayer. Experimentally, the single
crystalline ultrathin VTe2, NbTe2, and TaTe2 sheets were synthesized using
atmospheric pressure CVD approach.3535. J. Li, B. Zhao, P. Chen, R. Wu, B. Li,
Q. Xia, G. Guo, J. Luo, K. Zang, Z. Zhang, H. Ma, G. Sun, X. Duan, and X. Duan,
Adv. Mater. 30(36), 1801043 (2018). https://doi.org/10.1002/adma.201801043 The
magnetic hysteresis (M‐H) measurements demonstrated that VTe2 and NbTe2 exhibit
room‐temperature ferromagnetism [Figs. 9(d) and 9(e)]. The reported saturation
magnetization, coercivity, and remnant magnetization values of VTe2 were 0.3 emu
g−1, 1173.0 Oe, 0.16 emu g−1 at 10 K, and 0.21 emu g−1, 592 Oe, 0.10 emu g−1 at
300 K, respectively. However, the competition between CDW and magnetic
instability in VTe2 is still under debate. On the one hand, formation of the CDW
phase in 2D VSe2 was observed in experiment, and three possible CDW transitions
at 135, 240, and 186 K have been reported.35,197,19835. J. Li, B. Zhao, P. Chen,
R. Wu, B. Li, Q. Xia, G. Guo, J. Luo, K. Zang, Z. Zhang, H. Ma, G. Sun, X. Duan,
and X. Duan, Adv. Mater. 30(36), 1801043 (2018).
https://doi.org/10.1002/adma.201801043197. Y. Wang, J. Ren, J. Li, Y. Wang, H.
Peng, P. Yu, W. Duan, and S. Zhou, Phys. Rev. B 100(24), 241404 (2019).
https://doi.org/10.1103/PhysRevB.100.241404198. X. Ma, T. Dai, S. Dang, S. Kang,
X. Chen, W. Zhou, G. Wang, H. Li, P. Hu, Z. He, Y. Sun, D. Li, F. Yu, X. Zhou,
H. Chen, X. Chen, S. Wu, and S. Li, ACS Appl. Mater. Inter. 11(11), 10729
(2019). https://doi.org/10.1021/acsami.8b21442 The CDW effect would suppress the
magnetic instability and further lead to the absence of magnetic ordering.
Through a combination of in situ microscopic and spectroscopic techniques, Wong
et al.196196. P. K. J. Wong, W. Zhang, J. Zhou, F. Bussolotti, X. Yin, L. Zhang,
A. T. N'Diaye, S. A. Morton, W. Chen, J. Goh, M. P. de Jong, Y. P. Feng, and A.
T. S. Wee, ACS Nano 13(11), 12894 (2019).
https://doi.org/10.1021/acsnano.9b05349 observed a 4 × 4 CDW order and further
excluded the intrinsic ferromagnetic ordering in VTe2 by XMCD data. One the
other hand, Sugawara and coworkers199199. K. Sugawara, Y. Nakata, K. Fujii, K.
Nakayama, S. Souma, T. Takahashi, and T. Sato, Phys. Rev. B 99(24), 241404
(2019). https://doi.org/10.1103/PhysRevB.99.241404 recently found a large
triangular Fermi surface at the K point that satisfies a nearly perfect nesting
condition, whereas CDW is suppressed as highlighted by the observation of band
crossing of the Fermi level at low temperature, in contrast to monolayer VSe2.
Combining DFT calculations with scanning tunneling microscopy and spectroscopy
(STM/STS) measurements, ferrimagnetic ground state of 2D NbSe2 was demonstrated
with a magnetic moment of 1.09 μB.200200. S. Divilov, W. Wan, P. Dreher, M. M.
Ugeda, and F. Ynduráin, arXiv:005.06210 (2020). More importantly, their results
also inferred that substrate is the key to verifying the intrinsic
ferromagnetism of TMD materials.196,200196. P. K. J. Wong, W. Zhang, J. Zhou, F.
Bussolotti, X. Yin, L. Zhang, A. T. N'Diaye, S. A. Morton, W. Chen, J. Goh, M.
P. de Jong, Y. P. Feng, and A. T. S. Wee, ACS Nano 13(11), 12894 (2019).
https://doi.org/10.1021/acsnano.9b05349200. S. Divilov, W. Wan, P. Dreher, M. M.
Ugeda, and F. Ynduráin, arXiv:005.06210 (2020). It was shown that single-layer
NbSe2 does not display CDW instability unless a graphene layer is utilized as
substrate.
Motivated by DFT calculations,56,6056. C. Ataca, H. Şahin, and S. Ciraci, J.
Phys. Chem. C 116(16), 8983 (2012). https://doi.org/10.1021/jp212558p60. M. Kan,
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https://doi.org/10.1039/c3cp55146f O'Hara et al.3636. D. J. O'Hara, T. Zhu, A.
H. Trout, A. S. Ahmed, Y. K. Luo, C. H. Lee, M. R. Brenner, S. Rajan, J. A.
Gupta, D. W. McComb, and R. K. Kawakami, Nano Lett. 18(5), 3125 (2018).
https://doi.org/10.1021/acs.nanolett.8b00683 also observed room-temperature
ferromagnetism in manganese selenide (MnSex) films grown by MBE. Magnetic and
structural characterizations provided strong evidence that, at the monolayer
limit, ferromagnetism originates from the vdW MnSe2 monolayer. Using DFT
calculations combined with MC simulations, Kan et al.6060. M. Kan, S. Adhikari,
and Q. Sun, Phys. Chem. Chem. Phys. 16(10), 4990 (2014).
https://doi.org/10.1039/c3cp55146f have shown that 2D MnSe2 sheets are ideal
magnetic semiconductors with long-range magnetic ordering, where all Mn atoms
are ferromagnetically coupled and the estimated TC is 250 K [Fig. 9(f)].
Interestingly, first-principles calculations further revealed the great defect
tolerance in MnSe2. Despite the presence of high-density Se vacancies, the
defective MnSe2 monolayer can retain its stable ferromagnetic behavior.144144.
I. Eren, F. Iyikanat, and H. Sahin, Phys. Chem. Chem. Phys. 21(30), 16718
(2019). https://doi.org/10.1039/C9CP03112J Magnetic tunneling junctions based on
monolayer MnSe2 with room-temperature ferromagnetism were also observed with a
large tunneling magnetoresistance of 725%.201201. L. Pan, H. Wen, L. Huang, L.
Chen, H.-X. Deng, J.-B. Xia, and Z. Wei, Chin. Phys. B 28(10), 107504 (2019).
https://doi.org/10.1088/1674-1056/ab3e45 Moreover, by optical and electronic
measurements, Sun et al.147147. X. Sun, W. Li, X. Wang, Q. Sui, T. Zhang, Z.
Wang, L. Liu, D. L. Li, S. Feng, S. Zhong, H. Wang, V. Bouchiat, M. N. Regueiro,
N. Rougemaille, J. Coraux, Z. Wang, B. Dong, X. Wu, T. Yang, G. Yu, B. Wang, Z.
V. Han, X. Han, and Z. Zhang, Nano Res. 13, 3358 (2020).
https://doi.org/10.1007/s12274-020-3021-4 disclosed that the intrinsic
ferromagnetically aligned spin polarization can hold up to 316 K in a metallic
phase of 1T-CrTe2. Detailed spin transport measurements suggested
half-metallicity in its spin polarized band structure as well as in-plane
room-temperature negative anisotropic magnetoresistance. Importantly, their
study found that exchange coupling due to an enhancement of itinerant type was
the source of room-temperature ferromagnetism in both bulk and few-layered
Cr2Te3.2121. Y. Wen, Z. Liu, Y. Zhang, C. Xia, B. Zhai, X. Zhang, G. Zhai, C.
Shen, P. He, R. Cheng, L. Yin, Y. Yao, M. Getaye Sendeku, Z. Wang, X. Ye, C.
Liu, C. Jiang, C. Shan, Y. Long, and J. He, Nano Lett. 20(5), 3130 (2020).
https://doi.org/10.1021/acs.nanolett.9b05128
2. Other transition metal chalcogenides
Owing to the variable valence of transition metal elements, transition metal
chalcogenides have diverse stoichiometric compositions. In addition to TMDs with
M:X = 1:2, the 2D transition metal chalcogenide compounds with higher
stoichiometries (M:X = 5:8, 2:3, 3:4, and 1:1) also exhibit interesting magnetic
properties. In V5S8 nanosheets, an AFM to FM phase transition was observed when
the thickness is down to 3.2 nm. Using DFT calculations, Zhang et al.202202. R.
Z. Zhang, Y. Y. Zhang, and S. X. Du, Chin. Phys. B 29(7), 077504 (2020).
https://doi.org/10.1088/1674-1056/ab8db1 further investigated the
thickness-dependent magnetic ordering in V5S8 thin films, and confirmed an
antiferromagnetic to ferromagnetic phase transition when V5S8 is thinned down to
2.2 nm. The magnetic moments of the thin films in both antiferromagnetic and
ferromagnetic states are mainly located on V atoms in the intermediate layer.
Utilizing vdW epitaxy techniques, Cr2S3 sheets with one-unit cell thickness down
to 1.78 nm have been successfully synthesized, which exhibited ferrimagnetic
behavior with a Néel temperature of 120 K and the maximum saturation magnetic
momentum of up to 65 emu.203203. J. Chu, Y. Zhang, Y. Wen, R. Qiao, C. Wu, P.
He, L. Yin, R. Cheng, F. Wang, Z. Wang, J. Xiong, Y. Li, and J. He, Nano Lett.
19(3), 2154 (2019). https://doi.org/10.1021/acs.nanolett.9b00386 In addition, Lv
et al.204204. P. Lv, G. Tang, C. Yang, J. Deng, Y. Liu, X. Wang, X. Wang, and J.
Hong, 2D Mater. 5(4), 045026 (2018). https://doi.org/10.1088/2053-1583/aadb5a
identified Co2Se3 as a 2D half-metal among a series of M2Se3 candidate
materials, and the calculated TC from the mean field theory was about 600 K.
Using first-principles calculations, Ouyang and co-workers predicted a family of
stable 2D honeycomb lattices of Cr2X3 (X = O, S, Se). Cr2S3 and Cr2Se3 are
ferromagnetic half-metals with mirror symmetry protected nodal lines for the
spin-down channels, while Cr2O3 layers are ferromagnetic semiconductors with
large out-of-plane MAE.205205. J.-Y. Chen, X.-X. Li, W.-Z. Zhou, J.-L. Yang,
F.-P. Ouyang, and X. Xiong, Adv. Electron. Mater. 6(1), 2070001 (2020).
https://doi.org/10.1002/aelm.202070001 A hexagonal Ta2S3 sheet has also been
predicted as 2D magnet from the spin-wave theory, which possesses sizeable
out-of-plane MAE of 4.6 meV and high Curie temperature of 445 K.206206. L.
Zhang, C.-w. Zhang, S.-F. Zhang, W.-x. Ji, P. Li, and P.-j. Wang, Nanoscale
11(12), 5666 (2019). https://doi.org/10.1039/C9NR00826H
Based on first-principles calculations, a new composition of stable 2D
transition metal chalcogenides, i.e., Cr3X4 (X = S, Se, Te) monolayers, has been
predicted to possess fascinating magnetic properties.1515. X. Zhang, B. Wang, Y.
Guo, Y. Zhang, Y. Chen, and J. Wang, Nanoscale Horiz. 4(4), 859 (2019).
https://doi.org/10.1039/C9NH00038K Among them, Cr3S4 monolayer is a
ferrimagnetic semiconductor, while Cr3Se4 and Cr3Te4 monolayers are
ferromagnetic half-metals with TC of 370 and 460 K, respectively. Unlike the
d–p–d superexchange interaction found in the other transition metal compounds,
double exchange magnetic coupling mechanism is dominated in these 2D Cr3X4
sheets, finally leading to enhanced FM ordering and room temperature TC. That is
to say, a delocalized unpaired electron could hop between the two neighboring Cr
ions with different oxidation states in 2D Cr3X4.
Using PSO technique combined with first-principles calculations, Zhang et
al.207207. Y. Zhang, J. Pang, M. Zhang, X. Gu, and L. Huang, Sci. Rep. 7(1),
15993 (2017). https://doi.org/10.1038/s41598-017-16032-x predicted a new
transition metal chalcogenide monolayer composed of cobalt and sulfur
atoms—Co2S2. Their results revealed that a single-layer Co2S2 sheet is a
ferromagnetic metal with a Curie temperature of 404 K. Two-dimensional ultrathin
CrSe crystals were successfully synthesized on mica substrate via ambient
pressure CVD method.208208. Y. Zhang, J. Chu, L. Yin, T. A. Shifa, Z. Cheng, R.
Cheng, F. Wang, Y. Wen, X. Zhan, Z. Wang, and J. He, Adv. Mater. 31(19), 1900056
(2019). https://doi.org/10.1002/adma.201900056 Such CVD-grown 2D CrSe crystals
exhibit evident ferromagnetic behavior at temperatures below 280 K. Kang et
al.209209. L. Kang, C. Ye, X. Zhao, X. Zhou, J. Hu, Q. Li, D. Liu, C. M. Das, J.
Yang, D. Hu, J. Chen, X. Cao, Y. Zhang, M. Xu, J. Di, D. Tian, P. Song, G.
Kutty, Q. Zeng, Q. Fu, Y. Deng, J. Zhou, A. Ariando, F. Miao, G. Hong, Y. Huang,
S. J. Pennycook, K.-T. Yong, W. Ji, X. R. Wang, and Z. Liu, Nat. Commun. 11(1),
3729 (2020). https://doi.org/10.1038/s41467-020-17253-x reported the synthesis
of ultrathin FeTe film by CVD approach and discussed their structural and
magnetic transition. Transport measurements revealed that tetragonal FeTe is an
antiferromagnetic metal with TN of about 71.8 K, while hexagonal FeTe is a
ferromagnetic metal with TC of around 220 K. Very recently, Yuan et al.210210.
Q.-Q. Yuan, Z. Guo, Z.-Q. Shi, H. Zhao, Z.-Y. Jia, Q. Wang, J. Sun, D. Wu, and
S.-C. Li, Chin. Phys. Lett. 37(7), 077502 (2020).
https://doi.org/10.1088/0256-307X/37/7/077502 have grown FM MnSe monolayer on
silicon substrate using MBE method. The thickness dependence of Curie
temperature was found in the MnSe ultrathin films. The measured TC was 54 K for
monolayer, while it sharply increased to 225 K for three-layer MnSe, and 235 K
for four-layer MnSe.
C. MXene and MXene analogues
MXene, a category of 2D transition metal carbides, nitrides, and carbonitrides,
possibly terminated by functional group (T) on the surface, with a general
formula of Mn+1XnTx (M = transition metal; X = C and/or N; T = O, OH, F) are
attractive additions to the family of 2D materials. Since the discovery of Ti3C2
in 2011, MXenes of Ti2C, V2C, Nb2C, Mo2C, Zr3C2, Nb4C3, Ta4C3, and Ti4N3, as
well as of TiNbC, (Ti0.5Nb0.5)2C, (V0.5Cr0.5)3C2, Ti3CN, Mo2TiC2, Mo2ScC2,
Cr2TiC2, Mo2Ti2C3, (Nb0.8Ti0.2)4C3, and (Nb0.8Zr0.2)4C3 have already been
fabricated in a laboratory. According to the large number of M-X compositions,
more than a hundred MXenes have been theoretically predicted.211211. M. Khazaei,
A. Ranjbar, M. Arai, T. Sasaki, and S. Yunoki, J. Mater. Chem. C 5(10), 2488
(2017). https://doi.org/10.1039/C7TC00140A
The diverse compositions and controllable thickness of Mn+1XnTx systems provide
an ideal playground to achieve 2D intrinsic magnetism (Table III).92,211,21292.
X. Zhang and C. Gong, Science 363(6428), 4450 (2019).
https://doi.org/10.1126/science.aav4450211. M. Khazaei, A. Ranjbar, M. Arai, T.
Sasaki, and S. Yunoki, J. Mater. Chem. C 5(10), 2488 (2017).
https://doi.org/10.1039/C7TC00140A212. N. C. Frey, C. C. Price, A.
Bandyopadhyay, H. Kumar, and V. B. Shenoy, Predicted Magnetic Properties of
MXenes ( Springer, Cham, 2019). As mentioned above, transition metal M atom
usually has a partially filled d shell with unpaired electrons. First of all,
the magnetic properties of MXene are influenced by the total number of d
electrons. The M site can be also occupied by the ordered double transition
metal species, i.e., M′M″. Generally, M′ atoms locate in the outer layers and M″
atoms occupy the middle layer, yielding [M′X]nM″ arrangement. However, some
in-plane ordered double transition metal MXenes have also been reported. The
competition of electron hopping and electronic coupling between M′ and M″ ions
will further modify the magnetic ground state of MXene. Each X (C or N) anion
bounds with six transition metal M cations, forming XM6 octahedral
configuration. The transition metal atoms on the surface of M2X are subjected to
a C3v ligand field contributed by the neighboring X atoms. Hence, the five 3d
orbitals of transition metal atom would split into a single state a1 (dz2), two
twofold degenerate states e1 (dx2-y2/dxy) and e2 (dxz/dyz). The splitting
between a1 and e1/e2 is determined by the strengths of M–X interactions. In
addition, long-range spin interaction between the metal atoms always occurs
through X atoms in MXene, which plays an important role in mediating the
magnetic coupling.
TABLE III. A list of 2D magnets in MXene family with their compositions and key
electronic and magnetic properties, including the magnetic ground state (GS),
the values of Hubbard U, energy gap (Eg), magnetic moment on per transition
metal (Ms), Curie temperature (TC), and magnetic anisotropy energy per unit cell
(MAE).
Compositions GS U (eV) Eg (eV) Ms (μB) TC (K) MAE (meV) Ref. Mn+1Xn Cr2C FM – HM
3 – – 136136. C. Si, J. Zhou, and Z. Sun, ACS Appl. Mater. Inter. 7(31), 17510
(2015). https://doi.org/10.1021/acsami.5b05401 Ti2C FM – M 0.96 – – 222222. S.
Zhao, W. Kang, and J. Xue, Appl. Phys. Lett. 104(13), 133106 (2014).
https://doi.org/10.1063/1.4870515 Ti2C FM – HM 0.96 – – 223223. G. Gao, G. Ding,
J. Li, K. Yao, M. Wu, and M. Qian, Nanoscale 8(16), 8986 (2016).
https://doi.org/10.1039/C6NR01333C Ti2C AFM 2∼5 0.42 0.95 – – 224224. B. Akgenc,
A. Mogulkoc, and E. Durgun, J. Appl. Phys. 127(8), 084302 (2020).
https://doi.org/10.1063/1.5140578 Ti2C FM – M 0.97 146 – 243243. Y. Hu, X. L.
Fan, W. J. Guo, Y. R. An, Z. F. Luo, and J. Kong, J. Magn. Magn. Mater. 486,
165280 (2019). https://doi.org/10.1016/j.jmmm.2019.165280 2H-Ti2C FM 2∼5 HM 1.0
290 – 224224. B. Akgenc, A. Mogulkoc, and E. Durgun, J. Appl. Phys. 127(8),
084302 (2020). https://doi.org/10.1063/1.5140578 Cr2N AFM 3 M 4.45 – – 226226.
G. Wang, J. Phys. Chem. C 120(33), 18850 (2016).
https://doi.org/10.1021/acs.jpcc.6b05224 Ti2N FM – HM 0.5 – – 223223. G. Gao, G.
Ding, J. Li, K. Yao, M. Wu, and M. Qian, Nanoscale 8(16), 8986 (2016).
https://doi.org/10.1039/C6NR01333C V2N AFM – M 0.07 – – 223223. G. Gao, G. Ding,
J. Li, K. Yao, M. Wu, and M. Qian, Nanoscale 8(16), 8986 (2016).
https://doi.org/10.1039/C6NR01333C V2C AFM 4 M – – – 240240. J. Hu, B. Xu, C.
Ouyang, S. A. Yang, and Y. Yao, J. Phys. Chem. C 118(42), 24274 (2014).
https://doi.org/10.1021/jp507336x Zr2C FM – M 0.63 – – 222222. S. Zhao, W. Kang,
and J. Xue, Appl. Phys. Lett. 104(13), 133106 (2014).
https://doi.org/10.1063/1.4870515 Fe2C FM – M 1.96 861 (MFT) –0.114 9797. Y.
Yue, J. Magn. Magn. Mater. 434, 164 (2017).
https://doi.org/10.1016/j.jmmm.2017.03.058 Mn2C AFM – M 3 720 –0.025 220220. L.
Hu, X. Wu, and J. Yang, Nanoscale 8(26), 12939 (2016).
https://doi.org/10.1039/C6NR02417C Cr3C2 FM – M 1.3 886 (MFT) – 229229. Y. Zhang
and F. Li, J. Magn. Magn. Mater. 433, 222 (2017).
https://doi.org/10.1016/j.jmmm.2017.03.031 Ti3C2 PM i – – – 10 i – 233233. Y.
Yoon, T. A. Le, A. P. Tiwari, I. Kim, M. W. Barsoum, and H. Lee, Nanoscale
10(47), 22429 (2018). https://doi.org/10.1039/C8NR06854B Ti3C2 FM – M 228228. F.
Wu, K. Luo, C. Huang, W. Wu, P. Meng, Y. Liu, and E. Kan, Solid State Commun.
222, 9 (2015). https://doi.org/10.1016/j.ssc.2015.08.023 Ti3CN FM – M 228228. F.
Wu, K. Luo, C. Huang, W. Wu, P. Meng, Y. Liu, and E. Kan, Solid State Commun.
222, 9 (2015). https://doi.org/10.1016/j.ssc.2015.08.023 Ti4C3 FM – M 0.875 – –
236236. P. Urbankowski, B. Anasori, T. Makaryan, D. Er, S. Kota, P. L. Walsh, M.
Zhao, V. B. Shenoy, M. W. Barsoum, and Y. Gogotsi, Nanoscale 8(22), 11385
(2016). https://doi.org/10.1039/C6NR02253G Tin+1Cn (n=1∼9) FM 0.98∼0.15 217217.
Y. Xie and P. R. C. Kent, Phys. Rev. B 87(23), 235441 (2013).
https://doi.org/10.1103/PhysRevB.87.235441 Tin+1Nn (n=1∼9) FM 0.62∼0.06 217217.
Y. Xie and P. R. C. Kent, Phys. Rev. B 87(23), 235441 (2013).
https://doi.org/10.1103/PhysRevB.87.235441 Ta2C NM – – – – – 218218. N. J. Lane,
M. W. Barsoum, and J. M. Rondinelli, EPL-Europhys. Lett. 101(5), 57004 (2013).
https://doi.org/10.1209/0295-5075/101/57004 Ta2C AFM – – – – – 218218. N. J.
Lane, M. W. Barsoum, and J. M. Rondinelli, EPL-Europhys. Lett. 101(5), 57004
(2013). https://doi.org/10.1209/0295-5075/101/57004 Ta3C2 AFM – – – – – 218218.
N. J. Lane, M. W. Barsoum, and J. M. Rondinelli, EPL-Europhys. Lett. 101(5),
57004 (2013). https://doi.org/10.1209/0295-5075/101/57004 Ta3C2 FiM – – – – –
218218. N. J. Lane, M. W. Barsoum, and J. M. Rondinelli, EPL-Europhys. Lett.
101(5), 57004 (2013). https://doi.org/10.1209/0295-5075/101/57004 Ta4C3 AFM – –
– – – 218218. N. J. Lane, M. W. Barsoum, and J. M. Rondinelli, EPL-Europhys.
Lett. 101(5), 57004 (2013). https://doi.org/10.1209/0295-5075/101/57004 Ta4C3
AFM – – – – – 218218. N. J. Lane, M. W. Barsoum, and J. M. Rondinelli,
EPL-Europhys. Lett. 101(5), 57004 (2013).
https://doi.org/10.1209/0295-5075/101/57004 2H-Ru2C FM – M 0.86 – – 225225. B.
Akgenc, Solid State Commun. 303–304, 113739 (2019).
https://doi.org/10.1016/j.ssc.2019.113739 Fe2N AFM 4.0 227227. G. Wang and Y.
Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Co2N AFM 3.3 227227. G. Wang and Y.
Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Ni2N AFM 6.4 227227. G. Wang and Y.
Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Mn+1XnTx Sc2C(OH)xO2−x FM – 0.506
0.5 – – 234234. X.-H. Zha, J.-C. Ren, L. Feng, X. Bai, K. Luo, Y. Zhang, J. He,
Q. Huang, J. S. Francisco, and S. Du, Nanoscale 10(18), 8763 (2018).
https://doi.org/10.1039/C8NR01292J Mo3N2F2 FM 3 HM 2.1 237 0.1736 235235. S.-s.
Li, S.-j. Hu, W.-x. Ji, P. Li, K. Zhang, C.-w. Zhang, and S.-s. Yan, Appl. Phys.
Lett. 111(20), 202405 (2017). https://doi.org/10.1063/1.4993869 Mn2NF2 FM 4 HM
4.5 1877 – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V.
B. Shenoy, ACS Nano 11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578
Mn2NO2 FM 4 HM 3.8 1379 – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b02578 Mn2N(OH)2 FM 4 HM 4.4 1745 – 1212. H.
Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano
11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578 Cr2NF2 AFM 4 –
3.7/3.0 – – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V.
B. Shenoy, ACS Nano 11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578
Cr2NO2 FM 4 HM 2.8 566 – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b02578 Cr2N(OH)2 AFM 4 – 3.0 – – 1212. H.
Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano
11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578 V2NF2 AFM 3 –
2.5/2.0 – – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V.
B. Shenoy, ACS Nano 11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578
V2NO2 AFM 3 – 1.8/1.0 – – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b02578 V2N(OH)2 AFM 3 – 2.2 – – 1212. H. Kumar,
N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 11(8),
7648 (2017). https://doi.org/10.1021/acsnano.7b02578 Ti2NF2 AFM 4 – 1.3/1.0 – –
1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V. B. Shenoy,
ACS Nano 11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578 Ti2NO2 FM 4
HM 0.5 – – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y. Gogotsi, and V.
B. Shenoy, ACS Nano 11(8), 7648 (2017). https://doi.org/10.1021/acsnano.7b02578
Ti2N(OH)2 AFM 4 – 0.9 – – 1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b02578 Mn2CH2 FM 4 M 3.22 293 – 237237. X.
Zhang, T. He, W. Meng, L. Jin, Y. Li, X. Dai, and G. Liu, J. Phys. Chem. C
123(26), 16388 (2019). https://doi.org/10.1021/acs.jpcc.9b04445 Mn2CO2 FM 4 M
3.1 323 – 237237. X. Zhang, T. He, W. Meng, L. Jin, Y. Li, X. Dai, and G. Liu,
J. Phys. Chem. C 123(26), 16388 (2019). https://doi.org/10.1021/acs.jpcc.9b04445
Mn2CO1.5 FM 4 M 3.06 – – 237237. X. Zhang, T. He, W. Meng, L. Jin, Y. Li, X.
Dai, and G. Liu, J. Phys. Chem. C 123(26), 16388 (2019).
https://doi.org/10.1021/acs.jpcc.9b04445 Cr2CF2 FM – M 2.71 – – 525525. M.
Khazaei, M. Arai, T. Sasaki, C.-Y. Chung, N. S. Venkataramanan, M. Estili, Y.
Sakka, and Y. Kawazoe, Adv. Funct. Mater. 23(17), 2185 (2013).
https://doi.org/10.1002/adfm.201202502 Cr2C(OH)2 FM – M 2.24 – – 525525. M.
Khazaei, M. Arai, T. Sasaki, C.-Y. Chung, N. S. Venkataramanan, M. Estili, Y.
Sakka, and Y. Kawazoe, Adv. Funct. Mater. 23(17), 2185 (2013).
https://doi.org/10.1002/adfm.201202502 Cr2NF2 FM – M 3.23 – – 525525. M.
Khazaei, M. Arai, T. Sasaki, C.-Y. Chung, N. S. Venkataramanan, M. Estili, Y.
Sakka, and Y. Kawazoe, Adv. Funct. Mater. 23(17), 2185 (2013).
https://doi.org/10.1002/adfm.201202502 Cr2NOH2 FM – M 3.01 – – 525525. M.
Khazaei, M. Arai, T. Sasaki, C.-Y. Chung, N. S. Venkataramanan, M. Estili, Y.
Sakka, and Y. Kawazoe, Adv. Funct. Mater. 23(17), 2185 (2013).
https://doi.org/10.1002/adfm.201202502 Cr2NO2 FM – M 2.50 – – 525525. M.
Khazaei, M. Arai, T. Sasaki, C.-Y. Chung, N. S. Venkataramanan, M. Estili, Y.
Sakka, and Y. Kawazoe, Adv. Funct. Mater. 23(17), 2185 (2013).
https://doi.org/10.1002/adfm.201202502 Mn2NOF FM 4 SC 3.94 173 0.0241 252252. N.
C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy,
ACS Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201 Mn2NOF FM 4
SC 3.94 163 0.0488 252252. N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori,
Y. Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 Mn2NOF FM 4 HM 3.95 310 0.0202 252252.
N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B.
Shenoy, ACS Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201
Mn2NO0.5F1.5 FM 4 HM 4.17 187 0.0148 252252. N. C. Frey, A. Bandyopadhyay, H.
Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 Mn2NO1.5F0.5 FM 4 M 3.72 180 0.0326
252252. N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V.
B. Shenoy, ACS Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201
Cr2NOF AFM 4 HM 3.21 65 – 252252. N. C. Frey, A. Bandyopadhyay, H. Kumar, B.
Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 Cr2NOF AFM 4 HM 2.90 4 – 252252. N. C.
Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS
Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201 Cr2NOF AFM 4 HM
3.10 23 – 252252. N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 Cr2NO0.5F1.5 AFM 4 HM 3.29 86 – 252252.
N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B.
Shenoy, ACS Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201
Cr2NO1.5F0.5 AFM 4 M 3.03 335 – 252252. N. C. Frey, A. Bandyopadhyay, H. Kumar,
B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 V2NOF AFM 4 SC 1.64 347 – 252252. N. C.
Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS
Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201 V2NOF AFM 4 SC
1.64 62 – 252252. N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 V2NOF AFM 4 SC 1.63 128 – 252252. N. C.
Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS
Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201 V2NO0.5F1.5 AFM
4 HM 1.88 83 – 252252. N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 V2NO1.5F0.5 AFM 4 SC 1.53 281 – 252252.
N. C. Frey, A. Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B.
Shenoy, ACS Nano 13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201
Cr2CF2 AFM – 3.15 2.53 – – 244244. J. Yang, X. Zhou, X. Luo, S. Zhang, and L.
Chen, Appl. Phys. Lett. 109(20), 203109 (2016).
https://doi.org/10.1063/1.4967983 Cr2C(OH)2 AFM – 1.39 2.39 – – 244244. J. Yang,
X. Zhou, X. Luo, S. Zhang, and L. Chen, Appl. Phys. Lett. 109(20), 203109
(2016). https://doi.org/10.1063/1.4967983 Ni2NF2 FM 6.4 HM 1.60 1800 – 227227.
G. Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Ni2N(OH)2 FM 6.4 HM 1.60 2400 –
227227. G. Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Ni2NO2 FM 6.4 HM 0.97 3300 –
227227. G. Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Fe2NF2 AFM 4.0 – – – – 227227. G.
Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Fe2N(OH)2 FM 4.0 HM – – – 227227.
G. Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Fe2NO2 FM 4.0 HM – – – 227227. G.
Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Co2NF2 AFM 3.3 – – – – 227227. G.
Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Co2N(OH)2 AFM 3.3 – – – – 227227.
G. Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Co2NO2 FM 3.3 – – – – 227227. G.
Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Mn2NO2 FM 4 HM 3.8 67 63 238238. N.
C. Frey, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 12(6),
6319 (2018). https://doi.org/10.1021/acsnano.8b03472 Mn2N(OH)2 FM 4 HM 4.5 – 1.3
238238. N. C. Frey, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano
12(6), 6319 (2018). https://doi.org/10.1021/acsnano.8b03472 Mn2NF2 FM 4 HM 4.5
1148 2.0 238238. N. C. Frey, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy,
ACS Nano 12(6), 6319 (2018). https://doi.org/10.1021/acsnano.8b03472 Cr2NO2 FM 4
HM 2.9 53 22 238238. N. C. Frey, H. Kumar, B. Anasori, Y. Gogotsi, and V. B.
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Ti2NO2 FM 4 HM 0.51 – 0.78 238238. N. C. Frey, H. Kumar, B. Anasori, Y. Gogotsi,
and V. B. Shenoy, ACS Nano 12(6), 6319 (2018).
https://doi.org/10.1021/acsnano.8b03472 Cr2CFCl AFM 3 SM 3 395 – 239239. J. He,
P. Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J. Mater. Chem. C
4(27), 6500 (2016). https://doi.org/10.1039/C6TC01287F Cr2CHBr AFM 3 SM 3 320 –
239239. J. He, P. Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J.
Mater. Chem. C 4(27), 6500 (2016). https://doi.org/10.1039/C6TC01287F Cr2CClBr
AFM 3 SM 3 385 – 239239. J. He, P. Lyu, L. Z. Sun, Á. Morales García, and P.
Nachtigall, J. Mater. Chem. C 4(27), 6500 (2016).
https://doi.org/10.1039/C6TC01287F Cr2CFBr AFM 3 SM 3 310 – 239239. J. He, P.
Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J. Mater. Chem. C 4(27),
6500 (2016). https://doi.org/10.1039/C6TC01287F Cr2CBrOH AFM 3 SM 3 300 –
239239. J. He, P. Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J.
Mater. Chem. C 4(27), 6500 (2016). https://doi.org/10.1039/C6TC01287F Cr2CHCl
AFM 3 SM 3 430 – 239239. J. He, P. Lyu, L. Z. Sun, Á. Morales García, and P.
Nachtigall, J. Mater. Chem. C 4(27), 6500 (2016).
https://doi.org/10.1039/C6TC01287F Cr2CHF AFM 3 SM 3 380 – 239239. J. He, P.
Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J. Mater. Chem. C 4(27),
6500 (2016). https://doi.org/10.1039/C6TC01287F Cr2CClOH AFM 3 SM 3 375 –
239239. J. He, P. Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J.
Mater. Chem. C 4(27), 6500 (2016). https://doi.org/10.1039/C6TC01287F Cr2CFOH
AFM 3 SM 3 390 – 239239. J. He, P. Lyu, L. Z. Sun, Á. Morales García, and P.
Nachtigall, J. Mater. Chem. C 4(27), 6500 (2016).
https://doi.org/10.1039/C6TC01287F Cr2CHOH AFM 3 SM 3 270 – 239239. J. He, P.
Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J. Mater. Chem. C 4(27),
6500 (2016). https://doi.org/10.1039/C6TC01287F Mn2CF2 FM 3 HM 4 520 0.024 1313.
J. He, P. Lyu, and P. Nachtigall, J. Mater. Chem. C 4(47), 11143 (2016).
https://doi.org/10.1039/C6TC03917K Mn2CO2 AFM 3 SM 3 110 0.090 1313. J. He, P.
Lyu, and P. Nachtigall, J. Mater. Chem. C 4(47), 11143 (2016).
https://doi.org/10.1039/C6TC03917K Mn2C(OH)2 FM 3 HM 4 460 0.019 1313. J. He, P.
Lyu, and P. Nachtigall, J. Mater. Chem. C 4(47), 11143 (2016).
https://doi.org/10.1039/C6TC03917K Mn2CCl2 FM 3 HM 4 380 0.037 1313. J. He, P.
Lyu, and P. Nachtigall, J. Mater. Chem. C 4(47), 11143 (2016).
https://doi.org/10.1039/C6TC03917K Mn2CH2 AFM 3 M 3.03 120 0.233 1313. J. He, P.
Lyu, and P. Nachtigall, J. Mater. Chem. C 4(47), 11143 (2016).
https://doi.org/10.1039/C6TC03917K Cr2CH FM 4 HSC 3.83/2.36 – – 241241. X. Ma
and W. Mi, J. Phys. Chem. C 124(5), 3095 (2020).
https://doi.org/10.1021/acs.jpcc.9b10598 Cr2CF FM 4 HSC 3.83/3.15 – – 241241. X.
Ma and W. Mi, J. Phys. Chem. C 124(5), 3095 (2020).
https://doi.org/10.1021/acs.jpcc.9b10598 Cr2CF2 FM 4 HSC 3.11 – – 241241. X. Ma
and W. Mi, J. Phys. Chem. C 124(5), 3095 (2020).
https://doi.org/10.1021/acs.jpcc.9b10598 Cr2CO FM 4 HM 2.88 – – 241241. X. Ma
and W. Mi, J. Phys. Chem. C 124(5), 3095 (2020).
https://doi.org/10.1021/acs.jpcc.9b10598 Cr2CO2 FM 4 BHM 3.72/3.01 – – 241241.
X. Ma and W. Mi, J. Phys. Chem. C 124(5), 3095 (2020).
https://doi.org/10.1021/acs.jpcc.9b10598 Cr2CH2 FM 4 BMS 3.19 – – 241241. X. Ma
and W. Mi, J. Phys. Chem. C 124(5), 3095 (2020).
https://doi.org/10.1021/acs.jpcc.9b10598 M′M″XT TiZrC FM – M 0.57 418 – 243243.
Y. Hu, X. L. Fan, W. J. Guo, Y. R. An, Z. F. Luo, and J. Kong, J. Magn. Magn.
Mater. 486, 165280 (2019). https://doi.org/10.1016/j.jmmm.2019.165280 TiHfC FM –
M 0.52 329 – 243243. Y. Hu, X. L. Fan, W. J. Guo, Y. R. An, Z. F. Luo, and J.
Kong, J. Magn. Magn. Mater. 486, 165280 (2019).
https://doi.org/10.1016/j.jmmm.2019.165280 TiCrC AFM – M 0.2 – – 243243. Y. Hu,
X. L. Fan, W. J. Guo, Y. R. An, Z. F. Luo, and J. Kong, J. Magn. Magn. Mater.
486, 165280 (2019). https://doi.org/10.1016/j.jmmm.2019.165280 Cr2TiC2F2 AFM –
1.35 Cr: 2.59 – – 244244. J. Yang, X. Zhou, X. Luo, S. Zhang, and L. Chen, Appl.
Phys. Lett. 109(20), 203109 (2016). https://doi.org/10.1063/1.4967983
Cr2TiC2(OH)2 AFM – 0.84 Cr: 2.54 – – 244244. J. Yang, X. Zhou, X. Luo, S. Zhang,
and L. Chen, Appl. Phys. Lett. 109(20), 203109 (2016).
https://doi.org/10.1063/1.4967983 Cr2VC2F2 FM – M Cr: 2.49 696 – 244244. J.
Yang, X. Zhou, X. Luo, S. Zhang, and L. Chen, Appl. Phys. Lett. 109(20), 203109
(2016). https://doi.org/10.1063/1.4967983 Cr2VC2O2 FM – M Cr: 1.95 77 – 244244.
J. Yang, X. Zhou, X. Luo, S. Zhang, and L. Chen, Appl. Phys. Lett. 109(20),
203109 (2016). https://doi.org/10.1063/1.4967983 Cr2TiC2(OH)2 FM – M Cr: 2.41
618 – 244244. J. Yang, X. Zhou, X. Luo, S. Zhang, and L. Chen, Appl. Phys. Lett.
109(20), 203109 (2016). https://doi.org/10.1063/1.4967983 Ti2MnC2O2 FM 4/4 SM
0.99 495 – 245245. L. Dong, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy,
J. Phys. Chem. Lett. 8(2), 422 (2017).
https://doi.org/10.1021/acs.jpclett.6b02751 Ti2MnC2(OH)2 FM 4/4 M 1.3 1103 –
245245. L. Dong, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, J. Phys.
Chem. Lett. 8(2), 422 (2017). https://doi.org/10.1021/acs.jpclett.6b02751
Ti2MnC2F2 FM 4/4 M 1.413 109 – 245245. L. Dong, H. Kumar, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, J. Phys. Chem. Lett. 8(2), 422 (2017).
https://doi.org/10.1021/acs.jpclett.6b02751 Hf2MnC2O2 FM 2/4 0.238 1 829 –
245245. L. Dong, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, J. Phys.
Chem. Lett. 8(2), 422 (2017). https://doi.org/10.1021/acs.jpclett.6b02751
Hf2MnC2(OH)2 AFM 2/4 M 1.613 – – 245245. L. Dong, H. Kumar, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, J. Phys. Chem. Lett. 8(2), 422 (2017).
https://doi.org/10.1021/acs.jpclett.6b02751 Hf2MnC2F2 AFM 2/4 1.027 1.67 – –
245245. L. Dong, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, J. Phys.
Chem. Lett. 8(2), 422 (2017). https://doi.org/10.1021/acs.jpclett.6b02751
Hf2VC2O2 FM 2/3 0.055 0.33 1133 – 245245. L. Dong, H. Kumar, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, J. Phys. Chem. Lett. 8(2), 422 (2017).
https://doi.org/10.1021/acs.jpclett.6b02751 Hf2VC2(OH)2 AFM 2/3 M 0.443 – –
245245. L. Dong, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, J. Phys.
Chem. Lett. 8(2), 422 (2017). https://doi.org/10.1021/acs.jpclett.6b02751
Hf2VC2F2 AFM 2/3 M 0.423 – – 245245. L. Dong, H. Kumar, B. Anasori, Y. Gogotsi,
and V. B. Shenoy, J. Phys. Chem. Lett. 8(2), 422 (2017).
https://doi.org/10.1021/acs.jpclett.6b02751 Mo2TiC2Tx AFM 4/4 S – – – 246246. B.
Anasori, C. Shi, E. J. Moon, Y. Xie, C. A. Voigt, P. R. C. Kent, S. J. May, S.
J. L. Billinge, M. W. Barsoum, and Y. Gogotsi, Nanoscale Horiz. 1(3), 227
(2016). https://doi.org/10.1039/C5NH00125K Cr2Ti2C3O2 FM – – 1.983/0.011 721 –
247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L. Chen,
Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Ti2C3(OH)2 AFM – –
2.543/0.043 – – 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L.
Lv, and L. Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Ti2C3F2 AFM – – 2.602/0.036 –
– 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2V2C3O2 FM – – 2.015/0.049 247
– 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2V2C3(OH)2 AFM – – 2.399/0.271
– – 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2V2C3F2 AFM – – 2.498/0.289 –
– 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Nb2C3O2 AFM – – 2.259/0.040 –
– 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Nb2C3(OH)2 AFM – –
2.448/0.159 – – 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L.
Lv, and L. Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Nb2C3F2 AFM – – 2.539/0.172 –
– 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Ta2C3O2 AFM – – 2.285/0.041 –
– 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Ta2C3(OH)2 AFM – –
2.359/0.161 – – 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L.
Lv, and L. Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 Cr2Ta2C3F2 AFM – – 2.457/0.182 –
– 247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y. Xie, L. Lv, and L.
Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 TiV2C2O AFM 0/4 – ∼1.0 – –
249249. W. Sun, Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2C2 AFM 0/4 ∼0.45 ∼3.4 – – 249249. W.
Sun, Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2C2H AFM 0/4 ∼0.8 ∼3 – – 249249. W. Sun,
Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2C2O AFM 0/4 ∼0.9 ∼1.8 – – 249249. W.
Sun, Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2C2F AFM 0/4 ∼1.4 ∼3.3 – – 249249. W.
Sun, Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2C2OH AFM 0/4 ∼0.6 ∼3 – – 249249. W. Sun,
Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiMn2C2 AFM 0/4 – ∼3.75 – – 249249. W. Sun,
Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiMn2C2F FM 0/4 – ∼4 – – 249249. W. Sun, Y.
Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiV2C2H AFM 0/4 – ∼2.1 – – 249249. W. Sun, Y.
Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiV2C2OH AFM 0/4 – ∼2.4 – – 249249. W. Sun,
Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2N2 AFM 0/4 – ∼3.6 – – 249249. W. Sun, Y.
Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2N2O AFM 0/4 – ∼2.1 – – 249249. W. Sun,
Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiCr2N2OH AFM 0/4 – ∼3.5 – – 249249. W. Sun,
Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiMn2N2 AFM 0/4 – ∼4.3 – – 249249. W. Sun, Y.
Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiMn2N2H AFM 0/4 ∼0.2 ∼4.4 – – 249249. W.
Sun, Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C TiMn2N2F AFM 0/4 ∼1.9 ∼4.5 – – 249249. W.
Sun, Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C Cr2TiC2FCl BAFS 3 1.26 – – – 248248. J. He,
G. Ding, C. Zhong, S. Li, D. Li, and G. Zhang, Nanoscale 11(1), 356 (2019).
https://doi.org/10.1039/C8NR07692H Cr2TiC2F2 AFM 3 1.06 – – – 248248. J. He, G.
Ding, C. Zhong, S. Li, D. Li, and G. Zhang, Nanoscale 11(1), 356 (2019).
https://doi.org/10.1039/C8NR07692H Cr2TiC2Cl2 AFM 3 0.91 – – – 248248. J. He, G.
Ding, C. Zhong, S. Li, D. Li, and G. Zhang, Nanoscale 11(1), 356 (2019).
https://doi.org/10.1039/C8NR07692H Cr2TiC2FxCl2-x (x=0.25∼1.75) BAFS 3 – – – –
248248. J. He, G. Ding, C. Zhong, S. Li, D. Li, and G. Zhang, Nanoscale 11(1),
356 (2019). https://doi.org/10.1039/C8NR07692H (Ta2/3Fe1/3)2C AFM – SC 1.82 –
0.86 251251. Q. Gao and H. Zhang, Nanoscale 12(10), 5995 (2020).
https://doi.org/10.1039/C9NR10181K (Zr2/3Fe1/3)2C FM – SC 1.71 268 0.74 251251.
Q. Gao and H. Zhang, Nanoscale 12(10), 5995 (2020).
https://doi.org/10.1039/C9NR10181K (Hf2/3Fe1/3)2C FM – SC 1.79 894 1.39 251251.
Q. Gao and H. Zhang, Nanoscale 12(10), 5995 (2020).
https://doi.org/10.1039/C9NR10181K (Hf2/3Cr1/3)2C FM – SC 1.01 344 0.76 251251.
Q. Gao and H. Zhang, Nanoscale 12(10), 5995 (2020).
https://doi.org/10.1039/C9NR10181K (Ti2/3Hf1/3)2C FM – SC 0.3 190 0.71 251251.
Q. Gao and H. Zhang, Nanoscale 12(10), 5995 (2020).
https://doi.org/10.1039/C9NR10181K MBene MnB FM – M 3.2 345 0.025 215215. Z.
Jiang, P. Wang, X. Jiang, and J. Zhao, Nanoscale Horiz. 3(3), 335 (2018).
https://doi.org/10.1039/C7NH00197E MnBF FM – M 3.24 405 – 215215. Z. Jiang, P.
Wang, X. Jiang, and J. Zhao, Nanoscale Horiz. 3(3), 335 (2018).
https://doi.org/10.1039/C7NH00197E MnBOH FM – M 3.15 600 – 215215. Z. Jiang, P.
Wang, X. Jiang, and J. Zhao, Nanoscale Horiz. 3(3), 335 (2018).
https://doi.org/10.1039/C7NH00197E Ti2B FM – M 0.75 39 0.032 262262. I. Ozdemir,
Y. Kadioglu, O. Ü. Aktürk, Y. Yuksel, Ü. Akıncı, and E. Aktürk, J. Phys.:
Condens. Matter 31(50), 505401 (2019). https://doi.org/10.1088/1361-648X/ab3d1d
In experiments, MXene materials have been achieved by selective etching of the
A-layers (mostly Al) using acid solution in the bulk MAX phases [Fig.
10(a)].213,214213. B. Anasori, M. R. Lukatskaya, and Y. Gogotsi, Nat. Rev.
Mater. 2(2), 16098 (2017). https://doi.org/10.1038/natrevmats.2016.98214. M.
Naguib, V. N. Mochalin, M. W. Barsoum, and Y. Gogotsi, Adv. Mater. 26(7), 992
(2014). https://doi.org/10.1002/adma.201304138 Therefore, the majority of MXenes
are synthesized with mixed surface functional groups. In the functional group
terminated M2X MXene, each transition metal ion is subjected to a new octahedral
or distorted octahedral crystal field, which in turn splits the d orbitals into
the lower-energy t2g states (dxy, dyz, and dxz) and higher-energy eg states
(dx2-y2 and dz2).215,216215. Z. Jiang, P. Wang, X. Jiang, and J. Zhao, Nanoscale
Horiz. 3(3), 335 (2018). https://doi.org/10.1039/C7NH00197E216. D. Khomskii,
Transition Metal Compounds ( Cambridge University Press, Cambridge, 2014).
Moreover, the functional groups have strong affinity with MXene surface and may
serve as chemical dopants. The electron transfer from transition metal ions to
functional groups would directly affect the magnetic configuration of transition
metal ions, which can be interpreted as a competition between localized and
itinerant d states. Generally speaking, the itinerant d electrons in MXene favor
superexchange mechanism, while the localized d orbitals tend to have direct
exchange interaction. In addition to MXenes in M2XTx stoichiometry, thicker
MXene systems of Mn+1XnTx are also available in experiments, where the
dimensionality, n, describes the number of XM6 octahedral layers in Mn+1XnTx.
The stacking of octahedra and the number of occupied d orbitals would depend on
the ratio of M and X atoms. The sensitive correlation between the magnetic
ordering and dimensionality has also been observed.217,218217. Y. Xie and P. R.
C. Kent, Phys. Rev. B 87(23), 235441 (2013).
https://doi.org/10.1103/PhysRevB.87.235441218. N. J. Lane, M. W. Barsoum, and J.
M. Rondinelli, EPL-Europhys. Lett. 101(5), 57004 (2013).
https://doi.org/10.1209/0295-5075/101/57004
FIG. 10. (a) Structures of the MAX and MXene phases. (b) Band structure and
partial density of states of Cr d orbitals for Cr2C MXene, and the Fermi level
is set to zero. (c) Tunable magnetic anisotropy and noncollinear magnetism in
MXenes. (d) Structure of i-MXene (M2/3M'1/3)2X and classification of the
magnetic ground states for 319 kinds of i-MXene systems. (e) On-site magnetic
moments of Mn atom as function of temperature in bare and functionalized MnB
MBene. Panel (a) reproduced with permission from Naguib et al., Adv. Mater. 26,
992 (2014). Copyright 2014 John Wiley and Sons.214214. M. Naguib, V. N.
Mochalin, M. W. Barsoum, and Y. Gogotsi, Adv. Mater. 26(7), 992 (2014).
https://doi.org/10.1002/adma.201304138 Panel (b) reproduced with permission from
Si et al., ACS Appl. Mater. Inter. 7, 17510 (2015). Copyright 2015 American
Chemical Society.136136. C. Si, J. Zhou, and Z. Sun, ACS Appl. Mater. Inter.
7(31), 17510 (2015). https://doi.org/10.1021/acsami.5b05401 Panel (c) reproduced
with permission from Frey et al., ACS Nano 12, 6319 (2018). Copyright 2018
American Chemical Society.238238. N. C. Frey, H. Kumar, B. Anasori, Y. Gogotsi,
and V. B. Shenoy, ACS Nano 12(6), 6319 (2018).
https://doi.org/10.1021/acsnano.8b03472 Panel (d) reproduced with permission
from Gao et al., Nanoscale 12, 5995 (2020). Copyright 2020 Royal Society of
Chemistry.251251. Q. Gao and H. Zhang, Nanoscale 12(10), 5995 (2020).
https://doi.org/10.1039/C9NR10181K Panel (e) reproduced with permission from
Jiang et al., Nanoscale Horiz. 3, 335 (2018). Licensed under a Creative Commons
Attribution (CC-BY-3.0).215215. Z. Jiang, P. Wang, X. Jiang, and J. Zhao,
Nanoscale Horiz. 3(3), 335 (2018). https://doi.org/10.1039/C7NH00197E
* PPT
|
* High-resolution
1. Pristine MXene (Mn+1Xn)
We start from discussing the simplest case—the magnetic properties of pristine
MXenes. Although most MXenes have been synthesized with surface functional
groups, few pristine MXene materials Mn+1Xn are also evidenced by experimental
observations and theoretical calculations. For instance, a recent experiment
confirmed that the surface functional groups F and OH on Ti2C monolayer can be
eliminated by heat treatment at different temperatures.219219. J. Li, Y. Du, C.
Huo, S. Wang, and C. Cui, Ceram. Int. 41(2), 2631 (2015).
https://doi.org/10.1016/j.ceramint.2014.10.070 The Mn2C MXene as the global
minimum structure in two dimensions was confirmed by PSO structure search
combined with first-principles calculations.220220. L. Hu, X. Wu, and J. Yang,
Nanoscale 8(26), 12939 (2016). https://doi.org/10.1039/C6NR02417C Mo2C MXene was
successfully prepared with the traditional CVD method.221221. C. Xu, L. Wang, Z.
Liu, L. Chen, J. Guo, N. Kang, X.-L. Ma, H.-M. Cheng, and W. Ren, Nat. Mater.
14(11), 1135 (2015). https://doi.org/10.1038/nmat4374
Table III summarizes the unterminated MXenes that have been theoretically
predicted to be stable 2D intrinsic magnets, such as Fe2C, Cr2C, Cr2N, Mn2N,
Ru2C, Fe2N, Co2N, Ni2N, Ti2C, Zr2C, Ti2N, Ti3C2, Ti3CN, Cr3C2, Tan+1Cn, Tin+1Cn,
and Tin+1Nn. Si et al.136136. C. Si, J. Zhou, and Z. Sun, ACS Appl. Mater.
Inter. 7(31), 17510 (2015). https://doi.org/10.1021/acsami.5b05401 firstly
pointed out that Cr2C is a half-metallic ferromagnet with a bandgap of 2.85 eV.
The ferromagnetism arises from the itinerant Cr 3d electrons fractionally
occupied in the majority spin channel [Fig. 10(b)]. Similar to Cr2C, Fe2C is
also an itinerant ferromagnet, and Stoner model is able to explain the mechanism
to induce magnetic orderings.9797. Y. Yue, J. Magn. Magn. Mater. 434, 164
(2017). https://doi.org/10.1016/j.jmmm.2017.03.058 The corresponding exchange
interaction parameters are J1= 6.17 meV and J2 = 5.70 meV, which provide further
evidences for the robust ferromagnetic coupling of Fe atoms. Moreover, the
calculated MAE of Fe2C in reciprocal space is –22.8 μeV per unit cell, which has
an easy plane for the magnetization. From the distribution of MAEs, the negative
contributions around the sides of hexagonal Brillouin zone are responsible for
the in-plane magnetization. The Curie temperature within mean-field
approximation was 861 K. First-principles calculations were carried out to
investigate the electronic and magnetic properties of a series of M2C (M = Hf,
Nb, Sc, Ta, Ti, V, Zr) monolayers.222222. S. Zhao, W. Kang, and J. Xue, Appl.
Phys. Lett. 104(13), 133106 (2014). https://doi.org/10.1063/1.4870515 Among
them, Ti2C and Zr2C possesses magnetic moments of 1.92 and 1.25 μB/unit,
respectively. Gao et al. further confirmed that Ti2C exhibits nearly
half-metallicity with a magnetic moment of 0.96 μB/Ti.223223. G. Gao, G. Ding,
J. Li, K. Yao, M. Wu, and M. Qian, Nanoscale 8(16), 8986 (2016).
https://doi.org/10.1039/C6NR01333C Based on the spin-resolved partial density of
states, it is not surprising that the large exchange splitting of Ti 3d
electrons and the strong hybridization of Ti 3d electrons with C 2p electrons
are responsible for the formation of half-metallic magnetism. By considering all
possible spin configurations, Akgenc et al.224224. B. Akgenc, A. Mogulkoc, and
E. Durgun, J. Appl. Phys. 127(8), 084302 (2020).
https://doi.org/10.1063/1.5140578 indicated that Ti2C MXene is antiferromagnetic
metal that is 36 meV/cell lower in energy than FM state. However,
room-temperature half-metallic ferromagnetism was observed in 2H-Ti2C. The
room-temperature half-metallic ferromagnetism was also found in Ti2N MXene, and
the magnetic moments were mainly located at Ti ions with 1.00 μB per formula
unit. Although the individual atom of W, Mo, Ru, Os, Tc, and Re has a magnetic
moment of 4, 6, 2, 4, 5, and 3 μB, respectively, most MXenes with 4d/5d
transition metals are non-magnetic, except that 2H-Ru2C is a FM metal with
magnetic moment of 0.86 μB per Ru.225225. B. Akgenc, Solid State Commun.
303–304, 113739 (2019). https://doi.org/10.1016/j.ssc.2019.113739
Besides the above discussed ferromagnetic MXenes, Mn2C is an antiferromagnetic
metal with magnetic moment of 3 μB per Mn atom.220220. L. Hu, X. Wu, and J.
Yang, Nanoscale 8(26), 12939 (2016). https://doi.org/10.1039/C6NR02417C Both
high Néel temperature (720 K) and appreciable in-plane MAE (25 μeV) are
simultaneously observed in this system. Strong Mn–Mn coupling within the basal
plane is responsible for both AFM ordering and magnetic anisotropy. Cr2N is also
an AFM metal. Each Cr atom has a magnetic moment of 4.45 μB, while each N atom
possesses a magnetic moment of −0.30 μB. These values are quite different from
those of FM Cr2C system.226226. G. Wang, J. Phys. Chem. C 120(33), 18850 (2016).
https://doi.org/10.1021/acs.jpcc.6b05224 Ni2N MXene also prefers AFM ground
state.227227. G. Wang and Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 In bare Ni2N MXene, the direct
Ni–Ni exchange interaction within short distance is strong, which results in
antiferromagnetic coupling between Ni atoms. Similar results have also been
found in Fe2N and Co2N.227227. G. Wang and Y. Liao, Appl. Surf. Sci. 426, 804
(2017). https://doi.org/10.1016/j.apsusc.2017.07.249
To reveal the effect of dimensionality, we have compared the magnetic behavior
of different Mn+1Xn (n > 1) systems. In MXene with the formula of M3X2, there
are three metal atoms per unit cell, i.e., one in the middle (M_m) and two on
the surface (M_s). The distance between M_m and M_s is short, which enhances the
direct interaction between d orbitals and thus induces AFM coupling between
them. Since both M_s atoms couple antiferromagnetically with the M_m atom, these
two M_s atoms must couple ferromagnetically with each other. Additionally, after
insertion of X atoms, the M_m–M_s antiferromagnetic coupling is weakened.
Therefore, Ti3C2, Ti3CN, and Cr3C2 sheets are indeed ferromagnetic metals, and
their magnetism originates from Ti and Cr ions on the surface.228,229228. F. Wu,
K. Luo, C. Huang, W. Wu, P. Meng, Y. Liu, and E. Kan, Solid State Commun. 222, 9
(2015). https://doi.org/10.1016/j.ssc.2015.08.023229. Y. Zhang and F. Li, J.
Magn. Magn. Mater. 433, 222 (2017). https://doi.org/10.1016/j.jmmm.2017.03.031
The possible magnetic ground states of Tin+1Cn and Tin+1Nn (n = 1∼9) were
examined by DFT calculations with PBE functional.217217. Y. Xie and P. R. C.
Kent, Phys. Rev. B 87(23), 235441 (2013).
https://doi.org/10.1103/PhysRevB.87.235441 The results suggested that these
unterminated carbide and nitride MXenes with different thicknesses are all
magnetic. The magnetism still originates from the 3d electrons of Ti atoms on
the surface. However, Tin+1Cn and Tin+1Nn MXene show different magnetic
characteristics. The total magnetic moment of carbides increases from 2 to 3 μB
per f.u. with increasing n, while the total magnetic moment fluctuates with n
around 1.2 μB per f.u. for nitrides. Using first-principles calculations, Lane
et al.218218. N. J. Lane, M. W. Barsoum, and J. M. Rondinelli, EPL-Europhys.
Lett. 101(5), 57004 (2013). https://doi.org/10.1209/0295-5075/101/57004 have
also investigated the effect of dimensionality and electron correlations on the
magnetic ordering in Ta-C (Tan+1Cn, n = 1∼3). With LDA+U formulism, AFM
configurations were predicted for Ta2C and Ta4C3, whereas ferrimagnetism was
predicted for Ta3C2. Without U term, however, their magnetic ground states are
non-magnetic for Ta2C and antiferromagnetic for Ta3C2 and Ta4C3, respectively.
Using the salt-templating method, Xiao et al. recently synthesized ultrathin
Mn3N2 flakes on KCl substrate, which represent the first solution-processed 2D
transition metal nitride with intrinsic antiferromagnetism at room
temperature.230230. X. Xiao, P. Urbankowski, K. Hantanasirisakul, Y. Yang, S.
Sasaki, L. Yang, C. Chen, H. Wang, L. Miao, S. H. Tolbert, S. J. L. Billinge, H.
D. Abruña, S. J. May, and Y. Gogotsi, Adv. Funct. Mater. 29(17), 1809001 (2019).
https://doi.org/10.1002/adfm.201809001
2. Functional terminated MXenes (Mn+1XnTx)
As stated above, surface functional group is a key degree of freedom in MXene,
which are originally introduced during MXene synthesis. Experimentally, when
MXenes are chemically exfoliated by HF acid solutions, the outer layers are
often saturated with F, O, and/or OH groups. According to the distributions of
these functional groups, MXenes can be categorized as symmetrically
functionalized MXenes, asymmetrically functionalized MXene (namely, Janus
MXene), and mixed functionalized MXenes. The symmetrically functionalized MXenes
are terminated by identical groups on both sides of transition metal surfaces,
while the top and bottom transition metals surfaces are terminated by two
alternative functional groups in the asymmetrically functionalized MXenes. In
the mixed functionalized MXenes, the distribution of functional groups is random
and nonuniform. The positions and proportions of functional groups in MXene are
highly dependent on the synthesis route and post-synthesis treatments. It is
necessary to point out that the MXenes produced to date may prefer to have mixed
functional groups of F, OH, and O.231231. M. A. Hope, A. C. Forse, K. J.
Griffith, M. R. Lukatskaya, M. Ghidiu, Y. Gogotsi, and C. P. Grey, Phys. Chem.
Chem. Phys. 18(7), 5099 (2016). https://doi.org/10.1039/C6CP00330C From a
theoretical point of view, Singh et al.232232. P. Srivastava, A. Mishra, H.
Mizuseki, K.-R. Lee, and A. K. Singh, ACS Appl. Mater. Inter. 8(36), 24256
(2016). https://doi.org/10.1021/acsami.6b08413 also found that the mixed
functionalized Ti3C2Fx(OH)1–x (x = 0∼1) MXenes are very close in Gibbs free
energy.
Remarkably, magnetism has already been realized in the functional group
terminated MXene. Yoon et al.233233. Y. Yoon, T. A. Le, A. P. Tiwari, I. Kim, M.
W. Barsoum, and H. Lee, Nanoscale 10(47), 22429 (2018).
https://doi.org/10.1039/C8NR06854B developed a low-temperature solution based
synthetic method to reduce 2D Ti3C2Tx multilayers. The X-ray photoelectron
spectroscopy, electron spin resonance, and magnetization measurements implied
that the reduced Ti3C2Tx is Pauli paramagnetic, which is important experimental
evidence for magnetism in MXene. The presence of Ti3+ ions is the origin of the
electron spin resonance signal. At temperature less than 10 K, a Curie-like
concentration was observed, as indicative of singly occupied states at the Fermi
level.
The correlation between the magnetic properties of MXene and the functional
groups is a subject of increasing concern in the research of 2D MXenes. The
effects of different functional groups on the magnetic ground state of MXenes
have been widely investigated by theoretical simulations, which will be
described in detail below. Similar to the discussions about transition metal
trihalides in Sec. III A 1, the relative strengths of direct, superexchange, and
double exchange interactions can explain the diverse magnetic ground states of
the functionalized MXenes.12,1312. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b0257813. J. He, P. Lyu, and P. Nachtigall, J.
Mater. Chem. C 4(47), 11143 (2016). https://doi.org/10.1039/C6TC03917K
First, it is believed that magnetism can been introduced in the originally
non-magnetic MXene after the presence of functional groups. Zha et al.234234.
X.-H. Zha, J.-C. Ren, L. Feng, X. Bai, K. Luo, Y. Zhang, J. He, Q. Huang, J. S.
Francisco, and S. Du, Nanoscale 10(18), 8763 (2018).
https://doi.org/10.1039/C8NR01292J have investigated the mechanism for
structural conversion from Sc2C(OH)2 to Sc2CO2 MXene. The atomic configurations
and magnetic properties for all the intermediate states were determined. Bipolar
magnetic semiconductors were identified from these rearranged configurations
with inhomogeneous distribution of hydrogen atoms on different sides with x
approximately in the range of 0.188 ≤ x ≤ 0.812. First-principles calculations
predicted a novel ferrimagnetic half-metallic state in 2D F-terminated Mo3N2
with a Curie temperature of 237 K.235235. S.-s. Li, S.-j. Hu, W.-x. Ji, P. Li,
K. Zhang, C.-w. Zhang, and S.-s. Yan, Appl. Phys. Lett. 111(20), 202405 (2017).
https://doi.org/10.1063/1.4993869 Such ferrimagnetic coupling comes mainly from
the interactions of itinerant d electrons between different Mo layers, and thus
endows 100% spin polarization at the Fermi level with a sizable half-metallic
gap of 0.47 eV. Kumar et al.1212. H. Kumar, N. C. Frey, L. Dong, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, ACS Nano 11(8), 7648 (2017).
https://doi.org/10.1021/acsnano.7b02578 carried out a comprehensive theoretical
study on the magnetic properties of twelve nitride MXenes of M2NT2 (M = Ti, V,
Cr, Mn; T = F, OH, O). They identified a new series of Mn2NF2, Mn2NO2, and
Mn2N(OH)2 that exhibit FM half metallic behavior. The total magnetic moments are
9.0, 8.8, and 7.0 μB per f.u. for Mn2NF2, Mn2NO2, and Mn2N(OH)2, respectively.
Both the exchange parameters between intralayer nearest neighbors and interlayer
nearest neighbors are positive, indicating FM coupling. Impressively, the Curie
temperatures from MC simulations for Mn2NF2, Mn2NO2, and Mn2N(OH)2 are as high
as 1877, 1379, and 1745 K, respectively.
Second, theoretical calculations predicted that magnetism would disappear in
certain kinds of MXenes due to the presence of surface termination. In the bare
Tin+1Cn and Tin+1Nn, the magnetism originates mainly from the unpaired electrons
in Ti atoms. Upon functionalization of F, O, OH, and H, the unpaired electron on
each Ti atom would be completely donated to the functional group by forming
ionic bonds. Thus, the magnetically ordered ground states would be destroyed in
the functionalized Tin+1Cn and Tin+1Nn.217217. Y. Xie and P. R. C. Kent, Phys.
Rev. B 87(23), 235441 (2013). https://doi.org/10.1103/PhysRevB.87.235441 Once
two sides of Ti3C2 surfaces are saturated by external groups, a large number of
electronic states distributed around the Fermi level would be removed and the
whole system would become non-magnetic.228228. F. Wu, K. Luo, C. Huang, W. Wu,
P. Meng, Y. Liu, and E. Kan, Solid State Commun. 222, 9 (2015).
https://doi.org/10.1016/j.ssc.2015.08.023 Urbankowski et al.236236. P.
Urbankowski, B. Anasori, T. Makaryan, D. Er, S. Kota, P. L. Walsh, M. Zhao, V.
B. Shenoy, M. W. Barsoum, and Y. Gogotsi, Nanoscale 8(22), 11385 (2016).
https://doi.org/10.1039/C6NR02253G also reported that the magnetic moment of FM
bare Ti4N3 with 7.0 μB per unit cell is reduced to almost zero by OH
termination. One can therefore conclude that the magnetism of Ti atoms in MXene
would be destroyed by –1 valence functional group such as F, Cl, and OH.
Third, an interesting magnetic phase transition could be induced by
functionalization. Compared to pristine FM Cr2C, Cr2C terminated by F, H, OH, or
Cl groups are AFM. The underlying mechanism for such FM-AFM transition is that
the functional groups induce stronger localization behavior on the d electrons
of Cr atoms.136136. C. Si, J. Zhou, and Z. Sun, ACS Appl. Mater. Inter. 7(31),
17510 (2015). https://doi.org/10.1021/acsami.5b05401 Fe2N(OH)2, Fe2NO2, Co2NO2,
Ni2NF2, Ni2N(OH)2, and Ni2NO2 have FM ground state, which are different from the
AFM ground state in bare phase of Fe2N, Co2N, and Ni2N. In those functionalized
MXenes, the nearest interlayer distance of metal atoms increases after O, OH,
and F terminations.226226. G. Wang, J. Phys. Chem. C 120(33), 18850 (2016).
https://doi.org/10.1021/acs.jpcc.6b05224 The ferromagnetic coupling between
metal atoms is stronger than AFM coupling, which is explained by a superexchange
interaction mechanism mediated by the spin polarized N atoms.227227. G. Wang and
Y. Liao, Appl. Surf. Sci. 426, 804 (2017).
https://doi.org/10.1016/j.apsusc.2017.07.249 Although bare Cr2N MXene is an AFM
metal, Cr2NO2 has a ferromagnetic ground state that acts as a half-metal.226226.
G. Wang, J. Phys. Chem. C 120(33), 18850 (2016).
https://doi.org/10.1021/acs.jpcc.6b05224 The electrons with minority spin at the
Fermi level are suppressed by O groups. Similar to Cr2N, Mn2C monolayer also
transforms from AFM to FM state under hydrogenation and oxygenation.237237. X.
Zhang, T. He, W. Meng, L. Jin, Y. Li, X. Dai, and G. Liu, J. Phys. Chem. C
123(26), 16388 (2019). https://doi.org/10.1021/acs.jpcc.9b04445 The magnetic
moments are 3.22 μB per Mn atom under 100% degree of hydrogenation, and 3.10 and
3.06 μB per Mn atom under 75% and 100% oxygenation degrees, respectively, which
are slightly higher than that of bare Mn2C.220220. L. Hu, X. Wu, and J. Yang,
Nanoscale 8(26), 12939 (2016). https://doi.org/10.1039/C6NR02417C The Stoner
criterion can explain the AFM to FM transition in Mn2C well. From MC
simulations, the Curie temperatures are 293 and 323 K for fully hydrogenated and
oxygenated Mn2C, respectively.237237. X. Zhang, T. He, W. Meng, L. Jin, Y. Li,
X. Dai, and G. Liu, J. Phys. Chem. C 123(26), 16388 (2019).
https://doi.org/10.1021/acs.jpcc.9b04445 By modifying the surface termination,
the spin-orbit interaction and bond directionality of M2NTx nitride can be also
manipulated.238238. N. C. Frey, H. Kumar, B. Anasori, Y. Gogotsi, and V. B.
Shenoy, ACS Nano 12(6), 6319 (2018). https://doi.org/10.1021/acsnano.8b03472
These two important factors give rise to a rich diversity of noncollinear spin
structures and finely tunable magnetocrystalline anisotropy [Fig. 10(c)].
Specifically, Ti2NO2 and Mn2NF2 have continuous O(3) and O(2) spin symmetries,
respectively, while Cr2NO2 and Mn2NO2 are intrinsic Ising ferromagnets with
out-of-plane easy axes and magnetic anisotropy energies up to 63 μeV/atom. The
magnetic properties of Mn2CT2 (T = F, Cl, OH, O, H) have been computationally
investigated.1313. J. He, P. Lyu, and P. Nachtigall, J. Mater. Chem. C 4(47),
11143 (2016). https://doi.org/10.1039/C6TC03917K Depending on the
electronegativity of functional groups, the AFM Mn2C change to FM ground state
upon functionalization of F, Cl, or OH groups. They are intrinsic half metals
with high Curie temperature (280∼520 K) and sizable magnetic anisotropy (MAE =
24∼38 μeV). Theoretical studies of the asymmetrically functionalized MXenes have
predicted that Janus Cr2C behaves as a bipolar antiferromagnetic semiconductor
with zero magnetization and high Néel temperatures (270∼430 K).239239. J. He, P.
Lyu, L. Z. Sun, Á. Morales García, and P. Nachtigall, J. Mater. Chem. C 4(27),
6500 (2016). https://doi.org/10.1039/C6TC01287F With appropriate choice of
surface functional group pairs (H, F, Cl, Br, and OH), one can tailor the
bandgap of Cr2C from 0.15 to 1.51 eV. The itinerant d electrons in Cr2C are
favorable to Cr1↑-C↓-Cr2↑ superexchange mechanism, while the localized CrT′↑ d
orbitals can directly interact with the CrT″↓ one in Cr2CT′T″. In other words,
the distinct characteristics of d electrons in Cr2C and Cr2CT′T″ induce FM and
AFM ordering, respectively.239239. J. He, P. Lyu, L. Z. Sun, Á. Morales García,
and P. Nachtigall, J. Mater. Chem. C 4(27), 6500 (2016).
https://doi.org/10.1039/C6TC01287F
In addition, magnetism can be retained by precisely controlling the surface
functional groups on MXenes. For example, the fully covered functional groups of
O2– and H+ keep the magnetic properties of bare Mn2C, showing AFM semiconductor
and AFM metal, respectively.1313. J. He, P. Lyu, and P. Nachtigall, J. Mater.
Chem. C 4(47), 11143 (2016). https://doi.org/10.1039/C6TC03917K The stable
magnetic configurations of both V2C and V2C derivatives, i.e., V2CF2 and
V2C(OH)2, are antiferromagnetic coupling.240240. J. Hu, B. Xu, C. Ouyang, S. A.
Yang, and Y. Yao, J. Phys. Chem. C 118(42), 24274 (2014).
https://doi.org/10.1021/jp507336x Moreover, metal-semiconductor transition
behavior upon functionalization was also observed. Two-dimensional Fe2N, Co2N,
and Ni2N as well as their surface passivated structures were investigated using
DFT calculations. All the bare MXenes and functionalized systems, including
Fe2NF2, Co2NF2, and Co2NF2, prefer AFM state. For Cr2N, the bare system is an
antiferromagnetic metal, and passivation of F atoms or OH groups would not
change the antiferromagnetic characteristics.226226. G. Wang, J. Phys. Chem. C
120(33), 18850 (2016). https://doi.org/10.1021/acs.jpcc.6b05224 If only one side
is saturated, long-range FM ordering can be retained in F and H modified Ti3C2
monolayers. However, the strength of spin-spin coupling is weakened after
chemical modification, in comparison with that of pristine Ti3C2 monolayer. The
simulated Curie temperatures of Ti3C2, Ti3CN, and HTi3C2 were about 300, 350,
and 1000 K, respectively.228228. F. Wu, K. Luo, C. Huang, W. Wu, P. Meng, Y.
Liu, and E. Kan, Solid State Commun. 222, 9 (2015).
https://doi.org/10.1016/j.ssc.2015.08.023 CrC2 with single-side and two-side
functionalization (H, O, F) have also been investigated by first-principles
calculations. The CrC2 monolayer functionalized with O atoms on both sides shows
bipolar half-metallic characteristics, while it becomes a half-metal with O
atoms terminated on one side only. CrC2 monolayer with H/F at one side and F at
two sides are half semiconductors, while it is a bipolar magnetic semiconductor
(BMS) after being functionalized with H atoms on both sides.241241. X. Ma and W.
Mi, J. Phys. Chem. C 124(5), 3095 (2020).
https://doi.org/10.1021/acs.jpcc.9b10598
3. Double transition metal MXenes [(MM′)n+1XnTx]
The MXenes of double transition metal carbides are also synthesized and
predicted to be robust magnetic semiconductor or metal, making them desirable in
spintronics.242242. B. Anasori, Y. Xie, M. Beidaghi, J. Lu, B. C. Hosler, L.
Hultman, P. R. C. Kent, Y. Gogotsi, and M. W. Barsoum, ACS Nano 9(10), 9507
(2015). https://doi.org/10.1021/acsnano.5b03591 Hu et al.243243. Y. Hu, X. L.
Fan, W. J. Guo, Y. R. An, Z. F. Luo, and J. Kong, J. Magn. Magn. Mater. 486,
165280 (2019). https://doi.org/10.1016/j.jmmm.2019.165280 investigated the
magnetic properties of ordered double-metal MXenes MM'C for M being Ti and M'
being the other transition metal elements. They found that TiZrC and TiHfC are
FM metals with TC = 418 and 329 K, respectively, while TiCrC is an AFM metal.
The magnetic moments of TiZrC and TiHfC mainly come from the Ti atoms, and the
magnetic moments of Ti atoms in TiZrC (0.57 μB) and TiHrC (0.52 μB) are much
less than the value of Ti atoms in Ti2C (0.97 μB).243243. Y. Hu, X. L. Fan, W.
J. Guo, Y. R. An, Z. F. Luo, and J. Kong, J. Magn. Magn. Mater. 486, 165280
(2019). https://doi.org/10.1016/j.jmmm.2019.165280 The magnetic properties of
Cr2M′C2T2 (M' = Ti, V; T = O, OH, F) systems have been investigated by
first-principles calculations.244244. J. Yang, X. Zhou, X. Luo, S. Zhang, and L.
Chen, Appl. Phys. Lett. 109(20), 203109 (2016).
https://doi.org/10.1063/1.4967983 Cr2TiC2F2 and Cr2TiC2(OH)2 were predicted to
be antiferromagnetic, while Cr2VC2(OH)2, Cr2VC2F2, and Cr2VC2O2 were
ferromagnets with Curie temperatures of 618, 77, and 695 K, respectively. Among
Hf2MnC2Tx and Hf2MnC2Tx systems,245245. L. Dong, H. Kumar, B. Anasori, Y.
Gogotsi, and V. B. Shenoy, J. Phys. Chem. Lett. 8(2), 422 (2017).
https://doi.org/10.1021/acs.jpclett.6b02751 only Hf2MnC2O2 and Hf2MnC2O2 are
ferromagnetic semiconductors, while the ground states of the rest of OH and F
terminated ones are antiferromagnetic. More interestingly, the Curie
temperatures of four reported MXenes, i.e., Ti2MnC2O2, Ti2MnC2(OH)2, Hf2MnC2O2,
and Hf2VC2O2, are in the range from 495 to 1133 K, which are much higher than
room temperature. For the experimentally realized Mo2TiC2Tx, F and OH
terminations are shown to lead to antiferromagnetic semiconductors.246246. B.
Anasori, C. Shi, E. J. Moon, Y. Xie, C. A. Voigt, P. R. C. Kent, S. J. May, S.
J. L. Billinge, M. W. Barsoum, and Y. Gogotsi, Nanoscale Horiz. 1(3), 227
(2016). https://doi.org/10.1039/C5NH00125K The magnetism of Mo2TiC2Tx originates
from the unpaired Mo 3d orbitals that locate in the outer layer. The magnetic
properties of Cr2M2C3T2 (M = Ti, V, Nb, Ta; T = OH, O, F) were investigated
using DFT calculations.247247. J. Yang, X. Luo, X. Zhou, S. Zhang, J. Liu, Y.
Xie, L. Lv, and L. Chen, Comput. Mater. Sci. 139, 313 (2017).
https://doi.org/10.1016/j.commatsci.2017.08.016 It was shown that ferromagnetic
ordering is energetically more favorable for Cr2Ti2C3O2 and Cr2V2C3O2, while the
magnetic ground states of the rest of Cr2M2C3T2 systems prefer AFM ordering. The
Curie temperatures of FM Cr2Ti2C3O2 and Cr2V2C3O2 are 720 and 246 K,
respectively. For the asymmetrically functionalized double MXene Cr2TiC2FCl,
theoretical study by Sun et al. found that it behaves as bipolar
antiferromagnetic semiconductors (BAFS) with opposite spin character in the
conduction band minimum and valence band maximum.248248. J. He, G. Ding, C.
Zhong, S. Li, D. Li, and G. Zhang, Nanoscale 11(1), 356 (2019).
https://doi.org/10.1039/C8NR07692H The different chemical environments induce a
mismatch of d states for the Cr atoms in the upper and lower surfaces, thereby
resulting in the BAFS feature. Moreover, the mixed functionalized double MXenes
remain as a BAFS. Based on the experimental synthesis, Sun et al.249249. W. Sun,
Y. Xie, and P. R. C. Kent, Nanoscale 10(25), 11962 (2018).
https://doi.org/10.1039/C8NR00513C focused on the group of Ti-centered double
transition metal TiM2X2T (M = V, Cr, Mn; X = C, N; T = H, F, O, OH). After
screening various combinations of metal elements and terminating groups, only
TiMn2C2F showed FM ordering, whereas AFM state is energetically more favorable
in all other systems.
Motivated by the synthesis of in-plane ordered MAXs, i.e., (M2/3M1/3)2AX,250250.
Q. Tao, J. Lu, M. Dahlqvist, A. Mockute, S. Calder, A. Petruhins, R. Meshkian,
O. Rivin, D. Potashnikov, E. a. N. Caspi, H. Shaked, A. Hoser, C. Opagiste,
R.-M. Galera, R. Salikhov, U. Wiedwald, C. Ritter, A. R. Wildes, B. Johansson,
L. Hultman, M. Farle, M. W. Barsoum, and J. Rosen, Chem. Mater. 31(7), 2476
(2019). https://doi.org/10.1021/acs.chemmater.8b05298 the magnetic properties of
319 kinds of (M2/3M′1/3)2X MXene were investigated by high-throughput DFT
calculations [Fig. 10(d)], from which 40 FM compounds and 26 AFM compounds were
found.251251. Q. Gao and H. Zhang, Nanoscale 12(10), 5995 (2020).
https://doi.org/10.1039/C9NR10181K Among these magnetic systems, there are five
MXenes with out-of-plane MAE larger than 0.5 meV per f.u. Furthermore, the
predicted TC of (Zr2/3Fe1/3)2C and (Hf2/3Fe1/3)2C are higher than room
temperature.
Although the method to precisely control the surface functional group species
and/or double transition metal MXene is yet to be discovered, one can see that
the magnetism in a few MXenes, such as Ti2MnC2T2245245. L. Dong, H. Kumar, B.
Anasori, Y. Gogotsi, and V. B. Shenoy, J. Phys. Chem. Lett. 8(2), 422 (2017).
https://doi.org/10.1021/acs.jpclett.6b02751 and Mn2NTx,252252. N. C. Frey, A.
Bandyopadhyay, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano
13(3), 2831 (2019). https://doi.org/10.1021/acsnano.8b09201 is not sensitive to
the nature of surface terminations. Using first-principles and Monte Carlo
calculations, Frey et al.252252. N. C. Frey, A. Bandyopadhyay, H. Kumar, B.
Anasori, Y. Gogotsi, and V. B. Shenoy, ACS Nano 13(3), 2831 (2019).
https://doi.org/10.1021/acsnano.8b09201 have studied the effects of mixed
termination and characterized a wide variety of magnetic and transport behavior
in Janus M2X. Janus Mn2N systems were found to be robust ferromagnets regardless
of surface termination structures and compositions. By analyzing the electron
filling in transition metal cations and performing DFT calculations, Dong et
al.245245. L. Dong, H. Kumar, B. Anasori, Y. Gogotsi, and V. B. Shenoy, J. Phys.
Chem. Lett. 8(2), 422 (2017). https://doi.org/10.1021/acs.jpclett.6b02751
designed a series of 2D magnetic materials based on ordered double transition
metal MXenes. They revealed that Ti2MnC2Tx are ferromagnetic metals or
semimetals, regardless of their surface termination of O, OH, or F. In short,
the high Curie temperature and robust magnetism make these MXenes very
attractive for experimental realization of 2D magnets.
4. MBene
MAB phases, as boron analog of MAX phases, are formed by stacked M–B blocks and
interleaved A atomic planes.253–256253. W. Jeitschko, Monatshefte für Chemie und
verwandte Teile anderer Wissenschaften 97(5), 1472 (1966).
https://doi.org/10.1007/BF00902599254. W. Jeitschko, Acta Crystallograph. Sec. B
25(1), 163 (1969). https://doi.org/10.1107/S0567740869001944255. M. Ade and H.
Hillebrecht, Inorg. Chem. 54(13), 6122 (2015).
https://doi.org/10.1021/acs.inorgchem.5b00049256. Y. Liu, Z. Jiang, X. Jiang,
and J. Zhao, RSC Adv. 10(43), 25836 (2020). https://doi.org/10.1039/D0RA04385K
Recently, boride analogues of MXene termed as MBene were predicted
theoretically215,257215. Z. Jiang, P. Wang, X. Jiang, and J. Zhao, Nanoscale
Horiz. 3(3), 335 (2018). https://doi.org/10.1039/C7NH00197E257. Z. Guo, J. Zhou,
and Z. Sun, J. Mater. Chem. A 5(45), 23530 (2017).
https://doi.org/10.1039/C7TA08665B and were soon confirmed in experiment by
topochemical deintercalation of Al atoms from MAB structures (M2A2B2 and
M2AB2),258–261258. L. T. Alameda, P. Moradifar, Z. P. Metzger, N. Alem, and R.
E. Schaak, J. Am. Chem. Soc. 140(28), 8833 (2018).
https://doi.org/10.1021/jacs.8b04705259. H. Zhang, F.-Z. Dai, H. Xiang, X. Wang,
Z. Zhang, and Y. Zhou, J. Mater. Sci. Technol. 35(8), 1593 (2019).
https://doi.org/10.1016/j.jmst.2019.03.031260. L. T. Alameda, R. W. Lord, J. A.
Barr, P. Moradifar, Z. P. Metzger, B. C. Steimle, C. F. Holder, N. Alem, S. B.
Sinnott, and R. E. Schaak, J. Am. Chem. Soc. 141(27), 10852 (2019).
https://doi.org/10.1021/jacs.9b04726261. J. Wang, T.-N. Ye, Y. Gong, J. Wu, N.
Miao, T. Tada, and H. Hosono, Nat. Commun. 10(1), 2284 (2019).
https://doi.org/10.1038/s41467-019-10297-8 including 2D sheets of MoB, CrB, FeB,
and TiB. Since the number of valence electrons of boron is one/two less than
carbon/nitrogen, its electron deficiency and lower electronegativity would endow
MBene with distinctly different magnetic performance from the conventional
carbide or nitride based MXenes. For example, using DFT-based high-throughput
search, Jiang et al.215215. Z. Jiang, P. Wang, X. Jiang, and J. Zhao, Nanoscale
Horiz. 3(3), 335 (2018). https://doi.org/10.1039/C7NH00197E identified 12 stable
MBene nanosheets that are feasible to synthesize. Among them, 2D MnB MBene
exhibits robust metallic ferromagnetism with 3.2 μB per Mn atom and a high Curie
temperature of 345 K. After functionalization with F and OH groups, the
ferromagnetic ground state of 2D MnB is well preserved. More excitingly, the
Curie temperatures are even elevated to 405 K (with F groups) and 600 K (with OH
groups), respectively, suggesting that careful choice of functional groups might
be beneficial to the increase of TC in MBene [Fig. 10(e)]. Similar to MnB MBene,
the electronic and magnetic properties of Ti2B monolayer were also investigated
by DFT calculations. Its FM spin configuration corresponds to the magnetic
ground state, and the predicted Curie temperature of is TC = 39 K based on
Heisenberg model.262262. I. Ozdemir, Y. Kadioglu, O. Ü. Aktürk, Y. Yuksel, Ü.
Akıncı, and E. Aktürk, J. Phys.: Condens. Matter 31(50), 505401 (2019).
https://doi.org/10.1088/1361-648X/ab3d1d
D. Other binary transition metal compounds
The itinerant and localized behavior of the d electrons in transition metals and
the coupling between them are still the starting point of designing the 2D
magnetic materials for spintronics using other binary transition metal
compounds, such as carbides, nitrides, oxides, borides, phosphides, silicides,
arsenides, and hydrides. The local environments of the transition metal ions,
including symmetry, bonding types, and orbital hybridizations, are identified as
the key factors to understand the details of crystal field splitting. Therefore,
it is essential to establish a relationship between the local environment and
the spin-polarized orbital filling of the central transition metal ion. Most
reported binary transition metal compounds are listed in Table IV and
categorized by their compositions as well as magnetic properties for
discussions.
TABLE IV. A list of 2D magnets in binary transition metal compounds with their
compositions and key electronic and magnetic properties, including the magnetic
ground state (GS), the values of Hubbard U, energy gap (Eg), magnetic moment on
per transition metal (Ms), Curie temperature (TC), and magnetic anisotropy
energy per unit cell (MAE).
Compositions GS U (eV) Eg (eV) Ms (μB) TC (K) MAE (meV) Ref. Nitrides CrN FM 3
HM 3.19 675 – 263263. S. Zhang, Y. Li, T. Zhao, and Q. Wang, Sci. Rep. 4(1),
5241 (2014). https://doi.org/10.1038/srep05241 h-CrN FM 3 HM 3 – – 264264. A. V.
Kuklin, A. A. Kuzubov, E. A. Kovaleva, N. S. Mikhaleva, F. N. Tomilin, H. Lee,
and P. V. Avramov, Nanoscale 9(2), 621 (2017).
https://doi.org/10.1039/C6NR07790K h-MnN FM 5.5 HM 4 368 –0.134 266266. Z. Xu
and H. Zhu, J. Phys. Chem. C 122(26), 14918 (2018).
https://doi.org/10.1021/acs.jpcc.8b02323 h-VN FM – HM 1.46 768 –0.1 265265. A.
V. Kuklin, S. A. Shostak, and A. A. Kuzubov, J. Phys. Chem. Lett. 9(6), 1422
(2018). https://doi.org/10.1021/acs.jpclett.7b03276 t-VN FM – M 2.1 278 –0.021
265265. A. V. Kuklin, S. A. Shostak, and A. A. Kuzubov, J. Phys. Chem. Lett.
9(6), 1422 (2018). https://doi.org/10.1021/acs.jpclett.7b03276 Mn2N FM 4 HM 3.6
913 – 270270. K. Zhao and Q. Wang, Appl. Surf. Sci. 505, 144620 (2020).
https://doi.org/10.1016/j.apsusc.2019.144620 Carbides α-CoC AFM – 1.22 1.4 –
–0.545 267267. C. Zhu, H. Lv, X. Qu, M. Zhang, J. Wang, S. Wen, Q. Li, Y. Geng,
Z. Su, X. Wu, Y. Li, and Y. Ma, J. Mater. Chem. C 7(21), 6406 (2019).
https://doi.org/10.1039/C9TC00635D β-CoC AFM – M 1.61 – –0.216 267267. C. Zhu,
H. Lv, X. Qu, M. Zhang, J. Wang, S. Wen, Q. Li, Y. Geng, Z. Su, X. Wu, Y. Li,
and Y. Ma, J. Mater. Chem. C 7(21), 6406 (2019).
https://doi.org/10.1039/C9TC00635D α-NiC FM – M 0.43 – –0.166 267267. C. Zhu, H.
Lv, X. Qu, M. Zhang, J. Wang, S. Wen, Q. Li, Y. Geng, Z. Su, X. Wu, Y. Li, and
Y. Ma, J. Mater. Chem. C 7(21), 6406 (2019). https://doi.org/10.1039/C9TC00635D
β-NiC FM – M 0.27 – –0.107 267267. C. Zhu, H. Lv, X. Qu, M. Zhang, J. Wang, S.
Wen, Q. Li, Y. Geng, Z. Su, X. Wu, Y. Li, and Y. Ma, J. Mater. Chem. C 7(21),
6406 (2019). https://doi.org/10.1039/C9TC00635D FeC2 FM 5 HM 4 245 –0.98 268268.
T. Zhao, J. Zhou, Q. Wang, Y. Kawazoe, and P. Jena, ACS Appl. Mater. Inter.
8(39), 26207 (2016). https://doi.org/10.1021/acsami.6b07482 CrC2 FM 4 HM 8.0 – –
269269. B. Zhou, X. Wang, and W. Mi, J. Mater. Chem. C 6(15), 4290 (2018).
https://doi.org/10.1039/C7TC05383E VC2 FM 4 M 2.21 – – 269269. B. Zhou, X. Wang,
and W. Mi, J. Mater. Chem. C 6(15), 4290 (2018).
https://doi.org/10.1039/C7TC05383E MnC2 FM 4 M 6.81 – – 269269. B. Zhou, X.
Wang, and W. Mi, J. Mater. Chem. C 6(15), 4290 (2018).
https://doi.org/10.1039/C7TC05383E FeC2 FM 4 1.62 8 – – 269269. B. Zhou, X.
Wang, and W. Mi, J. Mater. Chem. C 6(15), 4290 (2018).
https://doi.org/10.1039/C7TC05383E CoC2 FM 4 SC 6.03 – – 269269. B. Zhou, X.
Wang, and W. Mi, J. Mater. Chem. C 6(15), 4290 (2018).
https://doi.org/10.1039/C7TC05383E NiC2 FM 4 M 2.85 – – 269269. B. Zhou, X.
Wang, and W. Mi, J. Mater. Chem. C 6(15), 4290 (2018).
https://doi.org/10.1039/C7TC05383E Borides t-MnB FM 3.32 M 2.65 406 0.218
271271. Y. Z. Abdullahi, Z. D. Vatansever, E. Aktürk, Ü. Akıncı, and O. Ü.
Aktürk, Phys. Chem. Chem. Phys. 22, 10893 (2020).
https://doi.org/10.1039/D0CP00503G CoB6 FM 3.5/6.0 D 1.377/1.382 – – 7878. X.
Tang, W. Sun, Y. Gu, C. Lu, L. Kou, and C. Chen, Phys. Rev. B 99(4), 045445
(2019). https://doi.org/10.1103/PhysRevB.99.045445 Oxides t-VO AFM – SC 2.27 – –
277277. H. van Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van
Huis, npj 2D Mater. Appl. 3(1), 18 (2019).
https://doi.org/10.1038/s41699-019-0100-z t-CrO AFM – SC 3.36 – – 277277. H. van
Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj 2D
Mater. Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z t-MnO
AFM – SC 4.42 – – 277277. H. van Gog, W.-F. Li, C. Fang, R. S. Koster, M.
Dijkstra, and M. van Huis, npj 2D Mater. Appl. 3(1), 18 (2019).
https://doi.org/10.1038/s41699-019-0100-z Sq-TiO AFM – M 1.22 – – 277277. H. van
Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj 2D
Mater. Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z Sq-VO
AFM – SC 3.66 – – 277277. H. van Gog, W.-F. Li, C. Fang, R. S. Koster, M.
Dijkstra, and M. van Huis, npj 2D Mater. Appl. 3(1), 18 (2019).
https://doi.org/10.1038/s41699-019-0100-z Sq-MnO AFM – SM 4.64 – – 277277. H.
van Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj 2D
Mater. Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z h-V2O3
FM – HM 2.1 – – 277277. H. van Gog, W.-F. Li, C. Fang, R. S. Koster, M.
Dijkstra, and M. van Huis, npj 2D Mater. Appl. 3(1), 18 (2019).
https://doi.org/10.1038/s41699-019-0100-z h-Mn2O3 FiM – SC 4.55–2.69 – – 277277.
H. van Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj
2D Mater. Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z RuO2
FM 1.5 – 1.60 38 –3.09 278278. Y. Wang, F. Li, H. Zheng, X. Han, and Y. Yan,
Phys. Chem. Chem. Phys. 20(44), 28162 (2018). https://doi.org/10.1039/C8CP05467C
OsO2 FM 0.5 – 1.34 197 –42.67 278278. Y. Wang, F. Li, H. Zheng, X. Han, and Y.
Yan, Phys. Chem. Chem. Phys. 20(44), 28162 (2018).
https://doi.org/10.1039/C8CP05467C MnO2 FM 3.9 3.41 3 140 – 273273. M. Kan, J.
Zhou, Q. Sun, Y. Kawazoe, and P. Jena, J. Phys. Chem. Lett. 4(20), 3382 (2013).
https://doi.org/10.1021/jz4017848 CrO2 FM 3.5 M 2.75 219 – 277277. H. van Gog,
W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj 2D Mater.
Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z MnO2 AFM 4 1.1
3.015 256 – 277277. H. van Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra,
and M. van Huis, npj 2D Mater. Appl. 3(1), 18 (2019).
https://doi.org/10.1038/s41699-019-0100-z TcO2 FM – 0.6 2.79 170 – 277277. H.
van Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj 2D
Mater. Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z FeO2 AFM
4 M 1.823 108 – 277277. H. van Gog, W.-F. Li, C. Fang, R. S. Koster, M.
Dijkstra, and M. van Huis, npj 2D Mater. Appl. 3(1), 18 (2019).
https://doi.org/10.1038/s41699-019-0100-z CoO2 FM 4.5 M 0.743 60 – 277277. H.
van Gog, W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj 2D
Mater. Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z Fe3O4
AFM 4.2 2.4 3.7∼4.1 – – 279279. P. A. T. Olsson, L. R. Merte, and H. Grönbeck,
Phys. Rev. B 101(15), 155426 (2020). https://doi.org/10.1103/PhysRevB.101.155426
Hydrides ScH2 FM 1 M 0.59 339 0.3 283283. Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and
X. C. Zeng, J. Phys. Chem. Lett. 9(15), 4260 (2018).
https://doi.org/10.1021/acs.jpclett.8b01976 TiH2 AFM 2.5 1.20 1.24 – – 283283.
Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J. Phys. Chem. Lett. 9(15),
4260 (2018). https://doi.org/10.1021/acs.jpclett.8b01976 VH2 AFM 2 2.43 2.40 – –
283283. Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J. Phys. Chem. Lett.
9(15), 4260 (2018). https://doi.org/10.1021/acs.jpclett.8b01976 CrH2 AFM 2.5
1.57 3.52 – – 283283. Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J.
Phys. Chem. Lett. 9(15), 4260 (2018).
https://doi.org/10.1021/acs.jpclett.8b01976 FeH2 AFM 2.5 0.09 3.02 – – 283283.
Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J. Phys. Chem. Lett. 9(15),
4260 (2018). https://doi.org/10.1021/acs.jpclett.8b01976 CoH2 FM 3 M 1.19 160
0.014 283283. Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J. Phys. Chem.
Lett. 9(15), 4260 (2018). https://doi.org/10.1021/acs.jpclett.8b01976 NiH2 AFM 4
1.90 1.17 – – 283283. Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J.
Phys. Chem. Lett. 9(15), 4260 (2018).
https://doi.org/10.1021/acs.jpclett.8b01976 Silicide Fe2Si FM 3.5 HM 3.037 780
0.325 280280. Y. Sun, Z. Zhuo, X. Wu, and J. Yang, Nano Lett. 17(5), 2771
(2017). https://doi.org/10.1021/acs.nanolett.6b04884 TiSi2 FM – M 0.563 – –
281281. N. Han, H. Liu, and J. Zhao, J. Supercond. Nov. Magn. 28(6), 1755
(2015). https://doi.org/10.1007/s10948-014-2940-2 VSi2 FM – M 2.148 – – 281281.
N. Han, H. Liu, and J. Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 CrSi2 FM – M 3.008 – – 281281. N. Han,
H. Liu, and J. Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 MnSi2 AFM – M 2.512 – – 281281. N.
Han, H. Liu, and J. Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 FeSi2 AFM – M 1.508 – – 281281. N.
Han, H. Liu, and J. Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 NbSi2 FM – M 0.618 – – 281281. N. Han,
H. Liu, and J. Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 MoSi2 FM – M 0.207 – – 281281. N. Han,
H. Liu, and J. Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 Ti2Si FM 1,2 M 0.685 – – 282282. Q.
Wu, J.-J. Zhang, P. Hao, Z. Ji, S. Dong, C. Ling, Q. Chen, and J. Wang, J. Phys.
Chem. Lett. 7(19), 3723 (2016). https://doi.org/10.1021/acs.jpclett.6b01731
Phosphide MnP FM 4 HM 4 495 0.166 7575. B. Wang, Y. Zhang, L. Ma, Q. Wu, Y. Guo,
X. Zhang, and J. Wang, Nanoscale 11(10), 4204 (2019).
https://doi.org/10.1039/C8NR09734H Fe2P AFM – M 1.6 23 –0.055 55. Q. Liu, J.
Xing, Z. Jiang, X. Jiang, Y. Wang, and J. Zhao, Nanoscale 12(12), 6776 (2020).
https://doi.org/10.1039/D0NR00092B Co2P FM – M 0.68 580 0.04 55. Q. Liu, J.
Xing, Z. Jiang, X. Jiang, Y. Wang, and J. Zhao, Nanoscale 12(12), 6776 (2020).
https://doi.org/10.1039/D0NR00092B Fe3P FM 2 M 2.55 420 0.356 7676. S. Zheng, C.
Huang, T. Yu, M. Xu, S. Zhang, H. Xu, Y. Liu, E. Kan, Y. Wang, and G. Yang, J.
Phys. Chem. Lett. 10(11), 2733 (2019).
https://doi.org/10.1021/acs.jpclett.9b00970 Arsenides MnAs FM 4 HM 4 711 0.281
7575. B. Wang, Y. Zhang, L. Ma, Q. Wu, Y. Guo, X. Zhang, and J. Wang, Nanoscale
11(10), 4204 (2019). https://doi.org/10.1039/C8NR09734H FeAs-I FM 4 M 3.07 645
0.645 7373. Y. Jiao, W. Wu, F. Ma, Z.-M. Yu, Y. Lu, X.-L. Sheng, Y. Zhang, and
S. A. Yang, Nanoscale 11(35), 16508 (2019). https://doi.org/10.1039/C9NR04338A
FeAs-II FM 4 M 3.02 170 – 7373. Y. Jiao, W. Wu, F. Ma, Z.-M. Yu, Y. Lu, X.-L.
Sheng, Y. Zhang, and S. A. Yang, Nanoscale 11(35), 16508 (2019).
https://doi.org/10.1039/C9NR04338A FeAs-III AFM 4 0.27 3.47 350 0.820 7373. Y.
Jiao, W. Wu, F. Ma, Z.-M. Yu, Y. Lu, X.-L. Sheng, Y. Zhang, and S. A. Yang,
Nanoscale 11(35), 16508 (2019). https://doi.org/10.1039/C9NR04338A
1. Transition metal carbides/nitrides
The magnetic properties of many transition metal carbides/nitrides have already
been described as the 2D MXene family in Sec. III C. However, they are not
limited to MXenes, which can be regarded as the transition metal rich compounds.
In contrast, the 2D transition metal carbides/nitrides with fewer metal atoms,
such as MN2, MN, MC2, and MC monolayers, show higher chemical stability.
Compared to MXenes, these monolayers of transition metal carbides/nitrides have
fewer exposed metal sites. As a result, the effect of surface modification might
be avoided in them, which is in principle more favorable than MXenes for
practical spintronic applications.
For the transition metal nitrides with stoichiometric ratio of 1:1, a monolayer
structure obtained from (100) surface of rocksalt-structured CrN crystal was
predicted to be a ferromagnet using PSO technique.263263. S. Zhang, Y. Li, T.
Zhao, and Q. Wang, Sci. Rep. 4(1), 5241 (2014).
https://doi.org/10.1038/srep05241 Analyses of its band structure and DOS
revealed that this material is a half-metal, and the origin of ferromagnetism
was ascribed to the p-d exchange interaction between Cr and N atoms. The
corresponding Curie temperature was about 675 K. Hexagonal CrN,264264. A. V.
Kuklin, A. A. Kuzubov, E. A. Kovaleva, N. S. Mikhaleva, F. N. Tomilin, H. Lee,
and P. V. Avramov, Nanoscale 9(2), 621 (2017).
https://doi.org/10.1039/C6NR07790K VN,265265. A. V. Kuklin, S. A. Shostak, and
A. A. Kuzubov, J. Phys. Chem. Lett. 9(6), 1422 (2018).
https://doi.org/10.1021/acs.jpclett.7b03276 and MnN266266. Z. Xu and H. Zhu, J.
Phys. Chem. C 122(26), 14918 (2018). https://doi.org/10.1021/acs.jpcc.8b02323
monolayers were also identified as intrinsic half-metallic ferromagnets. In
their flat atomically thin hexagonal lattices, the coordinate number of the
transition metal ions is three. Hence, the d orbital diagram is a typical
example of crystalline orbitals of trigonal-type complexes in terms of crystal
field theory, which is quite different from those of the octahedral Oh and C3v
crystal fields. In these transition metal mononitrides, the non-degenerate
non-bonding dz2 orbital has highest energy and is localized on the top of the
valence band. The next ones in energy are the doubly degenerate dxz and dyz
orbitals, which overlap with N pz states and form bonding π-dative orbitals
localized above and below the sheet. The lowest-lying orbitals in energy are
doubly degenerate dxy and dx2-y2 states, which hybridize with s orbitals of
transition metals to form sd22. X. Li and J. Yang, Natl. Sci. Rev. 3(3), 365
(2016). https://doi.org/10.1093/nsr/nww026 hybridization. Bader population and
orbital analyses revealed that the local magnetic moments on Cr, Mn, and V atom
are 3, 4, and 2.1 μB, respectively. The Curie temperatures of 368 K for h-MnN
monolayer and 768 K for h-VN monolayer were estimated from MC
simulations.265,266265. A. V. Kuklin, S. A. Shostak, and A. A. Kuzubov, J. Phys.
Chem. Lett. 9(6), 1422 (2018). https://doi.org/10.1021/acs.jpclett.7b03276266.
Z. Xu and H. Zhu, J. Phys. Chem. C 122(26), 14918 (2018).
https://doi.org/10.1021/acs.jpcc.8b02323 Their results indicated that the easy
axis for both 2D materials is in the in-plane direction, and the corresponding
MAE values are 134 and 100 μeV per transition metal atom for h-MnN and h-VN,
respectively. More interestingly, MnN monolayer can maintain FM half-metallicity
and constant magnetic moment even under ±10% strain, because 100% spin
polarization of the electronic states near the Fermi level is fully preserved by
the robust bandgap of the spin-down states.266266. Z. Xu and H. Zhu, J. Phys.
Chem. C 122(26), 14918 (2018). https://doi.org/10.1021/acs.jpcc.8b02323 The half
metallic nature of h-VN and h-CrN will be retained even after contact with
semiconducting 2D sheets of MoS2 or MoSe2, which can be used as the substrates
for h-VN and h-CrN devices.264,265264. A. V. Kuklin, A. A. Kuzubov, E. A.
Kovaleva, N. S. Mikhaleva, F. N. Tomilin, H. Lee, and P. V. Avramov, Nanoscale
9(2), 621 (2017). https://doi.org/10.1039/C6NR07790K265. A. V. Kuklin, S. A.
Shostak, and A. A. Kuzubov, J. Phys. Chem. Lett. 9(6), 1422 (2018).
https://doi.org/10.1021/acs.jpclett.7b03276 In addition, robust magnetic
behavior was also observed in the buckled tetragonal t-VN monolayer, which has
99.9% spin polarization at the Fermi level and shows a rare p2d2 hybridization
for V atoms.
Monolayer structures of transition metal carbides with 1:1 stoichiometry,
including CoC, NiC, and CuC, were predicted by PSO structure search method and
first-principles calculations. Among them, CoC monolayer is antiferromagnetic
and NiC monolayer is ferromagnetic, while CuC monolayer is non-magnetic.267267.
C. Zhu, H. Lv, X. Qu, M. Zhang, J. Wang, S. Wen, Q. Li, Y. Geng, Z. Su, X. Wu,
Y. Li, and Y. Ma, J. Mater. Chem. C 7(21), 6406 (2019).
https://doi.org/10.1039/C9TC00635D The local magnetic moments of α-CoC, β-CoC,
α-NiC and β-NiC are 1.4, 1.61, 0.43, and 0.27 μB per metal atom, respectively.
After considering magnetic anisotropy, the easy axis of α-CoC monolayer is [100]
direction and the easy axis of β-CoC, α-NiC and β-NiC monolayers is [010]
direction. Notably, the computed MAEs for antiferromagnetic CoC and
ferromagnetic NiC are 107∼545 μeV per metal atom, which are at least one order
of magnitude higher than those of Co (65 μeV per Co atom) and Ni (2.7 μeV per Ni
atom) crystals.
Two-dimensional FeC2268268. T. Zhao, J. Zhou, Q. Wang, Y. Kawazoe, and P. Jena,
ACS Appl. Mater. Inter. 8(39), 26207 (2016).
https://doi.org/10.1021/acsami.6b07482 and CrC2269269. B. Zhou, X. Wang, and W.
Mi, J. Mater. Chem. C 6(15), 4290 (2018). https://doi.org/10.1039/C7TC05383E
sheets with 1:2 stoichiometry were predicted as half metals, and their spin
polarization at the Fermi level is 100%. The C atoms in these 2D structures bind
with each other to form C2 dimers, which possess high electron affinity and gain
electrons from Fe/Cr atoms. The significant amount of charge accumulation
adjacent to C2 dimers indicates strong interaction between the Fe/Cr atoms and
the C2 units. Therefore, Fe and Cr atoms are in high-spin states in FeC2 and
CrC2 sheets, with magnetic moments of 4 and 3.83 μB per Fe/Cr atom,
respectively. Based on MC simulation and mean-field theory, their Curie
temperatures were estimated to be 245 and 965 K, respectively. N2 dimers are
also found in the penta-MnN2 monolayer. The ferromagnetic state of penta-MnN2 is
energetically more favorable than the antiferromagnetic one, with predicted
Curie temperature as high as 913 K.270270. K. Zhao and Q. Wang, Appl. Surf. Sci.
505, 144620 (2020). https://doi.org/10.1016/j.apsusc.2019.144620
2. Transition metal borides
Combining DFT calculations and MC simulations, Abdullahi et al.271271. Y. Z.
Abdullahi, Z. D. Vatansever, E. Aktürk, Ü. Akıncı, and O. Ü. Aktürk, Phys. Chem.
Chem. Phys. 22, 10893 (2020). https://doi.org/10.1039/D0CP00503G have presented
a new phase of freestanding tetragonal Mn2B2 monolayer. The 2D tetra-Mn2B2 sheet
showed metallic ferromagnetism with a magnetic moment of 2.65 μB per Mn atom and
a Curie temperature of 406 K. Using an advanced crystal structure search method
and extensive first-principles energetic and dynamic calculations, Tang et
al.7878. X. Tang, W. Sun, Y. Gu, C. Lu, L. Kou, and C. Chen, Phys. Rev. B 99(4),
045445 (2019). https://doi.org/10.1103/PhysRevB.99.045445 have identified a
planar CoB6 monolayer exhibiting robust ferromagnetic ground state, which
remains stable upon the adsorption of common environmental gases like O2, CO2,
and H2O. Electronic band structure calculations revealed remarkable features of
Dirac cones with characteristic linear dispersions and high Fermi velocities.
The atomically thin CoB6 monolayer could be fabricated by either depositing Co
atoms on the δ4 boron sheet or direct chemical growth based on precursors of
planar Co4B8+ cluster.
3. Transition metal oxides
In the on-going research of 2D materials, 2D metal oxides are tempting, owing to
their natural abundance, suitable bandgap in a wide range, and high chemical
inertness.272272. Y. Guo, L. Ma, K. Mao, M. Ju, Y. Bai, J. Zhao, and X. C. Zeng,
Nanoscale Horiz. 4(3), 592 (2019). https://doi.org/10.1039/C8NH00273H Originated
from the half-filled 3d shell of Mn atom, the magnetic properties of 2D
manganese oxide monolayer have been systematically investigated by
first-principles calculations as representatives of the transition metal oxides
family.273,274273. M. Kan, J. Zhou, Q. Sun, Y. Kawazoe, and P. Jena, J. Phys.
Chem. Lett. 4(20), 3382 (2013). https://doi.org/10.1021/jz4017848274. E. Kan, M.
Li, S. Hu, C. Xiao, H. Xiang, and K. Deng, J. Phys. Chem. Lett. 4(7), 1120
(2013). https://doi.org/10.1021/jz4000559 In experiment, 2D MnO2 sheets were
successfully synthesized by tetrabutylammonium intercalation and
exfoliation.275275. Y. Omomo, T. Sasaki, Wang, and M. Watanabe, J. Am. Chem.
Soc. 125(12), 3568 (2003). https://doi.org/10.1021/ja021364p It is an indirect
semiconductor with a bandgap of 3.41 eV. Each unit cell of this 2D material
possesses a magnetic moment of 3 μB, which is mainly contributed by the Mn
atoms. In 2D MnO2, Mn atoms prefer ferromagnetic coupling [Fig. 11(a)].
Furthermore, the Curie temperature is about 140 K, which can be further
increased by strain.273273. M. Kan, J. Zhou, Q. Sun, Y. Kawazoe, and P. Jena, J.
Phys. Chem. Lett. 4(20), 3382 (2013). https://doi.org/10.1021/jz4017848 Using
first-principles calculations, Kan et al.274274. E. Kan, M. Li, S. Hu, C. Xiao,
H. Xiang, and K. Deng, J. Phys. Chem. Lett. 4(7), 1120 (2013).
https://doi.org/10.1021/jz4000559 found that ultrathin films of the
experimentally realized wurtzite MnO transform into a stable graphitic structure
with ordered spin arrangement. Moreover, the AFM ordering of graphitic MnO
monolayer can be switched into half-metallic ferromagnetism by moderate doping.
They found that the Curie temperature is about 350 K when 0.25 hole/Mn is doped
in single-layer MnO.
FIG. 11. (a) Total magnetic moment per unit cell as a function of temperature
for 2D MnO2 monolayer. (b) Spin charge density, band structures, total DOS,
partial DOS, and simulated magnetic moment and specific heat as a function of
temperature for 2D Fe2Si monolayer. (c) Average magnetization per unit cell and
specific heat (Cv) as a function of temperature for Fe3P monolayer from MC
simulations. (d) Schematic diagram of electronic band structure, and the
magnetic susceptibility and magnetic moment as a function of temperature for
CoH2 monolayer from MC simulations. The atomic configurations of MnO2, Fe2Si,
Fe3P, and CoH2 monolayers are also shown in insets. Panel (a) reproduced with
permission from Kan et al., J. Phys. Chem. Lett. 4, 3382 (2013). Copyright 2013
American Chemical Society.273273. M. Kan, J. Zhou, Q. Sun, Y. Kawazoe, and P.
Jena, J. Phys. Chem. Lett. 4(20), 3382 (2013). https://doi.org/10.1021/jz4017848
Panel (b) reproduced with permission from Sun et al., Nano Lett. 17, 2771
(2017). Copyright 2017 American Chemical Society.280280. Y. Sun, Z. Zhuo, X. Wu,
and J. Yang, Nano Lett. 17(5), 2771 (2017).
https://doi.org/10.1021/acs.nanolett.6b04884 Panel (c) reproduced with
permission from Zheng et al., J. Phys. Chem. Lett. 10, 2733 (2019). Copyright
2019 American Chemical Society.7676. S. Zheng, C. Huang, T. Yu, M. Xu, S. Zhang,
H. Xu, Y. Liu, E. Kan, Y. Wang, and G. Yang, J. Phys. Chem. Lett. 10(11), 2733
(2019). https://doi.org/10.1021/acs.jpclett.9b00970 Panel (d) reproduced with
permission from Wu et al., J. Phys. Chem. Lett. 9, 4260 (2018). Copyright 2018
American Chemical Society.283283. Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C.
Zeng, J. Phys. Chem. Lett. 9(15), 4260 (2018).
https://doi.org/10.1021/acs.jpclett.8b01976
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Aguilera-Granja and Ayuela276276. F. Aguilera-Granja and A. Ayuela, J. Phys.
Chem. C 124(4), 2634 (2020). https://doi.org/10.1021/acs.jpcc.9b06496
investigated the magnetic properties of monolayer metal oxides under MO2
stoichiometry, including all 3d, 4d, and 5d transition metals. It is noteworthy
that CrO2 and FeO2 layers are half-metals, while MnO2 and TcO2 layers with
half-filled d orbitals in the transition metal elements behave as magnetic
semiconductors. A simple model, depending on the hybridization between d
orbitals of transition metals and 2p orbitals of oxygen, allows us to
rationalize the magnetic behavior of the complete series of 2D metal oxides. In
general, the Curie temperatures were estimated in the range of 170∼220 K for 3d
oxides and below 100 K for 4d oxides, respectively.276276. F. Aguilera-Granja
and A. Ayuela, J. Phys. Chem. C 124(4), 2634 (2020).
https://doi.org/10.1021/acs.jpcc.9b06496 Recently, Gog et al.277277. H. van Gog,
W.-F. Li, C. Fang, R. S. Koster, M. Dijkstra, and M. van Huis, npj 2D Mater.
Appl. 3(1), 18 (2019). https://doi.org/10.1038/s41699-019-0100-z carried out a
systematic DFT study (with HSE06 hybrid functional) on the atomically thin metal
oxide films with compositions of MO, M2O3, and MO2 for typical 3d transition
metal elements (M = Sc, Ti, V, Cr, Mn). Of 20 2D transition metal oxides
studied, a rich variety of magnetic properties were discovered for the thermally
stable TMOs. Among them, the square MnO (Sq-MnO) was predicted to be a semimetal
with antiferromagnetic ordering; h-V2O3, sq-ScO2, and sq-CrO2 were found to be
ferromagnetic half-metals; and sq-MnO2 was an antiferromagnetic half-metal. The
magnetic moments are mainly originated from transition metal atoms, varying from
0.95 to 4.6 μB. 1T-RuO2 and 1T-OsO2 monolayers were also assigned as intrinsic
2D ferromagnets with large MAE. Their magnetic moments of 1.60 and 1.34 μB per
transition metal atom and large MAE values come from Ru and Os atoms. In
particular, the MAE of monolayer 1T-OsO2 is as high as 42.67 meV per unit cell
along [100] direction due to the strong SOC of Os atom, which is two orders of
magnitude higher than the MAE of ferromagnetic monolayer materials composed of
3d transition metals (Table IV). According to the mean field theory, the Curie
temperatures were 38 K for 1T-RuO2 and 197 K for 1T-OsO2 monolayer,
respectively. By analyzing the density of states and d orbital resolved MAE of
Os atom based on second-order perturbation theory, it is revealed that the large
MAE of monolayer 1T-OsO2 is mainly contributed by the matrix element differences
between the opposite-spin dxy and dx2-y2 orbitals of Os atoms.278278. Y. Wang,
F. Li, H. Zheng, X. Han, and Y. Yan, Phys. Chem. Chem. Phys. 20(44), 28162
(2018). https://doi.org/10.1039/C8CP05467C Olsson et al.279279. P. A. T. Olsson,
L. R. Merte, and H. Grönbeck, Phys. Rev. B 101(15), 155426 (2020).
https://doi.org/10.1103/PhysRevB.101.155426 studied the magnetic order of a
novel three-layered Fe3O4 film by means of Hubbard-corrected DFT calculations.
The Fe3O4 film comprises a center layer with octahedrally coordinated Fe2+ ions
sandwiched between two layers with tetrahedrally coordinated Fe3+ ions. The film
exhibits an antiferromagnetic type I spin order.
4. Transition metal silicides
Two-dimensional Fe2Si crystal has a slightly buckled triangular lattice composed
of planar hexacoordinated Si and Fe atoms. DFT calculations with hybrid HSE06
functions indicated that 2D Fe2Si in its ground state is a ferromagnetic
half-metal with 100% spin-polarization ratio at the Fermi level. Its 2D lattice
can be retained at very high temperature up to 1200 K [Fig. 11(b)]. MC
simulations based on Ising model also predicted TC as high as 780 K, which can
be further modulated by biaxial strain. Moreover, the planar structure and
strong in-plane Fe–Fe interaction endow Fe2Si nanosheet sizable MAE (325 μeV per
Fe atom), which is at least one to two orders of magnitude larger than those of
Fe, Co, and Ni solids.280280. Y. Sun, Z. Zhuo, X. Wu, and J. Yang, Nano Lett.
17(5), 2771 (2017). https://doi.org/10.1021/acs.nanolett.6b04884
Twenty 3d and 4d TM silicides with a fixed chemical formula of MSi2 (M = Sc, Ti,
V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd) have been
investigated by first-principles calculations.281281. N. Han, H. Liu, and J.
Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 The transition metal silicides exhibit
a variety of magnetic properties. Among them, TiSi2, VSi2, CrSi2, NbSi2, and
MoSi2 are ferromagnetic; MnSi2 and FeSi2 are antiferromagnetic; and the rest
systems are nonmagnetic. The on-site moments of transition metal atom in TiSi2,
VSi2, CrSi2, NbSi2, and MoSi2 monolayers are 0.563, 2.148, 3.008, 0.618, and
0.207 μB, respectively, while the local spin moments of Mn and Fe atom in MnSi2
and FeSi2 are 2.512 and 1.508 μB, respectively.281281. N. Han, H. Liu, and J.
Zhao, J. Supercond. Nov. Magn. 28(6), 1755 (2015).
https://doi.org/10.1007/s10948-014-2940-2 Two-dimensional titanium silicide
monolayers with different chemical compositions were globally searched by PSO
simulations combined with DFT calculations. Among the explored 2D Ti-Si
structures, Ti2Si is ferromagnetic with a magnetic moment of 1.37 μB per unit
cell.282282. Q. Wu, J.-J. Zhang, P. Hao, Z. Ji, S. Dong, C. Ling, Q. Chen, and
J. Wang, J. Phys. Chem. Lett. 7(19), 3723 (2016).
https://doi.org/10.1021/acs.jpclett.6b01731
5. Transition metal phosphides/arsenides
Motivated by 2D FeSi2 and FeC2 as well as 3D Fe–P compounds, Zheng et al.7676.
S. Zheng, C. Huang, T. Yu, M. Xu, S. Zhang, H. Xu, Y. Liu, E. Kan, Y. Wang, and
G. Yang, J. Phys. Chem. Lett. 10(11), 2733 (2019).
https://doi.org/10.1021/acs.jpclett.9b00970 focused on the 2D materials with
Fe-rich compositions of FexP (x = 1∼3). With the aid of first-principles swarm
structural search calculations, they have identified an unreported planar Fe3P
monolayer in Kagome lattice, showing several desirable properties for its
application in spintronic devices, e.g., robust ferromagnetism with large MAE,
and high thermal stability. As shown in Fig. 11(c), MC simulation yielded a
Curie temperature of 420 K. In addition, five stable M2P monolayers (M = Fe, Co,
Ni, Ru, Pd) under P4/mmm symmetry group were predicted by high-throughput search
and DFT calculations, which showed peculiar features of coexistence of in-plane
M–P covalent bonds and M–M interlayer metallic bonds. Importantly, the distinct
electronic configurations of transition metal atoms under a tetragonal crystal
field lead to diverse magnetic properties in 2D M2Ps. Among them, Co2P is
ferromagnetic with a Curie temperature of 580 K, while Fe2P is antiferromagnetic
with a Néel temperature of 23 K. Their long-range magnetic orderings originate
from the interplay of M–P–M superexchange interactions and M–M direct exchange
interactions.55. Q. Liu, J. Xing, Z. Jiang, X. Jiang, Y. Wang, and J. Zhao,
Nanoscale 12(12), 6776 (2020). https://doi.org/10.1039/D0NR00092B Two
experimentally feasible 2D intrinsic ferromagnetic materials, MnP and MnAs
monolayers, were predicted by first-principles calculations,7575. B. Wang, Y.
Zhang, L. Ma, Q. Wu, Y. Guo, X. Zhang, and J. Wang, Nanoscale 11(10), 4204
(2019). https://doi.org/10.1039/C8NR09734H which posses appreciable out-of-plane
anisotropies with MAE of 166 and 281 μeV, respectively. These two monolayer
sheets exhibit remarkable half-metallicity with high Curie temperatures of 495 K
for MnP and 711 K for MnAs, respectively. Moreover, the excellent ferromagnetism
and half-metallicity can be well preserved in few-layer MnP and MnAs. Based on
DFT calculations combined with PSO algorithm, Jiao et al.7373. Y. Jiao, W. Wu,
F. Ma, Z.-M. Yu, Y. Lu, X.-L. Sheng, Y. Zhang, and S. A. Yang, Nanoscale 11(35),
16508 (2019). https://doi.org/10.1039/C9NR04338A identified three new monolayer
phases of iron arsenide with high stability. Specifically, monolayer FeAs-I and
FeAs-III sheets crystallize in a tetragonal lattice with space group of P4/nmm,
while 2D FeAs-II has a trigonal P-3m1 lattice. Among them, FeAs-I and FeAs-II
are ferromagnetic metals, while FeAs-III is an antiferromagnetic semiconductor.
FeAs-I and FeAs-III have Curie temperatures of 645 and 350 K, respectively, both
of which are above room temperature. Importantly, their magnetic anisotropy
energies of 645 and 820 μeV are comparable to the magnetic recording materials
such as FeCo alloy (700∼800 μeV per atom).
6. Transition metal hydrides
Wu et al.283283. Q. Wu, Y. Zhang, Q. Zhou, J. Wang, and X. C. Zeng, J. Phys.
Chem. Lett. 9(15), 4260 (2018). https://doi.org/10.1021/acs.jpclett.8b01976
predicted a stable family of 2D transition metal dihydride MH2 (M = Sc, Ti, V,
Cr, Fe, Co, Ni) monolayers featuring pyramidal symmetry (C3v). Among them, CoH2
and ScH2 monolayers are ferromagnetic metals, while the others are
antiferromagnetic semiconductors. CoH2 monolayer is a perfect half-metal with a
wide spin gap of 3.48 eV and an above-room-temperature TC of 339 K [Fig. 11(d)].
ScH2 monolayer also possesses half-metallicity through hole doping. Notably,
their half-metallicity can be well retained on some substrates such as Cu (111)
surface, BN, MoS2, and MoSe2.
E. Ternary transition metal compounds
Ternary transition metal compounds of type M-X′-X″, where M is transition metal
element, usually magnetic elements like Fe, Co, Ni, and X′/X″ is a nonmagnetic
main group element from main groups IV, V, VI, or VII of the Periodic Table.
These compounds exhibit a rich variety of compositions and diverse magnetic
properties. Only recently, ternary transition chalcogenides and halides with
common transition metal centered octahedral units have attracted attentions.
Compared with the above discussed binary magnetic materials, ternary ones are
composed of one type of metal cations and two kinds of non-metal elements. The
fascinating magnetism still mainly stems from the cations. The metal atoms
occupy different crystallographic sites and form distinct magnetic sublattices,
while the addition of two kinds of non-metal elements provides sufficient
flexibility to tune the structure and magnetic properties. For example, 2D
CrXTe3 systems (X = Si, Ge, Sn) have a layered structure with CrTe6 octahedra
forming a honeycomb lattice and are typically FM semiconductor. However, 2D
CrXTe3 for X = Sb and Ga as AFM semiconductors exhibit a pseudo-one-dimensional
crystalline structure, in which CrTe6 octahedra form an infinite, edge-sharing,
and double rutile chain.284284. T. Kong, K. Stolze, D. Ni, S. K. Kushwaha, and
R. J. Cava, Phys. Rev. Mater. 2(1), 014410 (2018).
https://doi.org/10.1103/PhysRevMaterials.2.014410 So far, the ongoing researches
in identifying the 2D ternary vdW magnets include CrGe(Si, Sn)Te3, FeGe(Si)Te3,
MnBi2Te4, MPS3, transition oxyhalides, transition nitrohalides, and CrSI. In
Secs. III E 1 through III E 5, we will review their important experimental and
theoretical progress.
1. CrXTe3 (X = Si, Ge, Sn)
The strong coupling between magnetic and lattice degrees of freedom was verified
by Raman spectroscopy in ternary CrGeTe3 and infrared spectroscopy in CrSiTe3,
respectively.49,28549. Y. Tian, M. J. Gray, H. Ji, R. J. Cava, and K. S. Burch,
2D Mater. 3(2), 025035 (2016). https://doi.org/10.1088/2053-1583/3/2/025035285.
L. D. Casto, A. J. Clune, M. O. Yokosuk, J. L. Musfeldt, T. J. Williams, H. L.
Zhuang, M. W. Lin, K. Xiao, R. G. Hennig, B. C. Sales, J. Q. Yan, and D.
Mandrus, APL Mater. 3(4), 041515 (2015). https://doi.org/10.1063/1.4914134
Actually, CrGeTe3 is the first reported 2D ternary ferromagnetic material. As a
representative of layered vdW materials, Cr2Ge2Te6 has been mechanically
exfoliated and the intrinsic long-range ferromagnetic ordering has persevered in
bilayer Cr2Ge2Te6, as revealed by scanning magneto-optic Kerr microscopy.3737.
C. Gong, L. Li, Z. Li, H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y.
Wang, Z. Q. Qiu, R. J. Cava, S. G. Louie, J. Xia, and X. Zhang, Nature
546(7657), 265 (2017). https://doi.org/10.1038/nature22060 The optical image of
the exfoliated Cr2Ge2Te6 atomic layer is shown in Fig. 12(a). From Fig. 12(b),
one can see that the long bilayer strip becomes clearly distinguishable at
liquid helium temperature from the bare surrounding substrate. Figure 12(c)
shows a monotonic decrease in Curie point with reducing thickness. The Curie
temperature of bulk Cr2Ge2Te6 is 68 K, and the bilayer value is about 30 K.3737.
C. Gong, L. Li, Z. Li, H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y.
Wang, Z. Q. Qiu, R. J. Cava, S. G. Louie, J. Xia, and X. Zhang, Nature
546(7657), 265 (2017). https://doi.org/10.1038/nature22060 Subsequently,
theoretical calculations have predicted that Cr2Ge2Te6 is a semiconductor with a
bandgap of 0.13 eV,286286. Y. Fang, S. Wu, Z.-Z. Zhu, and G.-Y. Guo, Phys. Rev.
B 98(12), 125416 (2018). https://doi.org/10.1103/PhysRevB.98.125416 and 2D
Cr2Ge2Te6 possesses a magnetic moment of 2.4 μB per Cr atom with out-of-plane
magnetic anisotropy.9999. W. Xing, Y. Chen, P. M. Odenthal, X. Zhang, W. Yuan,
T. Su, Q. Song, T. Wang, J. Zhong, S. Jia, X. C. Xie, Y. Li, and W. Han, 2D
Mater. 4(2), 024009 (2017). https://doi.org/10.1088/2053-1583/aa7034 Similar to
its bulk counterpart, the magnetic behavior of Cr2Ge2Te6 is well described by
Heisenberg model, where spins can freely rotate and adopt any direction.4444. N.
D. Mermin and H. Wagner, Phys. Rev. Lett. 17(22), 1133 (1966).
https://doi.org/10.1103/PhysRevLett.17.1133 The mechanism of ferromagnetism in
Cr2Ge2Te6 structure is dominated by the superexchange interaction between
half-filled Cr t2g and empty eg states via Te p orbitals.286286. Y. Fang, S. Wu,
Z.-Z. Zhu, and G.-Y. Guo, Phys. Rev. B 98(12), 125416 (2018).
https://doi.org/10.1103/PhysRevB.98.125416 The lengths of Cr–Cr bonds are too
long to support strong antiferromagnetic coupling between direct Cr t2g exchange
interaction.1111. Y. Sun, R. C. Xiao, G. T. Lin, R. R. Zhang, L. S. Ling, Z. W.
Ma, X. Luo, W. J. Lu, Y. P. Sun, and Z. G. Sheng, Appl. Phys. Lett. 112(7),
072409 (2018). https://doi.org/10.1063/1.5016568
FIG. 12. (a) Optical image of exfoliated Cr2Ge2Te6 atomic layer. (b) Kerr
rotation signal under different temperatures. (c) Temperature-dependent Kerr
rotation intensity of 2∼5 layer and bulk samples under a 0.075 T field. (d) The
atomic models and schematic diagram of three magnetic structures of MPS3. Type
I, type II, and type III are for NiPS3, MnPS3, and FePS3, respectively. (e) The
schematic diagram of exchange coupling in Fe3GeTe2. (f) The measured TC in
Fe3GeTe2 with different thickness. The left one uses three measures with
Remanent anomalous Hall resistance Rxyr Arrott plots and RMCD, and the right one
only chooses the RMCD measure. Panels (a)–(c) reproduced with permission from
Gong et al., Nature 546, 265 (2017). Copyright 2017 Springer Nature.3737. C.
Gong, L. Li, Z. Li, H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang,
Z. Q. Qiu, R. J. Cava, S. G. Louie, J. Xia, and X. Zhang, Nature 546(7657), 265
(2017). https://doi.org/10.1038/nature22060 Panel (d) reproduced with permission
from Wang et al., Adv. Funct. Mater. 28, 1802151 (2018). Copyright 2018 John
Wiley and Sons.297297. F. Wang, T. A. Shifa, P. Yu, P. He, Y. Liu, F. Wang, Z.
Wang, X. Zhan, X. Lou, F. Xia, and J. He, Adv. Funct. Mater. 28(37), 1802151
(2018). https://doi.org/10.1002/adfm.201802151 Panel (f) left reproduced with
permission from Deng et al., Nature 563, 94 (2018). Copyright 2018 Springer
Nature.5050. Y. Deng, Y. Yu, Y. Song, J. Zhang, N. Z. Wang, Z. Sun, Y. Yi, Y. Z.
Wu, S. Wu, J. Zhu, J. Wang, X. H. Chen, and Y. Zhang, Nature 563(7729), 94
(2018). https://doi.org/10.1038/s41586-018-0626-9 Panel (f) right reproduced
with permission from Fei et al., Nat. Mater. 17, 778 (2018). Copyright 2018
Springer Nature.4141. Z. Fei, B. Huang, P. Malinowski, W. Wang, T. Song, J.
Sanchez, W. Yao, D. Xiao, X. Zhu, A. F. May, W. Wu, D. H. Cobden, J. H. Chu, and
X. Xu, Nat. Mater. 17(9), 778 (2018). https://doi.org/10.1038/s41563-018-0149-7
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* High-resolution
Similar magnetic behavior also exists in ferromagnetic semiconductor Cr2Si2Te6
due to the identical geometry, especially the same Te ligands. However, larger
vdW interlayer gap and smaller in-plane Cr–Cr distance is presented in Cr2Si2Te6
with regard to Cr2Ge2Te6. In their bulk phase, the above two factors would
weaken the TC from 63 K for Cr2Ge2Te6 to 32 K for Cr2Si2Te6287287. Y. Liu and C.
Petrovic, Phys. Rev. Mater. 3(1), 014001 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.014001 and strength the magnetic
anisotropy simultaneously. Neutron scattering measurements revealed that bulk
Cr2Si2Te6 is a strongly anisotropic 2D Ising-like ferromagnet.109109. V.
Carteaux, F. Moussa, and M. Spiesser, Europhys. Lett. 29(3), 251 (1995).
https://doi.org/10.1209/0295-5075/29/3/011 The exfoliation of bulk Cr2Si2Te6 to
monolayer or few-layer 2D crystals and transfer onto Si/SiO2 substrate have been
achieved. Temperature-dependent resistivity measurements for the few-layer 2D
Cr2Si2Te6 FET devices observed a clear change in resistivity at 80∼120 K, which
corresponds to the theoretically predicted TC = 80 K.6262. M.-W. Lin, H. L.
Zhuang, J. Yan, T. Z. Ward, A. A. Puretzky, C. M. Rouleau, Z. Gai, L. Liang, V.
Meunier, B. G. Sumpter, P. Ganesh, P. R. C. Kent, D. B. Geohegan, D. G. Mandrus,
and K. Xiao, J. Mater. Chem. C 4(2), 315 (2016).
https://doi.org/10.1039/C5TC03463A The higher TC in monolayer Cr2Si2Te6 than
Cr2Ge2Te6 can be ascribed to the fact that intralayer Cr–Te–Cr superexchange
interaction becomes dominant at the monolayer limit. Moreover, the ferromagnetic
mechanism could be maintained when monolayer Cr2Si2Te6 is described by the
Heisenberg model.288288. M. S. Baranava, D. C. Hvazdouski, V. A. Skachkova, V.
R. Stempitsky, and A. L. Danilyuk, Mater. Today: Proc. 20, 342 (2020).
https://doi.org/10.1016/j.matpr.2019.10.072
Using first-principles calculations with HSE06 functional, Zhuang et al.289289.
H. L. Zhuang, Y. Xie, P. R. C. Kent, and P. Ganesh, Phys. Rev. B 92(3), 035407
(2015). https://doi.org/10.1103/PhysRevB.92.035407 predicted that single-layer
CrSnTe3 is also a ferromagnetic semiconductor. Moreover, the important magnetic
parameters of CrXTe3 (X = Si, Ge, Sn) have been comparatively analyzed within a
unified framework. The estimated Curie temperature of CrSnTe3 was 170 K, which
is significantly higher than that of single-layer CrSiTe3 (90 K) and CrGeTe3
(130 K). Such enhancement is originated from the shorter Sn–Te bond length and
stronger ionicity, which in turn increase the superexchange coupling between the
magnetic Cr atoms. The corresponding exchange integral J parameters for CrSnTe3,
CrSnTe3, and CrSnTe3 are 3.92, 3.07, and 2.10 meV, respectively. Considerable
magnitude of MAE was also obtained in these three CrXTe3 systems. The calculated
MAE values ranged from 69 to 419 μeV/f.u., whereas z axis is the easy direction
for the magnetization in CrXTe3 family.289289. H. L. Zhuang, Y. Xie, P. R. C.
Kent, and P. Ganesh, Phys. Rev. B 92(3), 035407 (2015).
https://doi.org/10.1103/PhysRevB.92.035407
In 2017, an in-depth DFT survey with vdW-D2 correction on the magnetic phases of
single-layer transition metal trichalcogenide ternary compounds (MAX3) with a
total of 54 compositions was performed, covering 3d transition metals (M = V,
Cr, Mn, Fe, Co, Ni), main group IV elements (A = Si, Ge, Sn), and chalcogen
elements (X = S, Se, Te).290290. B. L. Chittari, D. Lee, N. Banerjee, A. H.
MacDonald, E. Hwang, and J. Jung, Phys. Rev. B 101(8), 085415 (2020).
https://doi.org/10.1103/PhysRevB.101.085415 Besides the reported FM CrXTe3,
their results indicated that a variety of magnetic ground states, including AFM
phases in Néel, stripy, and zigzag configurations, as well as FM configurations,
may exist depending on material composition. Among them, 2D MnSiSe3 and MnGeSe3
are highly anticipated, since their Curie temperatures from DFT-D2+U
calculations are 345.4 and 310.1 K, respectively. Recently, You et al.291291.
J.-Y. You, Z. Zhang, X.-J. Dong, B. Gu, and G. Su, Phys. Rev. Res. 2(1), 013002
(2020). https://doi.org/10.1103/PhysRevResearch.2.013002 proposed three stable
2D ferromagnetic semiconductors TcSiTe3, TcGeSe3, and TcGeTe3, with TC of 538,
212, and 187 K, respectively, which were given by MC simulations. All of them
have a spin moment of about 2 μB and an extraordinarily large orbital moment of
about 0.5 μB per Tc atom. In addition, large MAE (26.5∼42.5 meV), high Kerr
rotation angle (3.6°), and anomalous Hall conductivity have also been found.
Replacing Si/Ge/Sn by Ga atom, CrGaTe3 monolayer is an intrinsic ferromagnetic
semiconductor with an indirect bandgap of 0.3 eV. Its Curie temperature
estimated by Monte Carlo simulations was 71 K.292292. M. Yu, X. Liu, and W. Guo,
Phys. Chem. Chem. Phys. 20(9), 6374 (2018). https://doi.org/10.1039/C7CP07912E
2. MPX3 (X = S, Se, Te)
Next, we discuss another series of ternary single-layer compounds MPX3, which
are structurally closely related to the above discussed transition metal
trichalcogenide cousins MAX3. The top and side view of 2D MPX3 are shown in Fig.
12(d). In detail, each unit cell of MPS3 is composed of two cations and one
[P2S6]4– cluster. The M atoms are coordinated with six S atoms, while the P
atoms are coordinated with three S atoms and one P atom to form a [P2S6]4–
skeleton, which is arranged in a 2D honeycomb structure.2828. B. Huang, G.
Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E.
Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero,
and X. Xu, Nature 546(7657), 270 (2017). https://doi.org/10.1038/nature22391 The
main difference between MPX3 and MAX3 compounds is that the main group IV atom
(A = Si, Ge, Sn) inside the (A2X6)6– bipyramids are replaced by the main group V
element (P) inside the (P2X6)4– skeleton. The change from group IV element to
group V element is responsible for the significant modifications in electronic
structures and especially magnetic properties. Because of the surface S atoms,
MPS3 layers exhibit strong van der Waals character and can be easily exfoliated
from the bulk phase. In 2015, Xiong et al.293293. K.-z. Du, X.-z. Wang, Y. Liu,
P. Hu, M. I. B. Utama, C. K. Gan, Q. Xiong, and C. Kloc, ACS Nano 10(2), 1738
(2015). https://doi.org/10.1021/acsnano.5b05927 first observed the mechanically
fabricated 2D FePSe3 and MnPS3 sheets in the MPS3 family, and finally they
successfully obtained monolayer FePS3. Soon after, bulk NiPS3 and MnPS3
materials were also mechanically exfoliated into 2D nanoflakes in the
laboratory.294,295294. C. T. Kuo, M. Neumann, K. Balamurugan, H. J. Park, S.
Kang, H. W. Shiu, J. H. Kang, B. H. Hong, M. Han, T. W. Noh, and J. G. Park,
Sci. Rep. 6, 20904 (2016). https://doi.org/10.1038/srep20904295. Y.-J. Sun,
Q.-H. Tan, X.-L. Liu, Y.-F. Gao, and J. Zhang, J. Phys. Chem. Lett. 10(11), 3087
(2019). https://doi.org/10.1021/acs.jpclett.9b00758 Therefore, it is natural to
investigate their magnetic properties and those of the other stable MPS3 systems
at the monolayer limit. As a new catalogue of 2D vdW magnets, in the following
we will discuss the details of magnetic properties of experimentally exfoliable
FePS3, NiPS3, and MnPS3, and recently predicted CoPS3, CrPS3, and V0.9PS3, and
correlated the critical magnetic parameters with the number of layers.
Considering their (P2X6)4– skeleton, the metal cations in 2D MPS3 have M2+
ionization states and are in their high-spin configurations. The Néel
temperatures of 2D FePS3, NiPS3, and MnPS3 were extracted from Raman
spectroscopy, which is a common means to probe the spin properties. In
principle, the appearance of two-magnon scattering and the change in Raman peak
positions or intensities suggest ordered spin states.4848. J.-U. Lee, S. Lee, J.
H. Ryoo, S. Kang, T. Y. Kim, P. Kim, C.-H. Park, J.-G. Park, and H. Cheong, Nano
Lett. 16(12), 7433 (2016). https://doi.org/10.1021/acs.nanolett.6b03052 All the
three MPS3 monolayers were predicted to be semiconductors with localized
magnetic moments of 1∼4 μB and long-range antiferromagnetic ordering.296296. B.
L. Chittari, Y. Park, D. Lee, M. Han, A. H. MacDonald, E. Hwang, and J. Jung,
Phys. Rev. B 94(18), 184428 (2016). https://doi.org/10.1103/PhysRevB.94.184428
The electronic structures are greatly affected by metal and chalcogenide atoms,
with bandgaps in range of 0.12 to 1.33 eV from PBE calculations.107107. A.
Hashemi, H.-P. Komsa, M. Puska, and A. V. Krasheninnikov, J. Phys. Chem. C
121(48), 27207 (2017). https://doi.org/10.1021/acs.jpcc.7b09634 The AFM ground
state is governed by the competition between direct M–M exchange and indirect
M–S–M superexchange interactions within atomic layers, as well as interlayer
exchange interactions.112112. D. Lançon, R. A. Ewings, T. Guidi, F. Formisano,
and A. R. Wildes, Phys. Rev. B 98(13), 134414 (2018).
https://doi.org/10.1103/PhysRevB.98.134414 Meanwhile, different metal atoms
induce different distributions of magnetic moments and magnetic coupling—FePS3
of Ising-type, NiPS3 of XXZ type, and MnPS3 of Heisenberg-type297297. F. Wang,
T. A. Shifa, P. Yu, P. He, Y. Liu, F. Wang, Z. Wang, X. Zhan, X. Lou, F. Xia,
and J. He, Adv. Funct. Mater. 28(37), 1802151 (2018).
https://doi.org/10.1002/adfm.201802151 [Fig. 12(d)].
Two-dimensional FePS3 with a honeycomb lattice and that behaves as a large spin
Mott insulator is an Ising-type antiferromagnetic material with Néel temperature
of about 120 K.298298. P. A. Joy and S. Vasudevan, Phys. Rev. B 46(9), 5425
(1992). https://doi.org/10.1103/PhysRevB.46.5425 Owing to the absence of
long-range superexchange interaction between the Fe atoms from adjacent layers,
multilayer FePS3 systems do not show stronger magnetic exchange interaction. As
a consequence, TN decreases from 117 K in bulk to 104 K in monolayer FePS3.
Fortunately, the Ising-type magnetic ordering of FePS3 is preserved down to the
monolayer limit, which is demonstrated by the emergence of a series of new Raman
modes pointing to antiferromagnetic ordering.3838. X. Wang, K. Du, Y. Y. F. Liu,
P. Hu, J. Zhang, Q. Zhang, M. H. S. Owen, X. Lu, C. K. Gan, P. Sengupta, C.
Kloc, and Q. Xiong, 2D Mater. 3(3), 031009 (2016).
https://doi.org/10.1088/2053-1583/3/3/031009 As 2D Ising magnets, the magnetic
ordering is mainly dominated by the in-plane third nearest-neighboring Fe–Fe
exchange interaction in 2D FePS3, and the spins are aligned along the
out-of-plane direction with MAE of 3.7 meV.299299. A. R. Wildes, K. C. Rule, R.
I. Bewley, M. Enderle, and T. J. Hicks, J. Phys.: Condens. Matter 24(41), 416004
(2012). https://doi.org/10.1088/0953-8984/24/41/416004 The intralayer spin
moments are arranged ferromagnetically in each chain but coupled
antiferromagnetically with their neighboring chains, and the neighboring planes
are also coupled antiferromagnetically along the out-of-plane direction.4848.
J.-U. Lee, S. Lee, J. H. Ryoo, S. Kang, T. Y. Kim, P. Kim, C.-H. Park, J.-G.
Park, and H. Cheong, Nano Lett. 16(12), 7433 (2016).
https://doi.org/10.1021/acs.nanolett.6b03052
The second member in the 2D MPS3 family is 2D MnPS3. Kim et al.3939. K. Kim, S.
Y. Lim, J. Kim, J.-U. Lee, S. Lee, P. Kim, K. Park, S. Son, C.-H. Park, J.-G.
Park, and H. Cheong, 2D Mater. 6(4), 041001 (2019).
https://doi.org/10.1088/2053-1583/ab27d5 discovered a unique feature of Raman
spectrum that correlates well with the stable antiferromagnetic ordering at the
bilayer limit of MnPS3. Its TN could maintain at 78 K from bulk phase to
five-layer system. The independence of number of layers stems from the weak
interlayer coupling in MnPS3. In the antiferromagnetic state of MnPS3, each Mn
atom is antiferromagnetically coupled with three nearest neighbors within the
basal plane. The direction of spin moments is ∼8° along c axis, and there exists
ferromagnetic coupling between the planes. By exploiting the spin-flop
transition, Long et al. have shown that the magnetoresistance persists as
thickness is reduced. The characteristic temperature and scale of magnetic field
are nearly unchanged, albeit with a different dependence on magnetic field,
indicating again the persistence of magnetism at the ultimate limit of
individual monolayer.300300. G. Long, H. Henck, M. Gibertini, D. Dumcenco, Z.
Wang, T. Taniguchi, K. Watanabe, E. Giannini, and A. F. Morpurgo, Nano Lett.
20(4), 2452 (2020). https://doi.org/10.1021/acs.nanolett.9b05165 Fascinatingly,
MnPS3 exhibits three spin-related critical transitions, including 2D single-ion
anisotropy antiferromagnetic phase transition at 120 K,
paramagnetic-antiferromagnetic transition at 80 K, and XY-like behavior at
55 K.295295. Y.-J. Sun, Q.-H. Tan, X.-L. Liu, Y.-F. Gao, and J. Zhang, J. Phys.
Chem. Lett. 10(11), 3087 (2019). https://doi.org/10.1021/acs.jpclett.9b00758 In
addition, long-distance magnon transport was also detected in MnPS3 crystal as
antiferromagnet.301301. W. Xing, L. Qiu, X. Wang, Y. Yao, Y. Ma, R. Cai, S. Jia,
X. C. Xie, and W. Han, Phys. Rev. X 9(1), 011026 (2019).
https://doi.org/10.1103/PhysRevX.9.011026 The antiferromagnetic transition at
Néel temperature of around 78 K in few-layered MnPS3 is completely suppressed by
Mn vacancy, which leads to a lower magnetic transition temperature of
38 K.302302. W. Bai, Z. Hu, C. Xiao, J. Guo, Z. Li, Y. Zou, X. Liu, J. Zhao, W.
Tong, W. Yan, Z. Qu, B. Ye, and Y. Xie, J. Am. Chem. Soc. 142, 10849 (2020).
https://doi.org/10.1021/jacs.0c04101 Moreover, long-range magnon transport over
several micrometers in the 2D antiferromagnet MnPS3 has been observed
experimentally.301301. W. Xing, L. Qiu, X. Wang, Y. Yao, Y. Ma, R. Cai, S. Jia,
X. C. Xie, and W. Han, Phys. Rev. X 9(1), 011026 (2019).
https://doi.org/10.1103/PhysRevX.9.011026
NiPS3 is the third member of 2D MPS3 family. In NiPS3, eight d electrons from Ni
atom occupy the split 3d shell under octahedral crystal field. The t2g orbitals
are fully occupied and two eg orbitals are half filled. Combined with its
honeycomb lattice, Gu et al.303303. Y. Gu, Q. Zhang, C. Le, Y. Li, T. Xiang, and
J. Hu, Phys. Rev. B 100(16), 165405 (2019).
https://doi.org/10.1103/PhysRevB.100.165405 suggested that 2D NiPS3 is a Dirac
material with strong electron-electron correlation. The unpaired electrons in
the two eg orbitals would “long-range” hop between two third nearest-neighboring
Ni sites in the Ni honeycomb lattice via superexchange interaction. Similar to
2D FePS3 and MnPS3, both DFT calculations296296. B. L. Chittari, Y. Park, D.
Lee, M. Han, A. H. MacDonald, E. Hwang, and J. Jung, Phys. Rev. B 94(18), 184428
(2016). https://doi.org/10.1103/PhysRevB.94.184428 and experimental
measurements4040. G. Le Flem, R. Brec, G. Ouvard, A. Louisy, and P. Segransan,
J. Phys. Chem. Solids 43(5), 455 (1982).
https://doi.org/10.1016/0022-3697(82)90156-1 suggested that the Ni honeycomb
lattice forms zigzag antiferromagnetic insulating ground state, which is
featured by AFM coupling between the double parallel ferromagnetic chains, while
the planes are ferromagnetically coupled along the out-of-plane
direction.112112. D. Lançon, R. A. Ewings, T. Guidi, F. Formisano, and A. R.
Wildes, Phys. Rev. B 98(13), 134414 (2018).
https://doi.org/10.1103/PhysRevB.98.134414 Unlike 2D FePS3 and MnPS3, on the one
hand, the single-ion anisotropy of NiPS3 would change from XY type to XXZ type
as the number of layers decreases.304304. K. Kim, S. Y. Lim, J.-U. Lee, S. Lee,
T. Y. Kim, K. Park, G. S. Jeon, C.-H. Park, J.-G. Park, and H. Cheong, Nat.
Commun. 10(1), 345 (2019). https://doi.org/10.1038/s41467-018-08284-6 On the
other hand, the antiferromagnetic ordering persists down to biayer NiPS3, and
its TN is about 130 K. However, the antiferromagnetic ordering is drastically
suppressed in the monolayer, indicating that intralayer exchange interactions
are much stronger than the interlayer ones. Such variation could be understood
by the strong spin fluctuations, which drastically suppress the bulk
antiferromagnetic ordering.304304. K. Kim, S. Y. Lim, J.-U. Lee, S. Lee, T. Y.
Kim, K. Park, G. S. Jeon, C.-H. Park, J.-G. Park, and H. Cheong, Nat. Commun.
10(1), 345 (2019). https://doi.org/10.1038/s41467-018-08284-6
With the aid of DFT calculations, the metal element M in 2D MPX3 family has been
further extended to 3d/4d/5d transition metals and the non-metal X element
extended to S, Se, and Te. Hence, a series of stable trichalcogenides were
predicted. Due to weak interlayer coupling, parts of them are exfoliable 2D
magnetic materials. For example, Chittari et al.296296. B. L. Chittari, Y. Park,
D. Lee, M. Han, A. H. MacDonald, E. Hwang, and J. Jung, Phys. Rev. B 94(18),
184428 (2016). https://doi.org/10.1103/PhysRevB.94.184428 have systemically
investigated the magnetic properties of 2D MPX3 (M = V, Cr, Mn, Fe, Co, Ni, Cu,
Zn; X = S, Se, Te). They concluded that the ground-state spin configuration
depends on the combination of transition metal and chalcogen elements. Besides
the reported Mn-, Fe-, and Ni-based 2D MPS3 antiferromagnetic semiconductors,
V-based compounds also exhibit semiconducting Néel antiferromagnetic states.
Interestingly, isostructural Mott transition was observed in VPS3.305305. M. J.
Coak, S. Son, D. Daisenberger, H. Hamidov, C. R. S. Haines, P. L. Alireza, A. R.
Wildes, C. Liu, S. S. Saxena, and J.-G. Park, npj Quantum Mater. 4(1), 38
(2019). https://doi.org/10.1038/s41535-019-0178-8 When M changes from the
strongly correlated 3d transition metals to the weakly correlated 4d and 5d
elements, the ground state would transform from FM to PM. In the case of 4d
PdPS3, the lowest-energy state is still AFM,306306. Y. Sugita, T. Miyake, and Y.
Motome, Phys. Rev. B 97(3), 035125 (2018).
https://doi.org/10.1103/PhysRevB.97.035125 while 2D PtPS3 with 5d element is PM.
Both Pt and Pd possess half-filled eg orbitals; thus, they may also exhibit
multiple Dirac cones at the same time. Moreover, replacing a smaller chalcogen
atom (S) with a larger chalcogen atom (Se or Te) reduces the energy bandgap as
well as the energy difference between FM and AFM states.107,296,307107. A.
Hashemi, H.-P. Komsa, M. Puska, and A. V. Krasheninnikov, J. Phys. Chem. C
121(48), 27207 (2017). https://doi.org/10.1021/acs.jpcc.7b09634296. B. L.
Chittari, Y. Park, D. Lee, M. Han, A. H. MacDonald, E. Hwang, and J. Jung, Phys.
Rev. B 94(18), 184428 (2016). https://doi.org/10.1103/PhysRevB.94.184428307. X.
Li, X. Wu, and J. Yang, J. Am. Chem. Soc. 136(31), 11065 (2014).
https://doi.org/10.1021/ja505097m
3. Fe-Ge-Te ternary compounds
Previously mentioned 2D ternary compounds are ferromagnetic/antiferromagnetic
semiconductors, while the series of Fe-Ge-Te (FGT) ternary compounds are
ferromagnetic metals with significant uniaxial magnetocrystaline anisotropy. As
a unique kind of itinerant ferromagnetic metals, the exchange mechanism in FGT
can be described by Stoner model, whose exchange splitting is induced by Coulomb
repulsion among itinerant electrons.308308. E. C. Stoner, Proc. R. Soc. London,
Ser. A 165, 372 (1938). https://doi.org/10.1098/rspa.1938.0066 In addition, the
itinerant ferromagnetism could be understood by mapping a classical Heisenberg
model with RKKY exchange interaction.309309. R. Prange and V. Korenman, Phys.
Rev. B 19(9), 4691 (1979). https://doi.org/10.1103/PhysRevB.19.4691
Advantageously, the metallic nature enables the interplay of both spin and
lattice degrees of freedom, which is the heart of various spintronic
architectures.310310. S. Bhatti, R. Sbiaa, A. Hirohata, H. Ohno, S. Fukami, and
S. N. Piramanayagam, Mater. Today 20(9), 530 (2017).
https://doi.org/10.1016/j.mattod.2017.07.007
The most widely studied 2D FGT materials is Fe3GeTe2. Bulk Fe3GeTe2 crystal is a
layered material with vdW gap of 2.95 Å. In a pioneer study in 2016, Zhuang et
al. predicted that mechanical exfoliated single-layer Fe3GeTe2 exhibited strong
out-plane magnetocrystalline anisotropy with MAE of 0.92 meV.1616. H. L. Zhuang,
P. R. C. Kent, and R. G. Hennig, Phys. Rev. B 93(13), 134407 (2016).
https://doi.org/10.1103/PhysRevB.93.134407 Very soon, this proposal was
confirmed by Chu et al., who successfully fabricated few-layered flakes of
Fe3GeTe2 by cleaving Fe3GeTe2 crystal onto a gold film. The RMCD measurement
probed the TC values to be 180 and 130 K for bilayer and monolayer Fe3GeTe2,
respectively. In addition, it was stated that monolayer Fe3GeTe2 with large
out-of-plane anisotropy is a truly 2D itinerant ferromagnet.4141. Z. Fei, B.
Huang, P. Malinowski, W. Wang, T. Song, J. Sanchez, W. Yao, D. Xiao, X. Zhu, A.
F. May, W. Wu, D. H. Cobden, J. H. Chu, and X. Xu, Nat. Mater. 17(9), 778
(2018). https://doi.org/10.1038/s41563-018-0149-7 Subsequently, Zhang et al.
used Al2O3-addisted exfoliation method instead of conventional mechanical
exfoliation to protect the intralayer bonding. The Curie temperature of
monolayer Fe3GeTe2 was determined to be 30 K by probing Remanent anomalous Hall
resistance and 68 K by RMCD measurement, respectively. Moreover, a definite
out-of-plane magnetocrystalline anisotropy energy of 2 meV was found, which is
large enough to protect the magnetic ordering below a finite TC.5050. Y. Deng,
Y. Yu, Y. Song, J. Zhang, N. Z. Wang, Z. Sun, Y. Yi, Y. Z. Wu, S. Wu, J. Zhu, J.
Wang, X. H. Chen, and Y. Zhang, Nature 563(7729), 94 (2018).
https://doi.org/10.1038/s41586-018-0626-9 This series of works opens a new era
in the development of high-temperature 2D magnets.
Different from many other 2D magnets, Fe3GeTe2 exhibits two values of on-site
spin moment because the Fe atoms occupy two different lattice sites, labeled as
Fe1 and Fe2. The magnetic moment of Fe13+ is about 1.7 μB and that of Fe22+ is
about 1 μB. The Stoner criterion states that formation of ferromagnetic ordering
is dictated by density of states at the Fermi level. In turn, the half-filled d
orbitals of Fe mainly affect the ferromagnetism in Fe3GeTe2. As shown in Fig.
12(e), the magnetic coupling parameters for interlayer Fe1-Fe1 coupling and
Fe1-Fe2 coupling are 23.48 meV and 20.41 meV, respectively, which collaborate to
determine the ferromagnetism in Fe3GeTe2. That is to say, the ferromagnetism in
Fe3GeTe2 is dominated by the coupling between perpendicular Fe atoms. As it is
known, the distance between adjacent Fe3GeTe2 layers also plays a crucial role
in modulating the magnetic interactions. Wang et al. substantiated that the
effective coupling becomes negligible when interlayer distance is increased by 1
Å in bilayer Fe3GeTe2.311311. C. Hu, D. Zhang, F. Yan, Y. Li, Q. Lv, W. Zhu, Z.
Wei, K. Chang, and K. Wang, Sci. Bull. 65(13), 1072 (2020).
https://doi.org/10.1016/j.scib.2020.03.035 Furthermore, Hwang et al. found that
formation of oxide at the interface of Fe3GeTe2 induces antiferromagnetic
coupling between pristine Fe3GeTe2 layer and oxidized Fe3GeTe2 layer in a
bilayer system. The interlayer distance is too large to generate direct Fe-Fe
coupling. Therefore, magnetic information between adjacent layers can only be
mediated by the indirect interaction between oxygen p orbitals.312312. D. Kim,
S. Park, J. Lee, J. Yoon, S. Joo, T. Kim, K. J. Min, S. Y. Park, C. Kim, K. W.
Moon, C. Lee, J. Hong, and C. Hwang, Nanotechnology 30(24), 245701 (2019).
https://doi.org/10.1088/1361-6528/ab0a37
As a kind of vdW ferromagnets, the effect of number of layers is clearly
manifested in Fe3GeTe2. As shown in Fig. 12(f), TC of Fe3GeTe2 closely depends
on the thickness of flakes. As the number of layers decreases to 7, a dramatic
drop of TC would occur.41,5041. Z. Fei, B. Huang, P. Malinowski, W. Wang, T.
Song, J. Sanchez, W. Yao, D. Xiao, X. Zhu, A. F. May, W. Wu, D. H. Cobden, J. H.
Chu, and X. Xu, Nat. Mater. 17(9), 778 (2018).
https://doi.org/10.1038/s41563-018-0149-750. Y. Deng, Y. Yu, Y. Song, J. Zhang,
N. Z. Wang, Z. Sun, Y. Yi, Y. Z. Wu, S. Wu, J. Zhu, J. Wang, X. H. Chen, and Y.
Zhang, Nature 563(7729), 94 (2018). https://doi.org/10.1038/s41586-018-0626-9
Experimental observation confirmed that TC decreases monotonically with
decreasing number of layers, while the strong perpendicular magnetic anisotropy
is retained. The difference of the probed TC might come from different
environments of Fe3GeTe2 during experimental synthesis.313,314313. L. Zhang, L.
Song, H. Dai, J.-H. Yuan, M. Wang, X. Huang, L. Qiao, H. Cheng, X. Wang, W. Ren,
X. Miao, L. Ye, K.-H. Xue, and J.-B. Han, Appl. Phys. Lett. 116(4), 042402
(2020). https://doi.org/10.1063/1.5142077314. J. Seo, D. Y. Kim, E. S. An, K.
Kim, G.-Y. Kim, S.-Y. Hwang, D. W. Kim, B. G. Jang, H. Kim, G. Eom, S. Y. Seo,
R. Stania, M. Muntwiler, J. Lee, K. Watanabe, T. Taniguchi, Y. J. Jo, J. Lee, B.
I. Min, M. H. Jo, H. W. Yeom, S.-Y. Choi, J. H. Shim, and J. S. Kim, Sci. Adv.
6(3), eaay8912 (2020). https://doi.org/10.1126/sciadv.aay8912 Han et al.
deposited Fe3GeTe2 flakes onto three types of substrates—Al, Au, and SiO2. The
change of substrate from Al to Au could elevate the value of TC significantly
from 105 to 180 K for Fe3GeTe2 film of 10 nm thickness. Such big modulation of
TC by substrates could be attributed to lattice distortion and charge
redistribution between the Fe3GeTe2 sample and the substrate.313313. L. Zhang,
L. Song, H. Dai, J.-H. Yuan, M. Wang, X. Huang, L. Qiao, H. Cheng, X. Wang, W.
Ren, X. Miao, L. Ye, K.-H. Xue, and J.-B. Han, Appl. Phys. Lett. 116(4), 042402
(2020). https://doi.org/10.1063/1.5142077 Recently, Kim et al. successful
synthesized and exfoliated Fe4GeTe2 flakes and obtained TC of about 270 K for
7-layer Fe4GeTe2.314314. J. Seo, D. Y. Kim, E. S. An, K. Kim, G.-Y. Kim, S.-Y.
Hwang, D. W. Kim, B. G. Jang, H. Kim, G. Eom, S. Y. Seo, R. Stania, M.
Muntwiler, J. Lee, K. Watanabe, T. Taniguchi, Y. J. Jo, J. Lee, B. I. Min, M. H.
Jo, H. W. Yeom, S.-Y. Choi, J. H. Shim, and J. S. Kim, Sci. Adv. 6(3), eaay8912
(2020). https://doi.org/10.1126/sciadv.aay8912
Fe5GeTe2 has a similar structure with Fe3GeTe2, which is also made up of 2D
slabs of Fe and Ge between layers of Te, but with two additional layers of Fe
atoms. The magnetic state in Fe5GeTe2 is even more complicated than Fe3GeTe2 due
to structural disorder and presence of short-range order associated with the
occupation of split sites. May et al. exfoliated Fe5GeTe2 nanoflakes (12 nm/4
unit-cell layers) on SiO2 substrates and determined TC to be in range of 270 to
300 K. The magnetic moment of Fe5GeTe2 along the out-of-plane is 0.8∼2.6 μB per
Fe atom on different Fe sites.4242. A. F. May, D. Ovchinnikov, Q. Zheng, R.
Hermann, S. Calder, B. Huang, Z. Fei, Y. Liu, X. Xu, and M. A. McGuire, ACS Nano
13(4), 4436 (2019). https://doi.org/10.1021/acsnano.8b09660 In contrast to
Fe3GeTe2, however, bulk Fe5GeTe2 crystal does not exhibit a perpendicular
magnetic anisotropy. Recently, Joe et al. predicted that both monolayer and
bilayer Fe5GeTe2 systems remain metallic and ferromagnetic. The ferromagnetism
originates from Fe atoms and the splitting of d orbitals occurs for both spin-up
and spin-down states, presenting exchange splitting to satisfy Stoner's theory
of ferromagnetism.143143. M. Joe, U. Yang, and C. Lee, Nano Mater. Sci. 1(4),
299 (2019). https://doi.org/10.1016/j.nanoms.2019.09.009 In addition, Zhang et
al. reported that Fe5-xGeTe2 shows glassy cluster behavior below 110 K and
revealed a transition from ferromagnet to ferrimagnet at 275 K. Meanwhile, they
observed that the Fe-Ge-Te crystal with more Fe contents favors an in-plane easy
magnetization at all temperatures up to TC.315315. H. Zhang, R. Chen, K. Zhai,
X. Chen, L. Caretta, X. Huang, R. V. Chopdekar, J. Cao, J. Sun, J. Yao, R.
Birgeneau, and R. Ramesh, Phys. Rev. B 102(6), 064417 (2020).
https://doi.org/10.1103/PhysRevB.102.064417
Compared to 2D MPX3 family, 2D MPX4 sheets show different electron configuration
formally with M3+[PX4]3–. In the monolayer structure of MPS4, six S atoms form a
slightly distorted octahedron encapsulated with a transition atom (M) in the
center. Meanwhile, the P atoms are in the center of tetrahedron consisting of
four S atoms, suggesting a distinct magnetic ordering. Experimentally, single-
and few-layered CrPS4 sheets were mechanically isolated in 2016.316316. J. Lee,
T. Y. Ko, J. H. Kim, H. Bark, B. Kang, S.-G. Jung, T. Park, Z. Lee, S. Ryu, and
C. Lee, ACS Nano 11(11), 10935 (2017). https://doi.org/10.1021/acsnano.7b04679
Further DFT calculations revealed that monolayer CrPS4 is a ferromagnetic
semiconductor, which is quite different from the antiferromagnetic ordering of
its bulk form.317317. H. L. Zhuang and J. Zhou, Phys. Rev. B 94(19), 195307
(2016). https://doi.org/10.1103/PhysRevB.94.195307 Later, Chen et al.9595. Q.
Chen, Q. Ding, Y. Wang, Y. Xu, and J. Wang, J. Phys. Chem. C 124(22), 12075
(2020). https://doi.org/10.1021/acs.jpcc.0c02432 systematically discussed the
magnetic ordering in monolayer MPS4 and proposed that VPS4, MnPS4, and NiPS4
prefer antiferromagnetic states while CrPS4 and FePS4 are ferromagnetic. The
calculated TC was 50 K for CrPS4.9595. Q. Chen, Q. Ding, Y. Wang, Y. Xu, and J.
Wang, J. Phys. Chem. C 124(22), 12075 (2020).
https://doi.org/10.1021/acs.jpcc.0c02432 From their first-principles
calculations, it was unveiled that V, Cr, Mn, Fe, and Ni in TMPS4 monolayers
carries local moment of 1.8, 2.9, 3.6, 0.9, and 0.5 μB, respectively. Indeed,
Fe, Co, and Ni atoms in TMPS4 are in the low-spin configuration because of the
relatively strong field ligands, while V, Mn, and Cr atoms adopt the high-spin
configurations. After replacing Cr (P) by Mn (As) atoms, the magnetic properties
of single-layer MnAsS4 have also been investigated.318318. T. Hu, W. Wan, Y. Ge,
and Y. Liu, J. Phys.: Condens. Matter 32, 385803 (2020).
https://doi.org/10.1088/1361-648X/ab95cc The half-metallic spin gap for
monolayer MnAsS4 is about 1.46 eV, and it has a large spin splitting energy of
about 0.49 eV in the conduction band. MC simulations predicted a rather high TC
of about 740 K.
4. MnBi2Te4 and CoGa2X4 (X = S, Se, or Te)
An emerging family of intrinsic magnets with tetrachalcogenides is also found on
a 2D triangular lattice, i.e., MnBi2Te4 and CoGa2X4 (X = S, Se, Te). Based on
DFT calculations, Li et al.1414. J. Li, Y. Li, S. Du, Z. Wang, B.-L. Gu, S.-C.
Zhang, K. He, W. Duan, and Y. Xu, Sci. Adv. 5(6), eaaw5685 (2019).
https://doi.org/10.1126/sciadv.aaw5685 predicted a series of novel magnetic
materials from MnBi2Te4 related ternary chalcogenides MB2T4, where M is
transition metal or rare earth metal; B is Bi or Sb; and T is Te, Se, or S [Fig.
13(a)]. In these materials, the intralayer exchange coupling is ferromagnetic,
giving rise to 2D ferromagnetism in the monolayer. By carefully controlling the
film thickness and external magnetic fields, many interesting topological
quantum states can be induced in MnBi2Te4 monolayer [Fig. 13(b)], including QAH
insulators, axion insulators, and quantum spin Hall insulators. Intriguingly,
magnetic and topological states are well combined in MnBi2Te4, where Mn atom
introduces magnetism and Bi–Te layers could generate topological properties. The
schematic mechanism is depicted in Fig. 13(c). The monolayer MnBi2Te4 is a
topologically trivial FM insulator with a direct bandgap of 0.70 eV [Fig.
13(d)]. Moreover, the magnetic and topological transitions in MnBi2Te4 are
thickness dependent.319319. M. M. Otrokov, I. P. Rusinov, M. Blanco-Rey, M.
Hoffmann, A. Y. Vyazovskaya, S. V. Eremeev, A. Ernst, P. M. Echenique, A. Arnau,
and E. V. Chulkov, Phys. Rev. Lett. 122(10), 107202 (2019).
https://doi.org/10.1103/PhysRevLett.122.107202 The MnBi2Te4 systems with odd
numbers of building blocks are uncompensated interlayer antiferromagnets, while
those with even numbers of building blocks are compensated interlayer
antiferromagnets. Thickness dependent wide-band-gap quantum anomalous Hall and
zero plateau quantum anomalous Hall states are observed. Li et al.320320. Y. Li,
Z. Jiang, J. Li, S. Xu, and W. Duan, Phys. Rev. B 100(13), 134438 (2019).
https://doi.org/10.1103/PhysRevB.100.134438 further analyzed its magnetic
anisotropy and found that the magnetic anisotropy comes mainly from single ion
anisotropy, which is caused by the SOC effect of Mn and Te atoms. The exchange
interaction in the monolayer MnBi2Te4 is nearly isotropic, which has no
contribution to the magnetic anisotropy. Thus, the Curie temperature was
estimated to be about 20 K.
FIG. 13. (a) Monolayer MnBi2Te4 with FM configurations. (b) Various topological
quantum states in MB2T4. (c) Schematic diagram of band magnetism and topology.
(d) Band structure of monolayer MnBi2Te4. (e) Curie temperatures of eight MXY 2D
ferromagnets, including ScCl, YCl, LaCl, LaBr2, CrSCl, CrSBr, CrSI, and CrSeBr.
(f) Exfoliation mechanism of 2D FM sheet from 3D vdW AFM crystal and the Curie
temperatures of CrOCl, CrOBr, and strained CrOCl. (g) Temperature dependence of
zero-field-cooled (ZFC) and field-cooled (FC) curves for magnetization of the
as-synthesized δ-FeOOH ultrathin nanosheets. Inset: the magnified part of ZFC
and FC curves in temperature range from 280 to 300 K. Panels (a)–(d) reproduced
with permission from Li et al., Sci. Adv. 5, eaaw5685 (2019). Copyright 2019
American Association for the Advancement of Science.1414. J. Li, Y. Li, S. Du,
Z. Wang, B.-L. Gu, S.-C. Zhang, K. He, W. Duan, and Y. Xu, Sci. Adv. 5(6),
eaaw5685 (2019). https://doi.org/10.1126/sciadv.aaw5685 Panel (e) reproduced
with permission from Jiang et al., ACS Appl. Mater. Inter. 10, 39032 (2018).
Copyright 2018 American Chemical Society.6565. Z. Jiang, P. Wang, J. Xing, X.
Jiang, and J. Zhao, ACS Appl. Mater. Inter. 10(45), 39032 (2018).
https://doi.org/10.1021/acsami.8b14037 Panel (f) reproduced with permission from
Miao et al., J. Am. Chem. Soc. 140, 2417 (2018). Copyright 2018 American
Chemical Society.322322. N. Miao, B. Xu, L. Zhu, J. Zhou, and Z. Sun, J. Am.
Chem. Soc. 140(7), 2417 (2018). https://doi.org/10.1021/jacs.7b12976 Panel (g)
reproduced with permission from Chen et al., Chem. Sci. 5, 2251 (2014).
Copyright 2014 Royal Society of Chemistry.325325. P. Chen, K. Xu, X. Li, Y. Guo,
D. Zhou, J. Zhao, X. Wu, C. Wu, and Y. Xie, Chem. Sci. 5(6), 2251 (2014).
https://doi.org/10.1039/C3SC53303D
* PPT
|
* High-resolution
Two-dimensional half-metallic ferromagnets, CoGa2X4 (X = S, Se, Te), are also
found to be stable against spin flipping at room temperature. Its robust HM
ferromagnetism originates from the superexchange interaction of Co-X–Co bonds
with bond angles close to 90°. The calculations of magnetic anisotropy with
inclusion of SOC indicated that CoGa2X4 systems have easy plane magnetizations,
which are expected to have Berezinsky-Kosterlitz-Thouless transitions following
the classical 2D XY model.9696. S. Zhang, R. Xu, W. Duan, and X. Zou, Adv.
Funct. Mater. 29(14), 1808380 (2019). https://doi.org/10.1002/adfm.201808380
Other 2D magnets in ternary sulfides with two nonmetal elements are also
observed for single-layer pentagonal CoAsS321321. L. Liu and H. L. Zhuang, APL
Mater. 7(1), 011101 (2019). https://doi.org/10.1063/1.5079867 and CrP2S7.6666.
H. Liu, J.-T. Sun, M. Liu, and S. Meng, J. Phys. Chem. Lett. 9(23), 6709 (2018).
https://doi.org/10.1021/acs.jpclett.8b02783
5. MXY-type compounds
A class of 2D magnetic materials, MXY (M = transition metal; X = O, S, Se, Te,
N; Y = Cl, Br, I) will crystalize in an orthorhombic structure with Pmmn space
group, which consists of M2X2 layers sandwiched by halogen atoms. The transition
metal M atoms are in the center of distorted octahedron (D2h symmetry) and bound
with X and Y atoms.
In H phase, five d orbitals split into three groups, i.e., dxy/yz, dx2-y2/xy,
and dz2 under trigonal prismatic ligand field. In T phase, the d orbitals can be
divided into t2g (dxy, dyz, dxz) and eg (dx2-y2, dz2) orbitals under octahedral
ligand field. In D2h phase of MX2, the degenerated d orbitals further split
owing to the reduction of symmetry. As observed in MoS2 monolayer with D2h
symmetry, it induces two spin-polarized electrons occupying dxy and dz2, leaving
dxz state unoccupied. Obviously, moderate splitting between the occupied and the
unoccupied d orbitals brings out a relatively small gap of 0.21 eV in D2h
symmetry instead of a severe splitting of ∼1.0 eV in semiconducting H phase. As
a result of such electronic structure, the magnetic moment per Mo atom reaches
2.0 μB at high-spin state (S = 1).
Chromium sulfide halides Cr-X-Y (X = S, Se, Te; Y = Cl, Br, I) with chemical
composition from transition metal dichalcohalides to di-halides have been
theoretically predicted. Similar to transition metal di-halides discussed in
Sec. III A 2, monolayer Cr-X-Y systems are ferromagnetic semiconductors, having
large spin polarization and high Curie temperature of 100∼500 K. Based on 1825
easily or potentially exfoliable compounds from high-throughput search of the
crystalline materials database, 36 monolayer 2D ferromagnets have been
identified.6767. N. Mounet, M. Gibertini, P. Schwaller, D. Campi, A. Merkys, A.
Marrazzo, T. Sohier, I. E. Castelli, A. Cepellotti, G. Pizzi, and N. Marzari,
Nat. Nanotechnol. 13(3), 246 (2018). https://doi.org/10.1038/s41565-017-0035-5
Among them, a noticeable system is CrSBr monolayer. By carefully examining its
magnetic behavior, Jiang et al.6565. Z. Jiang, P. Wang, J. Xing, X. Jiang, and
J. Zhao, ACS Appl. Mater. Inter. 10(45), 39032 (2018).
https://doi.org/10.1021/acsami.8b14037 demonstrated that 2D CrSBr is a
semiconductor possessing a large magnetic moment of ∼3 μB per Cr atom and a high
Curie temperature (TC = 290 K). The robust ferromagnetism of the CrSBr monolayer
has been ascribed to the halogen-mediated (Cr−Br−Cr) and chalcogen-mediated
(Cr−S−Cr) superexchange interactions. Based on that mechanism, they further
proposed an isoelectronic substitution strategy to tailor the magnetic coupling
strength. Finally, CrSI, CrSCl, and CrSeBr were also predicted as stable FM
semiconductors with appreciable Curie temperatures of 330, 500, and 500 K,
respectively [see Fig. 13(e)].6565. Z. Jiang, P. Wang, J. Xing, X. Jiang, and J.
Zhao, ACS Appl. Mater. Inter. 10(45), 39032 (2018).
https://doi.org/10.1021/acsami.8b14037 Several other theoretical studies also
found that CrTX (T = S, Se, Te; X = Cl, Br, I) monolayers are FM
semiconductors.26,70,13126. C. Wang, X. Zhou, L. Zhou, N.-H. Tong, Z.-Y. Lu, and
W. Ji, Sci. Bull. 64(5), 293 (2019).
https://doi.org/10.1016/j.scib.2019.02.01170. R. Han, Z. Jiang, and Y. Yan, J.
Phys. Chem. C 124(14), 7956 (2020). https://doi.org/10.1021/acs.jpcc.0c01307131.
Y. Guo, Y. Zhang, S. Yuan, B. Wang, and J. Wang, Nanoscale 10(37), 18036 (2018).
https://doi.org/10.1039/C8NR06368K Besides high Curie temperature (100∼500 K),
large perpendicular magnetic anisotropy, wide range of bandgaps, high carrier
mobilities, strong anisotropy of carrier effective mass, and large light
absorption are also found in these materials, suggesting that this 2D family
holds potential for high performance electronic and spintronic devices. From a
material database containing around 560 monolayer compounds of MXY (M = metal; X
= S, Se, Te; Y = F, Cl, Br, I), 46 potential magnetic semiconductors have been
further identified from HSE06 calculations.132132. J. Pan, J. Yu, Y.-F. Zhang,
S. Du, A. Janotti, C.-X. Liu, and Q. Yan, npj Computat. Mater. 6, 152 (2020).
https://doi.org/10.1038/s41524-020-00419-y Among them, Curie temperatures of the
newly reported TiTeI, VSI, VSeI, MoSI, WSeI, WTeI monolayers are 46, 1100, 913,
270, 76, 302, and 479 K, respectively.
Starting from 3D AFM transition metal oxyhalides, Miao et al.322322. N. Miao, B.
Xu, L. Zhu, J. Zhou, and Z. Sun, J. Am. Chem. Soc. 140(7), 2417 (2018).
https://doi.org/10.1021/jacs.7b12976 proposed that the 2D CrOCl and CrOBr
monolayers can be obtained by mechanical cleavage and further predicted that
they are intrinsic ferromagnetic semiconductors with bandgaps of 2.38 and 1.59
eV as well as Curie temperatures of 160 and 129 K [Fig. 13(f)], respectively.
Calculated with the same method, the TC of CrOF sheet may even exceed those of
CrOCl and CrOBr and reach up to ∼200 K.323323. T. Xiao, G. Wang, and Y. Liao,
Chem. Phys. 513, 182 (2018). https://doi.org/10.1016/j.chemphys.2018.08.007 The
spins in both CrOCl and CrOBr monolayers align along the out-of-plane direction
with considerably large MAE (0.03∼0.29 meV). These results have motivated some
successive investigations on the magnetic properties of transition metal
oxyhalides MOX (X = Cl, Br, I). All these FeOX (X = F, Cl, Br, I) monolayers are
theoretically stable and could be exfoliated from their bulk phase.133133. S.
Wang, J. Wang, and M. Khazaei, Phys. Chem. Chem. Phys. 22(20), 11731 (2020).
https://doi.org/10.1039/D0CP01767A These FeOX monolayers are revealed to be Mott
insulators with bandgaps from 2.73 to 0.48 eV as element X changes from F to I.
For all FeOX monolayers, the in-plane and inter-plane magnetic interactions
between Fe atoms are dominated by AFM coupling. The Néel temperatures of FeOF
and FeOI monolayers are 130 and 150 K, respectively. In addition, 2D VBrO,
TiClO, VClO were predicted to be FM half-metals,132132. J. Pan, J. Yu, Y.-F.
Zhang, S. Du, A. Janotti, C.-X. Liu, and Q. Yan, npj Computat. Mater. 6, 152
(2020). https://doi.org/10.1038/s41524-020-00419-y while 2D VOF was a FM
semiconductor.6666. H. Liu, J.-T. Sun, M. Liu, and S. Meng, J. Phys. Chem. Lett.
9(23), 6709 (2018). https://doi.org/10.1021/acs.jpclett.8b02783
Similar to transition halides, the ground state of 2D δ-FeOOH monolayer is AFM
with an indirect bandgap of 2.4 eV, which is derived from bulk Fe(OH)2 via
oxidation.324324. I. Khan, A. Hashmi, M. U. Farooq, and J. Hong, ACS Appl.
Mater. Inter. 9(40), 35368 (2017). https://doi.org/10.1021/acsami.7b08499 The
δ-FeOOH ultrathin films with thicknesses of 1.1∼1.3 nm have been experimentally
synthesized via a topochemical transformation process. These films exhibit
room-temperature ferromagnetism along with semiconducting behavior.325325. P.
Chen, K. Xu, X. Li, Y. Guo, D. Zhou, J. Zhao, X. Wu, C. Wu, and Y. Xie, Chem.
Sci. 5(6), 2251 (2014). https://doi.org/10.1039/C3SC53303D Besides the 2D
ternary sulfide halides and oxyhalides, intrinsic magnets have also been
observed in nitride halides, such as FeNF, MnNF, MnNCl, and MnNBr,26,13226. C.
Wang, X. Zhou, L. Zhou, N.-H. Tong, Z.-Y. Lu, and W. Ji, Sci. Bull. 64(5), 293
(2019). https://doi.org/10.1016/j.scib.2019.02.011132. J. Pan, J. Yu, Y.-F.
Zhang, S. Du, A. Janotti, C.-X. Liu, and Q. Yan, npj Computat. Mater. 6, 152
(2020). https://doi.org/10.1038/s41524-020-00419-y and the corresponding Curie
temperatures are 398, 238, 261, and 492 K, respectively.
Since the 3D AMnBi family (A = K, Rb, Cs) with layered structure have strong AFM
coupling between the Mn layers and weak interlayer coupling, AFM ordering is
also anticipated at their 2D limit.326326. Z. Zhu, C. Liao, S. Li, X. Zhang, W.
Wu, Z.-M. Yu, R. Yu, W. Zhang, and S. A. Yang, arXiv preprint arXiv:2003.10671
(2020). According to DFT calculations, AFM state is indeed favored for all of
the 2D KMnBi, RbMnBi, and CsMnBi as the ground state. The strong Coulomb
interaction arising from d orbitals of Mn atom results in their magnetic state.
The easy magnetic axis is along the z direction with MAE values of 0.86∼1.1 meV.
The Néel temperature is about 302–307 K, which is higher than that of the value
of 2D FePS3 (118 K). Moreover, the mobilities for both electron and hole
carriers are in the order of 103 cm2/(V·s), which is higher than 2D MoS2 at room
temperature.327327. X. Li, T. Gao, and Y. Wu, Sci. China Inform. Sci. 59(6),
061405 (2016). https://doi.org/10.1007/s11432-016-5559-z Two-dimensional K2CoS2
is also an in-plane antiferromagnetic insulator,328328. A. B. Sarkar, B. Ghosh,
B. Singh, S. Bhowmick, H. Lin, A. Bansil, and A. Agarwal, arXiv preprint
arXiv:2005.12868 (2020). and MC simulations predicted its transition temperature
to be TN ≈ 15 K. Remarkably, bulk K2CoS2 also hosts an in-plane AFM state, and
its magnetic ordering can persist even in ultrathin films down to monolayer
limit.
F. 2D f-electron magnets
Up to now, most reported 2D magnets are based on d electrons. Generally
speaking, d electrons are more localized and have stronger correlation effect
than s and p electrons, which is the basic requirement of magnetic state.
Compared to the d-electron based materials, f electrons are even more localized;
thus, the direct overlap of f orbitals between neighboring rare earth atoms as
well as the hybridization between f orbitals and p orbitals of neighbor anions,
are mostly negligible. As a result, the direct exchange and superexchange
interactions mediated by anions are usually very weak between rare earth
magnetic anions, which is the most serious drawback for finding high-temperature
f magnetism. However, the f-electron based 2D materials still have many
advantages for future spintronic applications. Especially, f electrons usually
have much stronger spin-orbit coupling than d electrons, which in turn leads to
stronger magnetocrystalline anisotropy. Hence, it is still meaningful to design
2D f-electron magnets with high TC. To this end, a few pioneer works have been
reported.
In the 2D f-electron magnets, the most attractive element is gadolinium (Gd),
which has half-filled and well-localized 4f subshell leading to ferromagnetic
behavior with high saturation magnetization. Experimentally, Ormaza et
al.329329. M. Ormaza, L. Fernández, M. Ilyn, A. Magaña, B. Xu, M. J. Verstraete,
M. Gastaldo, M. A. Valbuena, P. Gargiani, A. Mugarza, A. Ayuela, L. Vitali, M.
Blanco-Rey, F. Schiller, and J. E. Ortega, Nano Lett. 16(7), 4230 (2016).
https://doi.org/10.1021/acs.nanolett.6b01197 reported that single-layer GdAg2
grown on Ag(111) is ferromagnetic with a Curie temperature of 85 K [Fig. 14(a)].
All the bands are spin-polarized owing to the presence of half-filled Gd 4f
orbitals. The exchange interaction between Gd atoms is mediated by s, p–d Ag–Gd
hybrid bands, similar as the effective s–d hybrid bands in pure Gd
solid.329,330329. M. Ormaza, L. Fernández, M. Ilyn, A. Magaña, B. Xu, M. J.
Verstraete, M. Gastaldo, M. A. Valbuena, P. Gargiani, A. Mugarza, A. Ayuela, L.
Vitali, M. Blanco-Rey, F. Schiller, and J. E. Ortega, Nano Lett. 16(7), 4230
(2016). https://doi.org/10.1021/acs.nanolett.6b01197330. A. Correa, B. Xu, M. J.
Verstraete, and L. Vitali, Nanoscale 8(45), 19148 (2016).
https://doi.org/10.1039/C6NR06398E Twofold degenerate Weyl nodal lines in a 2D
single-layer Gd-Ag compound were observed by combining angle-resolved
photoemission spectroscopy measurements and theoretical calculations.331331. B.
Feng, R.-W. Zhang, Y. Feng, B. Fu, S. Wu, K. Miyamoto, S. He, L. Chen, K. Wu, K.
Shimada, T. Okuda, and Y. Yao, Phys. Rev. Lett. 123(11), 116401 (2019).
https://doi.org/10.1103/PhysRevLett.123.116401 Lei et al.332332. S. Lei, J. Lin,
Y. Jia, M. Gray, A. Topp, G. Farahi, S. Klemenz, T. Gao, F. Rodolakis, J. L.
McChesney, C. R. Ast, A. Yazdani, K. S. Burch, S. Wu, N. P. Ong, and L. M.
Schoop, Sci. Adv. 6(6), eaay6407 (2020). https://doi.org/10.1126/sciadv.aay6407
demonstrated that GdTe3 can be exfoliated to ultrathin flakes. The obtained
monolayer GdTe3 flake is an antiferromagnet, exhibiting a relatively high
carrier mobility.
FIG. 14. (a) STM image, remanent magnetization at zero applied field, and
angle-resolved photoemission of a GdAg2 monolayer alloy. (b)
Temperature-dependent spin moments of three different K2N AXenes phases. (c) 3D
band structure of the spin-down channel around the Fermi level. (d) PDOS of N
and Y atoms for 1T-YN2. (e) PDOS of s and p orbitals of N atom for 1T-YN2. (f)
Schematic diagram for the origin of magnetic moment of 1T-YN2 monolayer. Panel
(a) reproduced with permission from Ormaza et al., Nano Lett. 16, 4230 (2016).
Copyright 2016 American Chemical Society.329329. M. Ormaza, L. Fernández, M.
Ilyn, A. Magaña, B. Xu, M. J. Verstraete, M. Gastaldo, M. A. Valbuena, P.
Gargiani, A. Mugarza, A. Ayuela, L. Vitali, M. Blanco-Rey, F. Schiller, and J.
E. Ortega, Nano Lett. 16(7), 4230 (2016).
https://doi.org/10.1021/acs.nanolett.6b01197 Panel (b) reproduced with
permission from Jiang et al., J. Phys. Chem. Lett. 10, 7753 (2019). Copyright
2019 American Chemical Society.341341. X. Jiang, Q. Liu, J. Xing, and J. Zhao,
J. Phys. Chem. Lett. 10(24), 7753 (2019).
https://doi.org/10.1021/acs.jpclett.9b03030 Panels (c)–(f) reproduced with
permission from Liu et al., Nano Res. 10, 1972 (2017). Copyright 2017 Springer
Nature.66. Z. Liu, J. Liu, and J. Zhao, Nano Res. 10(6), 1972 (2017).
https://doi.org/10.1007/s12274-016-1384-3
* PPT
|
* High-resolution
Rare earth metal functionalized silicene MSi2 (M = Eu, Gd) were successfully
synthesized by the reaction of M with Si(111) substrate using MBE technique.
Strong magnetic response of transport was reported in EuSi2 and GdSi2
monolayers, suggesting indirect exchange interaction between localized f-shells
of Eu/Gd and p orbitals of silicene. The extended pz states of silicene can
mediate the long-range magnetic interactions.333333. O. E. Parfenov, A. M.
Tokmachev, D. V. Averyanov, I. A. Karateev, I. S. Sokolov, A. N. Taldenkov, and
V. G. Storchak, Mater. Today 29, 20 (2019).
https://doi.org/10.1016/j.mattod.2019.03.017 In addition, 4f-electron based
EuGe2 and GdGe2334334. A. M. Tokmachev, D. V. Averyanov, A. N. Taldenkov, O. E.
Parfenov, I. A. Karateev, I. S. Sokolov, and V. G. Storchak, Mater. Horiz. 6(7),
1488 (2019). https://doi.org/10.1039/C9MH00444K were also synthesized by direct
reaction between these elements, with thickness from bulk down to monolayer. The
common pattern is a transformation from 3D antiferromagnetism to 2D
ferromagnetism, as revealed by magnetization and electron transport
measurements.
Theoretically, high-temperature f-electron FM semiconductor has been found in
GdI2 monolayer.129129. B. Wang, X. Zhang, Y. Zhang, S. Yuan, Y. Guo, S. Dong,
and J. Wang, Mater. Horiz. 7, 1623 (2020). https://doi.org/10.1039/D0MH00183J
The highly localized 4f electrons often lead to weak direct exchange and
superexchange interactions. However, due to the coexistence of spin-polarized 5d
orbitals and 4f orbitals in Gd2+ cation, the strong direct interaction between
these two orbitals is found to determine the FM ground state of GdI2 monolayer,
whereas there still exists Gd 5d–I 5p–Gd 5d superexchange interaction. As a
result, high TC (241 K), large magnetization (8 μB/f.u.), and large magnetic
anisotropy energy (0.553 meV/Gd) were observed in GdI2 monolayer. According to a
recent DFT study, Gd2B2 monolayer is a ferromagnetic metal, and its
ferromagnetic state can sustain above room temperature as high as TC = 550 K
with a huge magnetic moment of μ = 7.30 μB per Gd atom.335335. T. Gorkan, E.
Vatansever, U. Akɪncɪ, G. Gökoglu, E. Aktürk, and S. Ciraci, J. Phys. Chem. C
124, 12816 (2020). https://doi.org/10.1021/acs.jpcc.0c03304
In addition to the 4f systems, Zhang et al.336336. S. Li, Z. Wang, M. Zhou, F.
Zheng, X. Shao, and P. Zhang, J. Phys. D: Appl. Phys. 53(18), 185301 (2020).
https://doi.org/10.1088/1361-6463/ab740c firstly reported a potential
5f-electron 2D magnet, i.e., hexagonal UI3 monolayer, using DFT calculations and
MC simulations. Non-SOC calculations revealed that UI3 monolayer is a Weyl
semi-metal, while it becomes a semiconductor with a bandgap of 0.18 eV after
inclusion of SOC effect. The projected density of states showed that the
magnetism comes from f electrons of U atom. Noticeably, its exchange parameter
and anisotropic energy are one order of magnitude larger than those of CrI3
monolayer. Hence, the estimated Curie temperature was 110 K.336336. S. Li, Z.
Wang, M. Zhou, F. Zheng, X. Shao, and P. Zhang, J. Phys. D: Appl. Phys. 53(18),
185301 (2020). https://doi.org/10.1088/1361-6463/ab740c
G. 2D p-electron magnets
Among the 2D magnets discussed above, a common feature is that the high-spin
states are guaranteed by the magnetic metal elements with partially filled 3d or
4f subshells. Apart from these conventional classes of 2D d/f ferromagnets,
robust magnetic coupling has also been observed in many materials with partially
occupied and localized/delocalized p orbitals, namely, p-electron magnets.
Similar to d and f orbitals, the partially occupied p orbitals with certain
localized character are mainly responsible for the magnetism. It is also
explained by the crystal symmetry protected flat bands model. Compared to the 2D
d/f-electron magnets, p orbitals in the superexchange interaction are more
delocalized, which is beneficial for the long-distance spin coupling. Thus,
these 2D p-electron magnetic materials are likely to possess higher Fermi
velocity and longer spin coherence length due to greater delocalization of p
orbitals and smaller SOC strength, which are prominent advantages for high-speed
and long-distance spin transport.
According to the origin of magnetism, 2D p-electron magnets can be divided into
two categories—d0 magnetism and dn magnetism with paired spin-antiparallel d
electron. Clearly, d0 magnetism would be observed in many 2D materials without
transition metal, rare earth, or actinide elements. First, these novel d0
magnetism may originate from their hosting flat bands, which resulted in high
density of states in the vicinity of the Fermi levels. Both theoretical and
experimental studies proposed that zigzag graphene nanoribbons with localized
edge states are room-temperature antiferromagnets.337–339337. Y.-W. Son, M. L.
Cohen, and S. G. Louie, Nature 444(7117), 347 (2006).
https://doi.org/10.1038/nature05180338. G. Z. Magda, X. Jin, I. Hagymási, P.
Vancsó, Z. Osváth, P. Nemes-Incze, C. Hwang, L. P. Biró, and L. Tapasztó, Nature
514(7524), 608 (2014). https://doi.org/10.1038/nature13831339. M. Slota, A.
Keerthi, W. K. Myers, E. Tretyakov, M. Baumgarten, A. Ardavan, H. Sadeghi, C. J.
Lambert, A. Narita, and K. Müllen, Nature 557(7707), 691 (2018).
https://doi.org/10.1038/s41586-018-0154-7 The intrinsic magnetic ordering
survives at room temperature in a novel B5N5 monolayer allotrope with decorated
bounce lattice, which is also a 2D antiferromagnetic insulator. The
antiferromagnetism arises from the nearly flat bands at the vicinity of the
Fermi energy.340340. D. Zhang, Q. Xiong, and K. Chang, Nanoscale Adv. 2, 4421
(2020). https://doi.org/10.1039/D0NA00270D
Second, d0 magnetism has been found in a few artificially designed 2D materials
with nonstoichiometric compositions.6,3416. Z. Liu, J. Liu, and J. Zhao, Nano
Res. 10(6), 1972 (2017). https://doi.org/10.1007/s12274-016-1384-3341. X. Jiang,
Q. Liu, J. Xing, and J. Zhao, J. Phys. Chem. Lett. 10(24), 7753 (2019).
https://doi.org/10.1021/acs.jpclett.9b03030 The periodic nonstoichiometric
compound refers to the order compound with either cation deficient or electron
deficient rather than the atomic defects, such as experimentally obtained 2D
Na2Cl and Na3Cl342342. G. Shi, L. Chen, Y. Yang, D. Li, Z. Qian, S. Liang, L.
Yan, L. H. Li, M. Wu, and H. Fang, Nat. Chem. 10(7), 776 (2018).
https://doi.org/10.1038/s41557-018-0061-4 and theoretically predicted K2N341341.
X. Jiang, Q. Liu, J. Xing, and J. Zhao, J. Phys. Chem. Lett. 10(24), 7753
(2019). https://doi.org/10.1021/acs.jpclett.9b03030 and YN2.66. Z. Liu, J. Liu,
and J. Zhao, Nano Res. 10(6), 1972 (2017).
https://doi.org/10.1007/s12274-016-1384-3 Such unconventional compounds may lead
to long-range ordered unpaired p electrons in the magnetic lattice. By carefully
considering the compositions and electronic configurations, a series of p-state
intrinsic ferromagnetic A2N compounds of alkali metal (A) and nitrogen (N),
namely, AXenes, have been proposed.341341. X. Jiang, Q. Liu, J. Xing, and J.
Zhao, J. Phys. Chem. Lett. 10(24), 7753 (2019).
https://doi.org/10.1021/acs.jpclett.9b03030 Taking 2D K2N as an example, all of
the three predicted phases (H, T, and I) are half metals with high Curie
temperature of 480∼1180 K [Fig. 14(b)]. As expected, the ferromagnetism in all
three phases is mainly contributed by N atoms, with on-site moment of 0.81,
0.72, and 0.79 μB, respectively. Meanwhile, their high Curie temperatures arise
from the coexistence of N–K–N superexchange and the carrier-mediated interaction
mechanism. Very recently, Jin et al.343343. L. Jin, X. Zhang, Y. Liu, X. Dai, X.
Shen, L. Wang, and G. Liu, Phys. Rev. B 102, 125118 (2020).
https://doi.org/10.1103/PhysRevB.102.125118 further found that K2N monolayer
with D3h point group shows two nodal lines in its low-energy band structures,
and these nodal lines are robust against weak SOC. Such feasible
nonstoichiometric strategy has also attained long-range p electron magnetic
ordering in 2D C3Ca2 and Na2C with Honeycomb-Kagome lattice.344,345344. W.-x.
Ji, B.-m. Zhang, S.-f. Zhang, C.-w. Zhang, M. Ding, P. Li, and P.-j. Wang, J.
Mater. Chem. C 5(33), 8504 (2017). https://doi.org/10.1039/C7TC02700A345. W.-X.
Ji, B.-M. Zhang, S.-F. Zhang, C.-W. Zhang, M. Ding, P.-J. Wang, and R. Zhang,
Nanoscale 10(28), 13645 (2018). https://doi.org/10.1039/C8NR02761G
First-principles and tight-binding calculations revealed that both of them are
intrinsic Dirac half-metals. Specifically, 2D C3Ca2 has a high-spin
ferromagnetic configuration of 8 μB per unit cell with a Curie temperature of
30.7 K, which is mainly contributed by the 2p orbitals of carbon atoms. The
mechanism of magnetism could be understood by the double exchange between carbon
anions using Ca2+ cations as bridges. In a similar manner, ferromagnetism of 2D
Na2C is also mainly contributed by the unpaired 2p electrons of carbon with an
estimated Curie temperature of 382 K. The origin of such 2p magnetism could be
explained by the superexchange mechanism between C2− anions with Na+ cations as
bridges. Indeed, the calculated Fermi velocities reach up to ∼105 ms−1, which
are promising for high-speed spintronic devices.
Novel 2D p-electron magnets with paired d electrons that have antiparallel spin
orientation have been reported in a few transition metal compounds, such as
MoN2, Y2N, MoN2, TcN2, TaN2, NbN2, and LaBr2. Using first-principles
calculations, 1H-MoN2 monolayer was theoretically proposed to be a 2D p-electron
intrinsically FM material with a high Curie temperature of 420 K,346346. F. Wu,
C. Huang, H. Wu, C. Lee, K. Deng, E. Kan, and P. Jena, Nano Lett. 15(12), 8277
(2015). https://doi.org/10.1021/acs.nanolett.5b03835 while it can be exfoliated
experimentally.347347. S. Wang, H. Ge, S. Sun, J. Zhang, F. Liu, X. Wen, X. Yu,
L. Wang, Y. Zhang, H. Xu, J. C. Neuefeind, Z. Qin, C. Chen, C. Jin, Y. Li, D.
He, and Y. Zhao, J. Am. Chem. Soc. 137(14), 4815 (2015).
https://doi.org/10.1021/jacs.5b01446 Liu et al. found that 2D Y2N with
octahedral coordination is a novel p-state Dirac half metal,66. Z. Liu, J. Liu,
and J. Zhao, Nano Res. 10(6), 1972 (2017).
https://doi.org/10.1007/s12274-016-1384-3 as shown in Fig. 14(c). From the PDOS
in Figs. 14(d) and 14(e), one can clearly see that N atoms instead of Y atoms
contribute to the Dirac states. More interestingly, the half-metallic gap is
1.53 eV, the Fermi velocity is 3.74 × 105 m/s, and the Curie temperature
estimated by mean-field approximation reaches over 332 K. Motivated by these
studies, 2D p-electron intrinsic magnets have also been explored by
first-principles computational search of thirty possible monolayer structures of
transition metal dinitride. Among them, 1H-MoN2 and 1H-TcN2 are 2D p-state
intrinsically ferromagnetic metals, while monolayer 1T-TaN2 and 1T-NbN2 are 2D
p-state intrinsically ferromagnetic half-metals.124,348124. J. Liu, Z. Liu, T.
Song, and X. Cui, J. Mater. Chem. C 5(3), 727 (2017).
https://doi.org/10.1039/C6TC04490E348. R. Li, B. Yang, F. Li, Y. Wang, X. Du,
and Y. Yan, J. Phys.: Condens. Matter 31(33), 335801 (2019).
https://doi.org/10.1088/1361-648X/ab1fbb For all these materials, the robust FM
ground state originates from the strong N–N direct exchange interaction.
Taking 1T-YN2 monolayer as an example, we discuss the origin of paired d
electrons and unpaired p electrons induced magnetism in Fig. 14(f).66. Z. Liu,
J. Liu, and J. Zhao, Nano Res. 10(6), 1972 (2017).
https://doi.org/10.1007/s12274-016-1384-3 The ground state electronic
configurations of neutral Y and N atoms are 4d15s2 and 3s23p3, respectively. In
YN2 monolayer, one of the two 5s electrons in Y atom occupies 4d orbital of Y
and the other one transfers to N-2p orbital, resulting in 4d25s0 electronic
configurations of Y2+ cation. Consequently, Y-4d orbitals are occupied by two
spin-antiparallel electrons, exhibiting nearly zero magnetic moment. Meanwhile,
3p states of the two N atoms are occupied by a total of eleven 3p electrons via
gaining one Y-5s electron. Following the eight-electron rule and assuming a
nearly electron-free gas model, one can conclude that these eleven 3p electrons
would exhibit an electronic shell configuration like: ↑↓ ↑↓ ↑↓ ↑↓ ↑ ↑ ↑. In
other words, the three unpaired electrons would result in a magnetic moment of
3 μB per YN2 formula unit, as obtained from spin-polarized DFT calculations.
H. 2D organic magnets
Apart from the rich family of 2D inorganic magnets, 2D organic magnetic
materials have also attracted considerable attention due to their molecular
diversity, flexibility in synthesis, easy processing, low cost, well-defined
geometry, and potential applications in quantum Hall effect, magnetic storage,
and spintronics. Research of the two fundamental physical concepts, i.e.,
exchange interaction and spin orbit coupling, represents the important branches
of 2D organic magnets. Generally speaking, the magnetic properties of two
conceptual classes of 2D organic materials have been discussed extensively in
recent literature. One class of 2D organic materials is 2D metal organic
frameworks (MOF), which is a kind of long-range network constructed by organic
linkers and metal ion centers. The magnetism can be implemented by incorporating
metal ions as the magnetic carriers. Another class of 2D organic magnets is
covalent organic frameworks (COFs), which are formed by covalent bonding of the
atoms of light elements (H, B, C, N and O). Without magnetic metal ions, the
magnetism in 2D COF can be implemented by incorporating open-shell organic
ligands. In addition, both ordered 2D MOF and COF conformations render the
connection between magnetic moment carriers within an interacting
distance.101,349–352101. J. Zhou and Q. Sun, J. Am. Chem. Soc. 133(38), 15113
(2011). https://doi.org/10.1021/ja204990j349. M. Kurmoo, Chem. Soc. Rev. 38(5),
1353 (2009). https://doi.org/10.1039/b804757j350. S. Yang, W. Li, C. Ye, G.
Wang, H. Tian, C. Zhu, P. He, G. Ding, X. Xie, and Y. Liu, Adv. Mater. 29(16),
1605625 (2017). https://doi.org/10.1002/adma.201605625351. A. Du, S. Sanvito,
and S. C. Smith, Phys. Rev. Lett. 108(19), 197207 (2012).
https://doi.org/10.1103/PhysRevLett.108.197207352. E. Kan, W. Hu, C. Xiao, R.
Lu, K. Deng, J. Yang, and H. Su, J. Am. Chem. Soc. 134(13), 5718 (2012).
https://doi.org/10.1021/ja210822c The key electronic and magnetic properties of
2D MOF and COF are listed in Table V for discussion.
TABLE V. A list of 2D organic magnets with their compositions and representative
electronic and magnetic properties, including the magnetic ground state (GS),
the values of Hubbard U term, energy gap (Eg), magnetic moment on per transition
metal (Ms), Curie temperature (TC), and magnetic anisotropy energy per unit cell
(MAE).
MOF Compositions GS U (eV) Eg (eV) Ms (μB) TC (K) MAE (meV) Ref. 3d@Pc Cr@Pc AFM
3 0.36 4 – – 101101. J. Zhou and Q. Sun, J. Am. Chem. Soc. 133(38), 15113
(2011). https://doi.org/10.1021/ja204990j Cr@Pc-kag AFM – 0.94 4 – 0.67 362362.
H. Chen, H. Shan, A. Zhao, and B. Li, Chinese J. Chem. Phys. 32(5), 563 (2019).
https://doi.org/10.1063/1674-0068/cjcp1810227 Mn@Pc FM 3 HM 3 150 – 101101. J.
Zhou and Q. Sun, J. Am. Chem. Soc. 133(38), 15113 (2011).
https://doi.org/10.1021/ja204990j Mn@Pc-kag FM – 0.09 3 125 1.18 362362. H.
Chen, H. Shan, A. Zhao, and B. Li, Chinese J. Chem. Phys. 32(5), 563 (2019).
https://doi.org/10.1063/1674-0068/cjcp1810227 Fe@Pc FM – Metal 1.95 – – 357357.
B. Białek, I. G. Kim, and J. I. Lee, Surf. Sci. 526(3), 367 (2003).
https://doi.org/10.1016/S0039-6028(03)00002-5 Fe@Pc AFM 3 0.24 2 – – 101101. J.
Zhou and Q. Sun, J. Am. Chem. Soc. 133(38), 15113 (2011).
https://doi.org/10.1021/ja204990j Fe@Pc-kag FM – 0.32 2 – – 362362. H. Chen, H.
Shan, A. Zhao, and B. Li, Chinese J. Chem. Phys. 32(5), 563 (2019).
https://doi.org/10.1063/1674-0068/cjcp1810227 Co@Pc AFM – Metal 1.01 – – 363363.
B. Białek, I. G. Kim, and J. I. Lee, Thin Solid Films 513(1–2), 110 (2006).
https://doi.org/10.1016/j.tsf.2006.01.050 Co@Pc AFM 3 0.10 1 – – 101101. J. Zhou
and Q. Sun, J. Am. Chem. Soc. 133(38), 15113 (2011).
https://doi.org/10.1021/ja204990j Co@Pc-kag FM – 1.09 1 – – 362362. H. Chen, H.
Shan, A. Zhao, and B. Li, Chinese J. Chem. Phys. 32(5), 563 (2019).
https://doi.org/10.1063/1674-0068/cjcp1810227 Ni@Pc PM – 0.7 0 – – 365365. B.
Białek, I. G. Kim, and J. I. Lee, Synth. Met. 129(2), 151 (2002).
https://doi.org/10.1016/S0379-6779(02)00042-5 Ni@Pc NM 3 0.34 0 – – 101101. J.
Zhou and Q. Sun, J. Am. Chem. Soc. 133(38), 15113 (2011).
https://doi.org/10.1021/ja204990j Ni@Pc-kag NM – 1.31 0 – – 362362. H. Chen, H.
Shan, A. Zhao, and B. Li, Chinese J. Chem. Phys. 32(5), 563 (2019).
https://doi.org/10.1063/1674-0068/cjcp1810227 Cu@Pc PM – 0.56 0.56 – – 364364.
B. Białek, I. G. Kim, and J. I. Lee, Thin Solid Films 436(1), 107 (2003).
https://doi.org/10.1016/S0040-6090(03)00521-2 Cu@Pc PMi – 1.5 – – – 358358. D.
M. Sedlovets, M. V. Shuvalov, Y. V. Vishnevskiy, V. T. Volkov, I. I. Khodos, O.
V. Trofimov, and V. I. Korepanov, Mater. Res. Bull. 48(10), 3955 (2013).
https://doi.org/10.1016/j.materresbull.2013.06.015 Cu@Pc AFMi – – – – – 359359.
Z. Honda, Y. Sakaguchi, M. Tashiro, M. Hagiwara, T. Kida, M. Sakai, T. Fukuda,
and N. Kamata, Appl. Phys. Lett. 110(13), 133101 (2017).
https://doi.org/10.1063/1.4979030 Cu@Pc AFM 3 0.31 1 – – 101101. J. Zhou and Q.
Sun, J. Am. Chem. Soc. 133(38), 15113 (2011). https://doi.org/10.1021/ja204990j
Cu@Pc-kag FM – 1.34 1 – – 362362. H. Chen, H. Shan, A. Zhao, and B. Li, Chinese
J. Chem. Phys. 32(5), 563 (2019). https://doi.org/10.1063/1674-0068/cjcp1810227
Zn@Pc NM 3 0.30 0 – – 101101. J. Zhou and Q. Sun, J. Am. Chem. Soc. 133(38),
15113 (2011). https://doi.org/10.1021/ja204990j Zn@Pc-kag NM – 1.34 0 – –
362362. H. Chen, H. Shan, A. Zhao, and B. Li, Chinese J. Chem. Phys. 32(5), 563
(2019). https://doi.org/10.1063/1674-0068/cjcp1810227 5d@Pc W@Pc AFM – – 2.4 –
19.9 360360. P. Wang, X. Jiang, J. Hu, X. Huang, and J. Zhao, J. Mater. Chem. C
4(11), 2147 (2016). https://doi.org/10.1039/C5TC04402B Re@Pc FM – – 2.4 626 20.7
360360. P. Wang, X. Jiang, J. Hu, X. Huang, and J. Zhao, J. Mater. Chem. C
4(11), 2147 (2016). https://doi.org/10.1039/C5TC04402B 5d2@Pc Ta2@Pc AFM – –
1.6/0.2 26.9 360360. P. Wang, X. Jiang, J. Hu, X. Huang, and J. Zhao, J. Mater.
Chem. C 4(11), 2147 (2016). https://doi.org/10.1039/C5TC04402B Os2@Pc FM – –
1.2/0 52 40.7 360360. P. Wang, X. Jiang, J. Hu, X. Huang, and J. Zhao, J. Mater.
Chem. C 4(11), 2147 (2016). https://doi.org/10.1039/C5TC04402B Ir2@Pc FM – –
1.6/0.2 91 47.2 360360. P. Wang, X. Jiang, J. Hu, X. Huang, and J. Zhao, J.
Mater. Chem. C 4(11), 2147 (2016). https://doi.org/10.1039/C5TC04402B Mo2@Pc AFM
– 0.93 0.88 – – 366366. G. Zhu, M. Kan, Q. Sun, and P. Jena, J. Phys. Chem. A
118(1), 304 (2014). https://doi.org/10.1021/jp4109255 NiM@OIPc NiCr@OIPc AFM 3
0.35 4 – – 367367. W. Li, L. Sun, J. Qi, P. Jarillo-Herrero, M. Dincă, and J.
Li, Chem. Sci. 8(4), 2859 (2017). https://doi.org/10.1039/C6SC05080H NiMn@OIPc
FM 3 HM 3 170 0.74 367367. W. Li, L. Sun, J. Qi, P. Jarillo-Herrero, M. Dincă,
and J. Li, Chem. Sci. 8(4), 2859 (2017). https://doi.org/10.1039/C6SC05080H
NiFe@OIPc AFM 3 0.28 2 – – 367367. W. Li, L. Sun, J. Qi, P. Jarillo-Herrero, M.
Dincă, and J. Li, Chem. Sci. 8(4), 2859 (2017).
https://doi.org/10.1039/C6SC05080H NiCo@OIPc AFM 3 0.35 1 – – 367367. W. Li, L.
Sun, J. Qi, P. Jarillo-Herrero, M. Dincă, and J. Li, Chem. Sci. 8(4), 2859
(2017). https://doi.org/10.1039/C6SC05080H NiCu@OIPc AFM 3 0.3 1 – – 367367. W.
Li, L. Sun, J. Qi, P. Jarillo-Herrero, M. Dincă, and J. Li, Chem. Sci. 8(4),
2859 (2017). https://doi.org/10.1039/C6SC05080H 3d@Pp V@Pp FM – HM 2.54 197 –
7979. H. K. Singh, P. Kumar, and U. V. Waghmare, J. Phys. Chem. C 119(45), 25657
(2015). https://doi.org/10.1021/acs.jpcc.5b09763 Cr@Pp FM 3 Metal 3 187 – 8080.
J. Tan, W. Li, X. He, and M. Zhao, RSC Adv. 3(19), 7016 (2013).
https://doi.org/10.1039/c3ra40502h Mn@Pp AFM 3 – 3.8 – – 8080. J. Tan, W. Li, X.
He, and M. Zhao, RSC Adv. 3(19), 7016 (2013). https://doi.org/10.1039/c3ra40502h
Fe@Pp AFM 3 – 2 – – 8080. J. Tan, W. Li, X. He, and M. Zhao, RSC Adv. 3(19),
7016 (2013). https://doi.org/10.1039/c3ra40502h Co@Pp AFM 3 – 1 – – 8080. J.
Tan, W. Li, X. He, and M. Zhao, RSC Adv. 3(19), 7016 (2013).
https://doi.org/10.1039/c3ra40502h Ni@Pp PM 3 – 0 – – 8080. J. Tan, W. Li, X.
He, and M. Zhao, RSC Adv. 3(19), 7016 (2013). https://doi.org/10.1039/c3ra40502h
Cu@Pp AFM 3 – 1.4 – – 8080. J. Tan, W. Li, X. He, and M. Zhao, RSC Adv. 3(19),
7016 (2013). https://doi.org/10.1039/c3ra40502h Zn@Pp PM 3 – 0 – – 8080. J. Tan,
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The rapid development of 2D organic magnets benefits from the synthesis of
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https://doi.org/10.1021/ja312380b So far, the majority of magnetic frameworks in
2D MOFs contain paramagnetic metal centers, in particular, the open-shell 3d and
5d transition metals. These metal ions, which may exist in different oxidation
states, allow variation of the two important parameters—spin quantum number and
magnetic anisotropy. In addition, the diversity of organic molecule frameworks
offers many opportunities to anchor the magnetic atoms, such as phthalocyanine
(Pc), polyporphyrin (poly-Pp), benzenehexathiolate (BHT), and
5,5′-bis(4-pyridyl)(2,2′-bipirimidine) (PBP). These organic frameworks act as
the medium to couple metal carriers. In addition, the organic ligands impose a
coordination environment on the metal ions, namely, ligand field, which is
important for determining the magnetic behavior. According to the symmetry of
typical molecular architectures, the possible coordination numbers of central
metal ions are 2, 3, 4, and 6.
Four is the most common coordination number for transition metal atoms in 2D
organic magnets, especially Pc, Pp, and BHT frameworks. As an 18-electron
conjugated system, Pc framework is a macrocyclic compound composed of an inner
porphyrazine ring that connects four isoindole groups, giving rise to the
characteristic cross-like shape. Pc framework has a cavity with a diameter of
about 2.70 Å in the center of the large conjugated ring. Transition metal
species embedded in the cavity can chelate with the Pc framework through
coordination bonds and form metal phthalocyanine sheet (M@Pcs) with appreciable
thermal stability. Both experimental and theoretical reports argued that a
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field of 2D magnetism.
Experimentally, Able et al. reported that 2D polymeric arrays of Fe@Pc can be
obtained by co-evaporation of Fe and 1,2,4,5-tetracyanobenzene (TCNB) with 2:1
stoichiometry in ultrahigh vacuum condition onto Au(111), Ag(111) and even
insulating NaCl substrates.361361. M. Abel, S. Clair, O. Ourdjini, M. Mossoyan,
and L. Porte, J. Am. Chem. Soc. 133(5), 1203 (2011).
https://doi.org/10.1021/ja108628r Cu@Pc film can be synthesized through the
reaction of pyromellitic acid tetranitrile (PMTN) with copper in a CVD
set-up.358358. D. M. Sedlovets, M. V. Shuvalov, Y. V. Vishnevskiy, V. T. Volkov,
I. I. Khodos, O. V. Trofimov, and V. I. Korepanov, Mater. Res. Bull. 48(10),
3955 (2013). https://doi.org/10.1016/j.materresbull.2013.06.015 As reported by
Honda et al.,359359. Z. Honda, Y. Sakaguchi, M. Tashiro, M. Hagiwara, T. Kida,
M. Sakai, T. Fukuda, and N. Kamata, Appl. Phys. Lett. 110(13), 133101 (2017).
https://doi.org/10.1063/1.4979030 XRD analysis, TEM characterization, and
magnetization measurements of 2D Cu@Pc sheets revealed the existence of
antiferromagnetic exchange interactions between neighboring Cu2+ ions.
According to the linking way, there are two kinds of TM@Pc structures. As
displayed in Figs. 15(a) and 15(b), they possess tetragonal symmetry with space
group of P4/mmm (M@Pc) and six-fold symmetry within a Kagome lattice (M@Pc-kag),
respectively. Among them, Cr@Pc and Mn@Pc have been proven to be stable
antiferromagnetic semiconductors, while Cr@Pc-kag and Mn@Pc-kag are
ferromagnetic half-metals. MC simulations within Ising model or Heisenberg model
have revealed phase transition between FM and PM states at a critical
temperature of 150 K and 125 K for Mn@Pc and Mn@Pc-kag, respectively.101,362101.
J. Zhou and Q. Sun, J. Am. Chem. Soc. 133(38), 15113 (2011).
https://doi.org/10.1021/ja204990j362. H. Chen, H. Shan, A. Zhao, and B. Li,
Chinese J. Chem. Phys. 32(5), 563 (2019).
https://doi.org/10.1063/1674-0068/cjcp1810227 However, Fe, Co and Cu-based
systems exhibit different magnetic couplings under tetragonal and hexagonal
lattices. The magnetic coupling in M@Pc-kag and M@Pc belongs to weak FM and weak
AFM, respectively. In Mn@Pc and Cr@Pc, dxz and dyz orbitals of metal atoms
hybridize strongly with p electrons of Pc in the proximity of Fermi level,
thereby leading to robust long-range ferromagnetic ordering.101,362101. J. Zhou
and Q. Sun, J. Am. Chem. Soc. 133(38), 15113 (2011).
https://doi.org/10.1021/ja204990j362. H. Chen, H. Shan, A. Zhao, and B. Li,
Chinese J. Chem. Phys. 32(5), 563 (2019).
https://doi.org/10.1063/1674-0068/cjcp1810227 In contrast, the other 2D TM@Pc
frameworks have larger bandgaps so that their magnetic couplings are relatively
weaker.
FIG. 15. Geometric structures of (a) M@Pc, (b) M@Pc-kag, (c) Mo2@Pc, (d)
NiM@OIPc, (e) M@Pp, (f) M@Pp0, (g) M@Pp45, (h) M@BHT-kag, (i) M@BHT, (j) Cr@DPP,
(k) TM@TCNQ, and (l) Mn@T and TM@PBP. Panel (a) reproduced from Honda et al.,
Appl. Phys. Lett. 110, 133101 (2017), with the permission of AIP
Publishing.359359. Z. Honda, Y. Sakaguchi, M. Tashiro, M. Hagiwara, T. Kida, M.
Sakai, T. Fukuda, and N. Kamata, Appl. Phys. Lett. 110(13), 133101 (2017).
https://doi.org/10.1063/1.4979030 Panel (b) reproduced with permission from Chen
et al., Chinese J. Chem. Phys. 32, 563 (2019). Copyright 2019 Chinese Physics
Society.362362. H. Chen, H. Shan, A. Zhao, and B. Li, Chinese J. Chem. Phys.
32(5), 563 (2019). https://doi.org/10.1063/1674-0068/cjcp1810227 Panel (c)
reproduced with permission from Zhu et al., J. Phys. Chem. A 118, 304 (2014).
Copyright 2014 American Chemical Society.366366. G. Zhu, M. Kan, Q. Sun, and P.
Jena, J. Phys. Chem. A 118(1), 304 (2014). https://doi.org/10.1021/jp4109255
Panel (d) reproduced with permission from Li et al., Chem. Sci. 8, 2859 (2017).
Licensed under a Creative Commons Attribution (CC-BY-3.0).367367. W. Li, L. Sun,
J. Qi, P. Jarillo-Herrero, M. Dincă, and J. Li, Chem. Sci. 8(4), 2859 (2017).
https://doi.org/10.1039/C6SC05080H Panel (e) reproduced with permission from
Singh et al., J. Phys. Chem. C 119, 25657 (2015). Copyright 2015 American
Chemical Society.7979. H. K. Singh, P. Kumar, and U. V. Waghmare, J. Phys. Chem.
C 119(45), 25657 (2015). https://doi.org/10.1021/acs.jpcc.5b09763 Panels (f) and
(g) reproduced with permission from Sun et al., J. Mater. Chem. C 3, 6901
(2015). Copyright 2015 Royal Society of Chemistry.368368. Q. Sun, Y. Dai, Y. Ma,
X. Li, W. Wei, and B. Huang, J. Mater. Chem. C 3(26), 6901 (2015).
https://doi.org/10.1039/C5TC01493J Panel (h) reproduced with permission from
Chakravarty et al., J. Phys. Chem. C 120, 28307 (2016). Copyright 2016 American
Chemical Society.372372. C. Chakravarty, B. Mandal, and P. Sarkar, J. Phys.
Chem. C 120(49), 28307 (2016). https://doi.org/10.1021/acs.jpcc.6b09416 Panel
(i) reproduced with permission from Zhang et al., Nano Lett. 17, 6166 (2017).
Copyright 2017 American Chemical Society.375375. X. Zhang, Y. Zhou, B. Cui, M.
Zhao, and F. Liu, Nano Lett. 17(10), 6166 (2017).
https://doi.org/10.1021/acs.nanolett.7b02795 Panel (j) reproduced with
permission from Li et al., J. Phys. Chem. Lett. 10, 2439 (2019). Copyright 2019
American Chemical Society.77. X. Li and J. Yang, J. Phys. Chem. Lett. 10(10),
2439 (2019). https://doi.org/10.1021/acs.jpclett.9b00769 Panel (k) reproduced
with permission from Ma et al., J. Phys. Chem. A 117, 5171 (2013). Copyright
2013 American Chemical Society.378378. Y. Ma, Y. Dai, W. Wei, L. Yu, and B.
Huang, J. Phys. Chem. A 117(24), 5171 (2013). https://doi.org/10.1021/jp402637f
Panel (l) left reproduced with permission from Wang et al., Rev. Lett. 110,
196801 (2013). Copyright 2013 American Physical Society.383383. Z. Wang, Z. Liu,
and F. Liu, Phys. Rev. Lett. 110(19), 196801 (2013).
https://doi.org/10.1103/PhysRevLett.110.196801 Panel (l) right reproduced with
permission from Zhang et al., Chem. Sci. 10, 10381 (2019). Licensed under a
Creative Commons Attribution (CC BY-NC 3.0).385385. L.-C. Zhang, L. Zhang, G.
Qin, Q.-R. Zheng, M. Hu, Q.-B. Yan, and G. Su, Chem. Sci. 10(44), 10381 (2019).
https://doi.org/10.1039/C9SC03816G
* PPT
|
* High-resolution
During 2002 and 2006, Białek's group investigated the electronic structures of a
series of M@Pcs (M = Fe, Co, Ni, and Cu) using the all-electron full-potential
linearized augmented plane wave method.357,363–365357. B. Białek, I. G. Kim, and
J. I. Lee, Surf. Sci. 526(3), 367 (2003).
https://doi.org/10.1016/S0039-6028(03)00002-5363. B. Białek, I. G. Kim, and J.
I. Lee, Thin Solid Films 513(1–2), 110 (2006).
https://doi.org/10.1016/j.tsf.2006.01.050364. B. Białek, I. G. Kim, and J. I.
Lee, Thin Solid Films 436(1), 107 (2003).
https://doi.org/10.1016/S0040-6090(03)00521-2365. B. Białek, I. G. Kim, and J.
I. Lee, Synth. Met. 129(2), 151 (2002).
https://doi.org/10.1016/S0379-6779(02)00042-5 They found that Fe@Pc and Co@Pc
prefer long-range FM and AFM orderings with on-site magnetic moment of about
1.95 and 1.01 μB per Fe and Co atom, respectively. The result of Co@Pc is
consistent with that of Zhou et al.101101. J. Zhou and Q. Sun, J. Am. Chem. Soc.
133(38), 15113 (2011). https://doi.org/10.1021/ja204990j According to the
spin-polarized density of state, Fe@Pc behaves as a half-metal and the other
three systems possess moderate bandgaps in the range of 0.56 to 1.45 eV.
In addition, 5d TM single atoms and dimers adsorbed on the central hollow site
of 2D Pc have been investigated using first-principles calculations.360360. P.
Wang, X. Jiang, J. Hu, X. Huang, and J. Zhao, J. Mater. Chem. C 4(11), 2147
(2016). https://doi.org/10.1039/C5TC04402B The on-site magnetic moments of Re@Pc
and W@Pc are approximately identical (c.a. 2.4 μB). The calculations of exchange
energy indicated that the magnetic ground state of W@Pc system is
antiferromagnetic. Attractively, Re@Pc exhibits stable FM state with a high
Curie temperature of about 626 K and a perpendicular MAE of about 20.7 meV. When
a homonuclear dimer is adsorbed on Pc framework (denoted as TM2@Pc), the MAE can
be greatly enhanced,360360. P. Wang, X. Jiang, J. Hu, X. Huang, and J. Zhao, J.
Mater. Chem. C 4(11), 2147 (2016). https://doi.org/10.1039/C5TC04402B i.e.,
26.9, 40.7, and 47.2 meV for Ta2@Pc, Os2@Pc, and Re2@Pc, respectively. Among
those systems, Os2@Pc and Ir2@Pc are FM with TC of 52 and 91 K, respectively.
For 4d transition metals, Mo dimer embedded in Pc with another kind of atomic
arrangement shown in Fig. 15(c) has been discussed by Sun's group.366366. G.
Zhu, M. Kan, Q. Sun, and P. Jena, J. Phys. Chem. A 118(1), 304 (2014).
https://doi.org/10.1021/jp4109255 In this 2D material, each Mo atom has a
magnetic moment of about 0.88 μB, and the entire Mo2@Pc system adopts AFM ground
state. Using DFT calculations with the hybrid HSE06 functional, a direct gap of
about 0.93 eV was obtained.
By replacing Pc with octaamino-substituted phthalocyanines (OIPc) and square
planar Ni2+ ions, a new kind of square 2D MOFs with two different 4-coordinated
transition metal centers in each unit cell has been predicted.367367. W. Li, L.
Sun, J. Qi, P. Jarillo-Herrero, M. Dincă, and J. Li, Chem. Sci. 8(4), 2859
(2017). https://doi.org/10.1039/C6SC05080H The OIPc organic molecule enables
strong conjugation of π electrons, having a critical impact on the magnetic
properties of the 2D lattices [Fig. 15(d)]. Among these charge neutral 2D MOFs
candidates, NiMn@OIPc exhibits a half-metallic and ferromagnetic ground state.
The large exchange energy of NiMn@OIPc results from the unique strong
hybridization between d/π orbitals of Mn, Pc ring, and Ni-bisphenylenediimine
nodes. This picture is consistent with the TM@Pc mentioned above. In addition,
CrNi, FeNi, CoNi, and CuNi-based systems are all narrow-band-gap semiconductors
(0.28∼0.35 eV) with weak AFM coupling.367367. W. Li, L. Sun, J. Qi, P.
Jarillo-Herrero, M. Dincă, and J. Li, Chem. Sci. 8(4), 2859 (2017).
https://doi.org/10.1039/C6SC05080H
The porphyrin (Pp) ligand consisting of four pyrroles, is a planar, dianionic
macrocycle with four nitrogen donors in a square planar arrangement with a hole
size of around 2.0 Å in radius. Using Pp molecule as building block, a series of
stable four-coordination 2D periodic metal porphyrin frameworks (M@Pp) have been
proposed in recent years,79,8079. H. K. Singh, P. Kumar, and U. V. Waghmare, J.
Phys. Chem. C 119(45), 25657 (2015). https://doi.org/10.1021/acs.jpcc.5b0976380.
J. Tan, W. Li, X. He, and M. Zhao, RSC Adv. 3(19), 7016 (2013).
https://doi.org/10.1039/c3ra40502h which are formed by embedding transition
metal atoms in poly-Pp framework. Based on different bridged ligands, the Pp
frameworks can be labeled as Pp, Pp0, and Pp45, which are shown in Figs. 15(e),
15(f), and 15(g), respectively. Benefiting from the abundant combinations of
metal atoms, Pp frameworks, and bridged ligands, many 2D MOFs with diverse
magnetic ground states can be designed.
When Fe and Cu atoms are embedded in Pp and Pp0 frameworks, AFM ground state is
more favorable than FM ground state. For the other TM elements, magnetic
coupling of ground state is sensitive to the organic frameworks and transition
metals. As summarized in Table V, V@Pp, Cr@Pp, and Mn@Pp0 are FM; Mn@Pp and
Co@Pp0 are AFM; while Ni@Pp, Zn@Pp, and Co@Pp0 are PM. In Cr@Pp8080. J. Tan, W.
Li, X. He, and M. Zhao, RSC Adv. 3(19), 7016 (2013).
https://doi.org/10.1039/c3ra40502h and V@Pp sheets,7979. H. K. Singh, P. Kumar,
and U. V. Waghmare, J. Phys. Chem. C 119(45), 25657 (2015).
https://doi.org/10.1021/acs.jpcc.5b09763 robust FM ordering is obtained with
considerable TC of 187 and 197 K, respectively. Interestingly, half-metallicity
can be achieved in these 2D systems as the Fermi level is lifted up via electron
doping.8080. J. Tan, W. Li, X. He, and M. Zhao, RSC Adv. 3(19), 7016 (2013).
https://doi.org/10.1039/c3ra40502h The magnetic coupling in Mn, Fe, Co, and
Cu-based M@Pp systems are all AFM with on-site magnetic moment of 3.8, 2, 1 and
1.4 μB, respectively. For Ni@Pp and Zn@Pp nanosheets, the interactions between
the local magnetic moments are almost negligible due to the large lattice
constant, thereby exhibiting PM feature. Among the metal-embedded Pp0
frameworks, only Mn@Pp0 framework exhibits half-metallic nature as well as
long-range FM coupling with room-temperature TC of about 320 K, whereas 2D
Cr@Pp0, Fe@Pp0, and Cu@Pp0 MOFs prefer weak antiferromagnetic coupling and
Co@Pp0 exhibits PM behavior.368368. Q. Sun, Y. Dai, Y. Ma, X. Li, W. Wei, and B.
Huang, J. Mater. Chem. C 3(26), 6901 (2015). https://doi.org/10.1039/C5TC01493J
In Pp45 frameworks incorporated with 3d TM atoms, the inter-spin coupling is
identified to be PM, mainly arising from their long spin coherence length.
Meanwhile, different magnetic coupling states and large perpendicular magnetic
anisotropy (PMA) in Pp45 frameworks are induced by various 5d TM atoms.369369.
P. Wang, X. Jiang, J. Hu, and J. Zhao, Adv. Sci. 4(10), 1700019 (2017).
https://doi.org/10.1002/advs.201700019
Moreover, the magnetic moment and magnetic anisotropy can be modified by
replacing the peripheral H atoms of Pp framework by methyl (-CH3), hydroxyl
(-OH), and amino (-NH2) radicals, denoted as TM@M-Pp, TM@H-Pp, and TM@A-Pp,
respectively. Among them, W@Pp based systems prefer AFM coupling, and the
coupling strength is gradually strengthened with the electron donating capacity
increasing, which is ascribed to the functional radicals. The on-site magnetic
moment and PMA can be tailored in range of 2.3∼2.7 μB and 24∼36.7 meV,
respectively.369369. P. Wang, X. Jiang, J. Hu, and J. Zhao, Adv. Sci. 4(10),
1700019 (2017). https://doi.org/10.1002/advs.201700019 In contrast, Re@Pp based
systems prefer FM state with exchange energy over 180 meV, and the coupling
strength changes slightly with functional radical. Amazingly, MAE of Re@Pp
framework can be greatly enhanced from 23.9 to 60.8 meV when H atoms are
replaced by NH2 radicals. Furthermore, MC simulations yielded an estimated TC of
∼200 K for Re@Pp, suggesting that its ferromagnetism can be retained at
relatively high temperature.
A group of stable 2D tetra-coordination polymer complexes linked by conjugated
benzenehexathiolate following two different Kagome frameworks (M3@BHT-1 and
M3@BHT-2) were also experimentally prepared or theoretically designed, and parts
of them showed magnetic behavior. In them, the metal ions (M = Mg, Zn, Ni, Cu)
with sixfold symmetry were successfully synthesized by the coordination reaction
between benzenehexathiol (BHT) and metal acetate [M-(OAc)2], and the system with
Co ions was calculated by first-principles method.355,356,370355. A. J. Clough,
J. W. Yoo, M. H. Mecklenburg, and S. C. Marinescu, J. Am. Chem. Soc. 137(1), 118
(2015). https://doi.org/10.1021/ja5116937356. T. Kambe, R. Sakamoto, K. Hoshiko,
K. Takada, M. Miyachi, J. H. Ryu, S. Sasaki, J. Kim, K. Nakazato, and M. Takata,
J. Am. Chem. Soc. 135(7), 2462 (2013). https://doi.org/10.1021/ja312380b370. X.
Huang, P. Sheng, Z. Tu, F. Zhang, J. Wang, H. Geng, Y. Zou, C.-a. Di, Y. Yi, and
Y. Sun, Nat. Commun. 6(1), 1 (2015). Since BHT is a polyphosphine ligand with
three dithiolene donor groups (C6S66–), a series of M3@BHT complexes were
studied using first-principles calculations by considering all 3d transition
metal elements. Indeed, three kinds of systems, such as Mn3C12S12,371371. M.
Zhao, A. Wang, and X. Zhang, Nanoscale 5(21), 10404 (2013).
https://doi.org/10.1039/c3nr03323f M3C6S6 (M3@BHT-1),372372. C. Chakravarty, B.
Mandal, and P. Sarkar, J. Phys. Chem. C 120(49), 28307 (2016).
https://doi.org/10.1021/acs.jpcc.6b09416 and MMg2C12S12 (M-Mg2@BHT-2),373373. L.
Tang, Q. Li, C. Zhang, F. Ning, W. Zhou, L. Tang, and K. Chen, J. Magn. Magn.
Mater. 488, 165354 (2019). https://doi.org/10.1016/j.jmmm.2019.165354 have been
reported to possess magnetic properties. Their structural models are depicted in
Figs. 15(h) and 15(i). Among Mn3C12S12 and M3@BHT-1 systems with planar
spin-frustrated Kagome lattice, Mn, Fe, and Co complexes have half-metallicity
and FM ground state, whereas Cr@BHT-1 is stabilized in AFM ground state and
behaves as a semimetal.372372. C. Chakravarty, B. Mandal, and P. Sarkar, J.
Phys. Chem. C 120(49), 28307 (2016). https://doi.org/10.1021/acs.jpcc.6b09416 A
similar study was reported by Liu and Sun in 2015,374374. J. Liu and Q. Sun,
Chemphyschem 16(3), 614 (2015). https://doi.org/10.1002/cphc.201402713 and they
predicted 2D Mn3C12N12H12 sheet to exhibit stronger FM coupling with TC of
450 K. In 2D Mn3C12N12H12 sheet, the isoelectronic NH groups take the position
of sulfur atoms, which result in smaller lattice constant and larger magnetic
coupling strength. Furthermore, d-p hybridization not only makes the main
contribution to the stable π-conjugated planar Kagome framework structure but
also results in favorable FM magnetic coupling.
Based on DFT calculations, it was found that Cu3C6H6 monolayer (Cu@BHT-2) is a
FM metal with an altralow TC of about 4 K,375375. X. Zhang, Y. Zhou, B. Cui, M.
Zhao, and F. Liu, Nano Lett. 17(10), 6166 (2017).
https://doi.org/10.1021/acs.nanolett.7b02795 while 2D Mg3C6S6 is a nonmagnetic
semiconductor.376376. L. Tang, L. Tang, D. Wang, H. Deng, and K. Chen, J. Phys.:
Condens. Matter 30(46), 465301 (2018). https://doi.org/10.1088/1361-648X/aae618
The atomic model is shown in Fig. 15(i). When one of the Mg atoms in nonmagnetic
Mg3C6S6 is substituted by a 3d transition metal atom (i.e., Sc∼Co),
intrinsically magnetic and semiconducting properties can be observed. Among
these 2D systems, FM state is more stable than PM state for V-Mg2@BHT-2 with
rather high TC of 471 K.373373. L. Tang, Q. Li, C. Zhang, F. Ning, W. Zhou, L.
Tang, and K. Chen, J. Magn. Magn. Mater. 488, 165354 (2019).
https://doi.org/10.1016/j.jmmm.2019.165354 The π-d coupling between central
metal atom and organic ligands effectively regulates the spin-polarized state of
these systems and the coupling strength is greatly influenced by the magnetic
moment of TM atoms and interatomic distance.
Apart from the above systems, magnetic MOFs have also been identified in other
2D materials with tetra-coordination, such as TM3HITP2 (HITP = 2, 3, 6, 7, 10,
11-hexaiminotriphenylene),377377. L. Dong, Y. Kim, D. Er, A. M. Rappe, and V. B.
Shenoy, Phys. Rev. Lett. 116(9), 096601 (2016).
https://doi.org/10.1103/PhysRevLett.116.096601 TM@DPP (DPP =
diketopyrrolopyrrole),125125. X. Li and J. Yang, J. Am. Chem. Soc. 141(1), 109
(2019). https://doi.org/10.1021/jacs.8b11346 TM@TCNQ (TCNQ =
7,7,8,8-tetracyanoquinodimethane),378378. Y. Ma, Y. Dai, W. Wei, L. Yu, and B.
Huang, J. Phys. Chem. A 117(24), 5171 (2013). https://doi.org/10.1021/jp402637f
TM-naphthalene,8282. B. Mandal and P. Sarkar, Phys. Chem. Chem. Phys. 17(26),
17437 (2015). https://doi.org/10.1039/C5CP01359C Fe@CMP,379379. A. Pimachev, R.
D. Nielsen, A. Karanovich, and Y. Dahnovsky, Phys. Chem. Chem. Phys. 21(46),
25820 (2019). https://doi.org/10.1039/C9CP04509K and Fe@PTC (PTC = 1, 2, 3, 4,
5, 6, 7, 8, 9, 10, 11, 12-perthiolated coronene).380380. R. Dong, Z. Zhang, D.
C. Tranca, S. Zhou, M. Wang, P. Adler, Z. Liao, F. Liu, Y. Sun, W. Shi, Z.
Zhang, E. Zschech, S. C. B. Mannsfeld, C. Felser, and X. Feng, Nat. Commun.
9(1), 2637 (2018). https://doi.org/10.1038/s41467-018-05141-4 In 2014 and 2015,
the experimentally synthesized Ni3(HITP)2 and Cu3(HITP)2 were reported to be
semiconductors with high electrical conductivity.353,381353. D. Sheberla, L.
Sun, M. A. Blood-Forsythe, S. l. Er, C. R. Wade, C. K. Brozek, A. n.
Aspuru-Guzik, and M. Dinca˘, J. Am. Chem. Soc. 136(25), 8859 (2014).
https://doi.org/10.1021/ja502765n381. M. G. Campbell, D. Sheberla, S. F. Liu, T.
M. Swager, and M. Dincă, Angew. Chem. Int. Ed. 54(14), 4349 (2015).
https://doi.org/10.1002/anie.201411854 Later, QAH effect in TM3(HIPT)2 was
theoretically predicted by Dong et al.377377. L. Dong, Y. Kim, D. Er, A. M.
Rappe, and V. B. Shenoy, Phys. Rev. Lett. 116(9), 096601 (2016).
https://doi.org/10.1103/PhysRevLett.116.096601 In their study, Ta3(HITP)2,
Re3(HITP)2, and Ir3(HITP)2 monolayers are ferromagnetic narrow-band-gap
semiconductors with magnetic moment of 1 μB per TM atom. When all N atoms in
TM3(HITP)2 are substituted by O atoms, the resulted Ta3(C18H12O6)2 and
Ir3(C18H12O6)2 systems retain FM ground state. Meanwhile, QAH effect could be
successfully realized at much higher temperature by chemical modification in
Ta3(C18H12O6)2. The magnetic moment of Ta ion is greatly enhanced to 3 μB as
compared to that of Ta3(HITP)2 (1 μB).377377. L. Dong, Y. Kim, D. Er, A. M.
Rappe, and V. B. Shenoy, Phys. Rev. Lett. 116(9), 096601 (2016).
https://doi.org/10.1103/PhysRevLett.116.096601 The ground state of 2D
V(Cr)-diketopyrrolopyrrole (DPP), as shown in Fig. 15(j), was theoretically
predicted to be ferrimagnetic with an exchange energy more than 426 meV
(328 meV).77. X. Li and J. Yang, J. Phys. Chem. Lett. 10(10), 2439 (2019).
https://doi.org/10.1021/acs.jpclett.9b00769 The robust ferrimagnetic ordering
originates from strong d-p direct exchange interactions between conjugated
electron acceptors and electron providers.125125. X. Li and J. Yang, J. Am.
Chem. Soc. 141(1), 109 (2019). https://doi.org/10.1021/jacs.8b11346
TM-7,7,8,8-tetracyanoquinodimethane (TM@TCNQ), Cr@TCNQ, Mn@TCNQ, and Fe@TCNQ
exhibit long-range AFM coupling [Fig. 15(k)], while Co@TCNQ is PM according to
DFT calculations.378378. Y. Ma, Y. Dai, W. Wei, L. Yu, and B. Huang, J. Phys.
Chem. A 117(24), 5171 (2013). https://doi.org/10.1021/jp402637f For a simple 2D
MOF composed of substituted naphthalene moieties and transition metals, FM
coupling can be achieved in Cr- and Mn-based systems with high on-site magnetic
moment of about 4 μB. Except that Mn complex is metallic, Cr, Fe, and Co
complexes are all half-metallic in nature. Furthermore, Fe and Cr complexes
exhibit remarkable 100% spin-filtering efficiency. Therefore, TCNQ and
TM-naphthalene could serve as ideal building blocks for the candidate materials
of spintronic devices.8282. B. Mandal and P. Sarkar, Phys. Chem. Chem. Phys.
17(26), 17437 (2015). https://doi.org/10.1039/C5CP01359C Another 2D
ferromagnetic MOF, i.e., Fe@PTC [PTC = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
12)-perthiolated coronene] compound was synthesized from reaction of PTC with
ammoniacal solutions of iron acetate [Fe(OAc)2] and exhibited a high electrical
conductivity of about 10 S/cm at 300 K through the measured I–V curves, which
decreased upon cooling and suggested typical feature of semiconductor.380380. R.
Dong, Z. Zhang, D. C. Tranca, S. Zhou, M. Wang, P. Adler, Z. Liao, F. Liu, Y.
Sun, W. Shi, Z. Zhang, E. Zschech, S. C. B. Mannsfeld, C. Felser, and X. Feng,
Nat. Commun. 9(1), 2637 (2018). https://doi.org/10.1038/s41467-018-05141-4
Reducing the coordination number of transition metal centers from 4 to 3, there
are still many 2D MOFs with intrinsic magnetism through rational design, such as
M@triphenyl (M2C18H12, M= Co, Ni and Mn),382,383382. Y. Ma, Y. Dai, X. Li, Q.
Sun, and B. Huang, Carbon 73, 382 (2014).
https://doi.org/10.1016/j.carbon.2014.02.080383. Z. Wang, Z. Liu, and F. Liu,
Phys. Rev. Lett. 110(19), 196801 (2013).
https://doi.org/10.1103/PhysRevLett.110.196801 indium-phenylene
(In@IPOF),384384. Z. Liu, Z. Wang, J. Mei, Y. Wu, and F. Liu, Phys. Rev. Lett.
110(10), 106804 (2013). https://doi.org/10.1103/PhysRevLett.110.106804 TM@PBP
[PBP = 5,5′-bis(4-pyridyl)(2,2′-bipirimidine],385385. L.-C. Zhang, L. Zhang, G.
Qin, Q.-R. Zheng, M. Hu, Q.-B. Yan, and G. Su, Chem. Sci. 10(44), 10381 (2019).
https://doi.org/10.1039/C9SC03816G bilayer Fe@T4PT [T4PT
= 2,4,6-tris(4-pyridyl)-1,3,5-triazine],386386. T. Umbach, M. Bernien, C. F.
Hermanns, A. Krüger, V. Sessi, I. Fernandez-Torrente, P. Stoll, J. I. Pascual,
K. Franke, and W. Kuch, Phys. Rev. Lett. 109(26), 267207 (2012).
https://doi.org/10.1103/PhysRevLett.109.267207 and Ni-thiophene
(Ni2C24S6H12).387387. L. Wei, X. Zhang, and M. Zhao, Phys. Chem. Chem. Phys.
18(11), 8059 (2016). https://doi.org/10.1039/C6CP00368K Among these compounds,
Fe@PBP framework has already been synthesized on Au(111) substrate by molecular
self-assembly.354354. D. Grumelli, B. Wurster, S. Stepanow, and K. Kern, Nat.
Commun. 4(1), 2904 (2013). https://doi.org/10.1038/ncomms3904 In@IPOF and M@T
(M=Co, Ni, and Mn) share the same geometry, as shown in Fig. 15(l). Except for
Fe@T4PT and TM@PBP, most of the above mentioned three-coordinated 2D MOFs
possess amazing half-metallic Dirac point and are novel topologically nontrivial
materials. Meanwhile, In@IPOF, Mn@T, Mn@PBP, Fe@T4PT, and Ni2C24S6H12 have been
reported as 2D ferromagnets. Mn@PBP is the first ferromagnetic 2D MOF with the
Shastry-Sutherland lattice [Fig. 15(m)], and its TC was predicted to be about
105 K, while Fe@PBP and TM@PBP (TM = Cr, Co, Ni) were found to be AFM and
magnetic-dimerized, respectively.385385. L.-C. Zhang, L. Zhang, G. Qin, Q.-R.
Zheng, M. Hu, Q.-B. Yan, and G. Su, Chem. Sci. 10(44), 10381 (2019).
https://doi.org/10.1039/C9SC03816G
Other 2D MOFs with even lower coordination of two have been theoretically
designed. One kind of two-coordinated organometallic framework composed by
(1,3,5)-benzenetricarbonitrile (TCB) molecules and noble metals (Au, Ag, and Cu)
have been shown as FM half-metals with TC as high as 325 K. Besides, TCB-Re
exhibits intrinsically ferromagnetic ordering with a high TC of 613 K predicted
by MFT and a considerable MAE of 19 meV/atom.388388. J. Xing, P. Wang, Z. Jiang,
X. Jiang, Y. Wang, and J. Zhao, APL Mater. 8(7), 071105 (2020).
https://doi.org/10.1063/5.0010822 In these organometallic frameworks, the strong
electronegativity of C–N groups drives the charge transfer from metal atoms to
the organic molecules, forming the local magnetic centers. These magnetic
centers experience strong FM and AFM couplings through d–p covalent
bonding.8181. H. Sun, B. Li, and J. Zhao, J. Phys.: Condens. Matter 28(42),
425301 (2016). https://doi.org/10.1088/0953-8984/28/42/425301
In addition, many researchers have also observed 2D magnetic MOFs with
hexa-coordination. Experimentally, 2D Cu-TP-1 has been synthesized under
solvothermal conditions from the Cu2+ and 2-tetrazole pyrimidine (C5H5N6, H-TP).
According to the field cooled (FC) and zero field cooled (ZFC) measurements, the
system exhibited an antiferromagnetically coupled Cu–Cu interaction down to 8 K
with a Weiss temperature around 108 K.389389. P. Pachfule, R. Das, P. Poddar,
and R. Banerjee, Cryst. Growth Des. 10(6), 2475 (2010).
https://doi.org/10.1021/cg1003726 Another polynuclear Cu compound showed
predominant ferromagnetic coupling even at 300 K.390390. X. Zhu, J. Zhao, B. Li,
Y. Song, Y. Zhang, and Y. Zhang, Inorg. Chem. 49(3), 1266 (2010).
https://doi.org/10.1021/ic902404b Theoretically, Kan et al.391391. E. Kan, X.
Wu, C. Lee, J. H. Shim, R. Lu, C. Xiao, and K. Deng, Nanoscale 4(17), 5304
(2012). https://doi.org/10.1039/c2nr31074k found that one kind of freestanding
organometallic sheets (Ps) can be assembled by 3d TM atoms and benzene
molecules. Among them, V@Ps and Mn@Ps are FM systems with TC of 279 K and 96 K
based on MFT, respectively, while Ti@Ps, Co@Ps, and Cr@Ps exhibit AFM
ordering.391391. E. Kan, X. Wu, C. Lee, J. H. Shim, R. Lu, C. Xiao, and K. Deng,
Nanoscale 4(17), 5304 (2012). https://doi.org/10.1039/c2nr31074k In these
frameworks, their magnetic coupling can be explained by maximization of the
virtual hopping between separated magnetic centers caused by the half-occupied
dxz and dyz bands. In addition, another hexa-coordinate compound, i.e.,
Mn2C6S12, has also been predicted. It is a honeycomb structure and possesses
stable FM state at room temperature along with spin-polarized Dirac cone.
Interestingly, the exchange interaction between Mn ions is achieved by the
conduction electrons of C atoms acting as the intermediate.392392. A. Wang, X.
Zhang, Y. Feng, and M. Zhao, J. Phys. Chem. Lett. 8(16), 3770 (2017).
https://doi.org/10.1021/acs.jpclett.7b01187
Due to large magnetic moments and remarkable magnetic anisotropy in the
lanthanide ions, a series of 2D lanthanide-MOFs displayed strong magnetic
interactions. Using hydrothermal method, a new 2D Dy3+ MOF was synthesized from
4-hydroxypyridine-2,6-dicarboxylic acid and showed intramolecular ferromagnetic
interaction and two-step thermal magnetic relaxation.393393. C. Liu, J. Xiong,
D. Zhang, B. Wang, and D. Zhu, RSC Adv. 5(127), 104854 (2015).
https://doi.org/10.1039/C5RA23638J Another dysprosium layered compound from
reaction with 2-(3-pyridyl) pyrimidine-4-carboxylic acid also presented FM
interactions between Dy3+ ions and slowed magnetic relaxation behavior.394394.
D. Yin, Q. Chen, Y. Meng, H. Sun, Y. Zhang, and S. Gao, Chem. Sci. 6(5), 3095
(2015). https://doi.org/10.1039/C5SC00491H
Another important member of 2D organic materials is COFs, which have been
investigated widely as catalysts, energy storage and gas adsorption materials,
and so on. Two-dimensional COFs are crystalline porous polymers formed by
molecular building units and organic linkers via covalent bond without any
transition metal.395,396395. X. Li, P. Yadav, and K. P. Loh, Chem. Soc. Rev.
49(14), 4835 (2020). https://doi.org/10.1039/D0CS00236D396. S. B. Alahakoon, S.
D. Diwakara, C. M. Thompson, and R. A. Smaldone, Chem. Soc. Rev. 49(5), 1344
(2020). https://doi.org/10.1039/C9CS00884E Due to their transition metal free
nature, it is hardly to found 2D magnets in COFs. However, some pioneer studies
have proven that they can exhibit magnetic properties in the presence of some
special atomic arrangements. In a theoretical study by Yang et al.,352352. E.
Kan, W. Hu, C. Xiao, R. Lu, K. Deng, J. Yang, and H. Su, J. Am. Chem. Soc.
134(13), 5718 (2012). https://doi.org/10.1021/ja210822c single-layer organic
porous sheets dimethylmethylene-bridged triphenylamine (DTPA) is a ferromagnetic
half-metal with a bandgap in semiconducting channel of about 1 eV. In comparison
with the boron-doped (FM) and pure graphene nanoflakes (GFs) (AFM), both of
which possess similar molecular architecture with DTPA, the FM state can be
explained by half-occupied π orbital and allowed virtual hopping for FM
configurations. Graphitic carbon nitride (g-C4N3) as a novel material with FM
ground state and intrinsic half-metallicity has also been theoretically
predicted by Du et al.351351. A. Du, S. Sanvito, and S. C. Smith, Phys. Rev.
Lett. 108(19), 197207 (2012). https://doi.org/10.1103/PhysRevLett.108.197207
According the analysis of orbital-resolved DOS, the p orbitals of N atoms rather
than C atoms make the main contributions to the half-metallicity and magnetism
of g-C4N3. A 2D COF with D6h symmetry, C3N, was shown to have an intrinsic
indirect bandgap of 0.39 eV under FM ground state from both experimental and
theoretical studies.350350. S. Yang, W. Li, C. Ye, G. Wang, H. Tian, C. Zhu, P.
He, G. Ding, X. Xie, and Y. Liu, Adv. Mater. 29(16), 1605625 (2017).
https://doi.org/10.1002/adma.201605625 No doubt, the above studies open a door
for fundamental research and potential applications toward realistic metal-free
spintronics.
IV. MODIFICATIONS
Section:
ChooseTop of pageABSTRACTI. INTRODUCTIONII. THE ORIGIN OF MAGNETI...III. THE 2D
VDW MAGNETS D...IV. MODIFICATIONS <<V. CONCLUSION AND OUTLOOKAUTHORS'
CONTRIBUTIONSCITING ARTICLESChoose
In the above discussions, we mainly focused on the database of 2D magnets and
their dominant magnetic exchange interactions. For practical applications, we
have to address the other issues, such as how to enhance the critical transition
temperature (TC or TN) to room temperature in these existing 2D magnetic
materials by post-treating with various manipulations. We have already shown
that the magnetic ground state and exchange coupling strength are very sensitive
to symmetry, charge distributions, Fermi level, valence states, orbital
occupation, orbital hybridizations, energy level, hopping paths, and so on.
According to these target parameters, many strategies have been proposed,
including strain engineering, intercalation, external electronic/magnetic field,
interfacial engineering, defect engineering, Janus structuring, and optical
controlling. A schematic diagram of these modification mechanisms is displayed
in Fig. 16.
FIG. 16. The schematic diagram of the modification mechanisms of Curie
temperature of 2D magnets. Many strategies have been included, such as strain
engineering, intercalation, external electronic/magnetic field, interfacial
engineering, defect engineering, Janus structuring, and optical controlling.
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* High-resolution
A. Strain engineering
Strain engineering is a simple and efficient means for tailoring the magnetic
properties. Experimentally, the strain effect on 2D magnets is inevitable during
the synthesis processes. After exfoliation, 2D magnets (e.g., 2D CrI32828. B.
Huang, G. Clark, E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D.
Zhong, E. Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P.
Jarillo-Herrero, and X. Xu, Nature 546(7657), 270 (2017).
https://doi.org/10.1038/nature22391) could be transferred on SiO2 substrate and
other 2D materials. In addition, there are also direct MBE/CVD growth of VSe2,
MnSe2, and MnSe on Si(111)/SiO2 substrates.34,3534. M. Bonilla, S. Kolekar, Y.
Ma, H. C. Diaz, V. Kalappattil, R. Das, T. Eggers, H. R. Gutierrez, M.-H. Phan,
and M. Batzill, Nature Nanotechnol. 13(4), 289 (2018).
https://doi.org/10.1038/s41565-018-0063-935. J. Li, B. Zhao, P. Chen, R. Wu, B.
Li, Q. Xia, G. Guo, J. Luo, K. Zang, Z. Zhang, H. Ma, G. Sun, X. Duan, and X.
Duan, Adv. Mater. 30(36), 1801043 (2018). https://doi.org/10.1002/adma.201801043
As a consequence, the lattice mismatch of substrate with 2D magnets would cause
certain lattice strain. Even so, one should note that we mainly discussed 2D van
der Waals materials in this review. Strain from the neighboring layers could be
either non-uniform or quite small, despite the large lattice mismatch.
The effect of in-plane biaxial strain on the TC of 2D magnetic materials has
been thoroughly investigated, especially on the experimentally reported systems.
From the structural point of view, biaxial tensile strain would immediately
increase the bond lengths and widen the bond angle between the magnetic atoms,
while compressive strain would reduce the bond length and narrow the bond angle.
These two factors undoubtedly affect orbital hybridization and change the
magnetic exchange parameters, which are described by the distance and angle
dependent GKA rules.126–128126. J. B. Goodenough, Phys. Rev. 100(2), 564 (1955).
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115(1), 2 (1959). https://doi.org/10.1103/PhysRev.115.2 Taking monolayer
CrSi(Ge)Te3 as an example, herein we further explain the strain effect. As shown
in Fig. 17(a), the direct AFM interaction is short-ranged and decreases rapidly
with the increase of Cr–Cr distance, while the superexchange FM interaction is
comparatively long-ranged and decreases relatively slowly with the increase of
Cr–Te–Cr distance. Under biaxial tensile strain, the Cr–Te–Cr angle gradually
approaches to normative 90°, and the FM exchange interaction is
strengthened.397397. X. Chen, J. Qi, and D. Shi, Phys. Lett. A 379(1–2), 60
(2015). https://doi.org/10.1016/j.physleta.2014.10.042 These combined effects
lead to increase of the energy difference between FM and AFM states. Moreover,
the magnetic moment on Cr atoms increases monotonically with tensile
strain.398,399398. X.-J. Dong, J.-Y. You, B. Gu, and G. Su, Phys. Rev. Appl.
12(1), 014020 (2019). https://doi.org/10.1103/PhysRevApplied.12.014020399. K.
Wang, T. Hu, F. Jia, G. Zhao, Y. Liu, I. V. Solovyev, A. P. Pyatakov, A. K.
Zvezdin, and W. Ren, Appl. Phys. Lett. 114(9), 092405 (2019).
https://doi.org/10.1063/1.5083992 Thus, the calculated TC of CrSiTe3 by MFA
dramatically increases from 22.5 K in strain-free state to 290 K under 8%
strain. Figure 17(b) displays TC as a function of strain for CrGeTe3. Under 3%
and 5% tensile strains, the TC can be enhanced to 326 and 421 K, respectively,
in comparison to TC = 144 K for strain-free state. Under a compressive strain of
1%, the TC will decrease to 67 K.398398. X.-J. Dong, J.-Y. You, B. Gu, and G.
Su, Phys. Rev. Appl. 12(1), 014020 (2019).
https://doi.org/10.1103/PhysRevApplied.12.014020 The strain effect on the
magnetic properties of VSe2 was investigated by DFT calculations.400400. S. Feng
and W. Mi, Appl. Surf. Sci. 458, 191 (2018).
https://doi.org/10.1016/j.apsusc.2018.07.070 Figures 17(c) and 17(d) show the
energy difference and individual magnetic moment in strained VSe2, respectively.
As the strain increases, the calculated magnetic moment of V atom increases from
0.77 to 1.18 μB. The TC is monotonously enhanced from 290 to 812 K under a
strain of 6%. Similar results and underlying mechanisms have also been found in
monolayer CrI3401401. Z. Wu, J. Yu, and S. Yuan, Phys. Chem. Chem. Phys. 21(15),
7750 (2019). https://doi.org/10.1039/C8CP07067A and Fe3GeTe2402402. X. Hu, Y.
Zhao, X. Shen, A. V. Krasheninnikov, Z. Chen, and L. Sun, ACS Appl. Mater.
Inter. 12(23), 26367 (2020). https://doi.org/10.1021/acsami.0c05530 sheets.
FIG. 17. (a) A schematic diagram of the nearest neighboring Cr–Cr direct AFM
interaction and FM superexchange interaction mediated by the middle Te atom with
Cr–Cr distance in monolayer CrSi(Ge)Te3. (b) Normalized magnetization as a
function temperature. (c) Energy difference between FM and AFM states for
monolayer VSe2 and the TC as a function of strain. (d) The calculated magnetic
moments of V and Se atoms in ferromagnetic VSe2 monolayer as a function of
strain. Panel (a) reproduced with permission from Chen et al., Phys. Lett. A
379, 60 (2015). Copyright 2015 Elsevier.397397. X. Chen, J. Qi, and D. Shi,
Phys. Lett. A 379(1–2), 60 (2015).
https://doi.org/10.1016/j.physleta.2014.10.042 Panel (b) reproduced with
permission from Dong et al., Rev. Appl. 12, 014020 (2019). Copyright 2019
American Physical Society.398398. X.-J. Dong, J.-Y. You, B. Gu, and G. Su, Phys.
Rev. Appl. 12(1), 014020 (2019).
https://doi.org/10.1103/PhysRevApplied.12.014020 Panels (c) and (d) reproduced
with permission from Feng et al., Surf. Sci. 458, 191 (2018). Copyright 2018
Elsevier.400400. S. Feng and W. Mi, Appl. Surf. Sci. 458, 191 (2018).
https://doi.org/10.1016/j.apsusc.2018.07.070
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* High-resolution
The applied strain also yields a pronounced transition of magnetic ground state
from AFM to FM under tensile strain or FM to AFM under compressive
strain.104,156,401,403–405104. L. Webster and J.-A. Yan, Phys. Rev. B 98(14),
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https://doi.org/10.1063/1.5078713 Depending on their ground states, the critical
strains for phase transition in 2D CrTe3, RuCl3, and CrI3 are 3%, 2%, and –5%,
respectively. However, the tensile strain induced TC enhancement and magnetic
transition are not found in the ferromagnetic NiX3 (X = I, Br, Cl) sheets. In
these materials, compressive strain will enhance their TC and induce a FM to AFM
transition (8%). In fact, this is still consistent with GKA rules. In NiX3 (X =
I, Br, Cl), the intrinsic angle of Ni–X–Ni is about 95°, which deviates from
90°. The tensile strain would further enlarge the Ni–X–Ni angle, which weakens
FM coupling and strengthens AFM coupling.406406. Z. Li, B. Zhou, and C. Luan,
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In addition to the magnetic coupling parameters and TC, MAE is another
significant magnetic characteristic that can be successfully modulated by strain
engineering. According to the second-order perturbation theory, the value of MAE
is very sensitive to strain, and no consistent mechanism has been found. For
example, MAE increases with compressive strain in monolayer 1T-FeCl2 because of
the enhanced positive contribution to MAE from the SOC interaction between
dx2-y2 (dxy) and dyz (dxz) orbitals of Fe atom.407407. H. Zheng, J. Zheng, C.
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https://doi.org/10.1016/j.jmmm.2017.08.005 Han et al. reported that MAE of
monolayer CrSI is mainly contributed by spin-polarized p-orbitals of nonmetal I
atoms. The compressive strain enhances the MAE value to 0.52 meV/atom, which
originates from the positive contribution of matrix element difference between
spin-up px and py orbitals as well as spin-up py and pz orbitals of I
atoms.408408. R. Han and Y. Yan, Phys. Chem. Chem. Phys. 21(37), 20892 (2019).
https://doi.org/10.1039/C9CP03535D
B. Intercalation
Owing to the existence of VDW gap and weak interlayer interaction, 2D materials
are ideal host materials for various intercalant species, including small ions,
atoms, and molecules. Chemical intercalation and electrochemical intercalation
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as a powerful approach to induce local spin and long-range magnetic ordering in
the 2D non-magnetic materials.417–422417. X. Zhao, P. Song, C. Wang, A. C.
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https://doi.org/10.1088/1361-6528/aac320 In the 2D intrinsic magnets,
intercalation can further modulate the electronic structure, reduce the
interlayer coupling, and change the valence state of magnetic ions by doping
electron/hole.
Wang et al.423423. N. Wang, H. Tang, M. Shi, H. Zhang, W. Zhuo, D. Liu, F. Meng,
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https://doi.org/10.1021/jacs.9b06929 successfully intercalated organic
tetrabutyl ammonium (TBA) cations into Cr2Ge2Te6 [Fig. 18(a)] and obtained a
hybrid superlattice of (TBA)Cr2Ge2Te6. Such electron doping leads to metallic
behavior of (TBA)Cr2Ge2Te6 at low temperature. Meanwhile, the Curie temperature
has been significantly elevated to 208 K [Fig. 18(b)]. For comparison, pristine
Cr2Ge2Te6 is a FM semiconductor with TC of 67 K only. The underlying exchange
coupling mechanism is illustrated in Fig. 18(c). A weak superexchange mechanism
is found in Cr2Ge2Te6 semiconductor, while double exchange mechanism is
dominated in FM metallic (TBA)Cr2Ge2Te6. Weber et al. demonstrated that
Fe2.78GeTe2 can be topotactically intercalated with sodium in the presence of
benzophenone to yield NaFe2.78GeTe2.424424. D. Weber, A. H. Trout, D. W. McComb,
and J. E. Goldberger, Nano Lett. 19(8), 5031 (2019).
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both Fe2.78GeTe2 and NaFe2.78GeTe2 from SQUID magnetometry were about 150 K.
Even though the Curie temperature has not been appreciably changed, positive
evidence of room-temperature magnetism resulted from the presence of Fe2−xGe
impurity was observed. Moreover, Qiu et al. have revealed sensitive dependence
of Fe3GeTe2 TC on its environmental change by Ga implantation. The TC of bulk
Fe3GeTe2 could increase up to 450 K by controlling the fluence of Ga
irradiation, while the magnetic anisotropy could be inversed to in-plane
direction.425425. M. Yang, Q. Li, R. V. Chopdekar, C. Stan, S. Cabrini, J. W.
Choi, S. Wang, T. Wang, N. Gao, A. Scholl, N. Tamura, C. Hwang, F. Wang, and Z.
Qiu, Adv. Quantum Technolog. 3(4), 2000017 (2020).
https://doi.org/10.1002/qute.202000017
FIG. 18. (a) Side view plot of crystal structure of pristine Cr2Ge2Te6 and
(TBA)Cr2Ge2Te6. Organic ion tetrabutyl ammonium is added into the van der Waals
gap via electrochemical intercalated. (b) Temperature-dependent magnetic
susceptibility of Cr2Ge2Te6 and (TBA)Cr2Ge2Te6. The Curie temperature is largely
increased from 67 K in Cr2Ge2Te6 to 208 K in intercalated (TBA)Cr2Ge2Te6. (c)
Schematic diagram of intercalation-induced magnetic exchange mechanism
transition (from superexchange interaction to double exchange interaction).
Reproduced with permission from Wang et al., J. Am. Chem. Soc. 141, 17166
(2019). Copyright 2019 American Chemical Society.423423. N. Wang, H. Tang, M.
Shi, H. Zhang, W. Zhuo, D. Liu, F. Meng, L. Ma, J. Ying, and L. Zou, J. Am.
Chem. Soc. 141(43), 17166 (2019). https://doi.org/10.1021/jacs.9b06929
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* High-resolution
In a recent theoretical study, Guo et al.426426. Y. Guo, N. Liu, Y. Zhao, X.
Jiang, Si Zhou, and J. Zhao, Chin. Phys. Lett. 37(10), 107506 (2020).
https://doi.org/10.1088/0256-307X/37/10/107506 found that self-filling either Cr
or I atoms into the vdW gap of stacked and twisted CrI3 bilayer sheets can
significantly strengthen the interlayer FM coupling. The exchange energy
increases with the intercalated Cr concentration, reaching about 40 meV per
formula compared to the values of pristine CrI3 bilayer (2.95 meV/f.u. for the
low-temperature stacked phase and −0.14 meV/f.u. for the high-temperature
phase). Such strong ferromagnetism of self-intercalated CrI3 bilayer results
from the mid-gap states due to eg-t2g hybridization, which is related to the
additional electron hoping paths through intercalated atoms, bridging I atoms,
and intralayered Cr atoms. On the other hand, the intercalated and intralayer Cr
atoms show different oxidation states. In turn, double exchange interaction
among them would take over superexchange interaction to determine the magnetic
behavior. Thus, much higher TC with regard to that of pristine CrI3 bilayer
(low-temperature phase) is highly anticipated.
C. Electric control magnetism
As the essential tools for electric control of magnetism, the effects of
electrostatic/ion liquid gating and electric field on the magnetic properties of
the recently emerged 2D magnets have been investigated. These studies offer
exciting prospects of 2D magnets for processing and storing information in
future spintronics. Due to the ultrathin nature of 2D magnets, electrostatic/ion
liquid gating and electric field could largely modify the carrier concentration,
electron population, orbital occupation, symmetry, and bandgap, which would
further lead to the adjustment of magnetic ground state, exchange parameters,
and magnetic anisotropy.
Jiang et al.427427. S. Jiang, L. Li, Z. Wang, K. F. Mak, and J. Shan, Nat.
Nanotechnol. 13(7), 549 (2018). https://doi.org/10.1038/s41565-018-0135-x
demonstrated control of the magnetic properties in both monolayer and bilayer
CrI3 by electrostatic doping. The doping effect on the magnetic properties of
monolayer CrI3 is illustrated in Fig. 19(a). From the saturation magnetization,
coercive force, and Curie temperature, one can see strengthened/weakened
magnetic ordering with hole/electron doping (–3 × 1013 to 3 × 1013). The TC of
monolayer was finally modulated between 40 and 50 K. The doping effect on the
magnetic properties of bilayer CrI3 is shown in Fig. 19(b). The AFM phase
shrinks continuously with increasing electron doping density. As electron doping
concentration increases to 2.5 × 1013 cm−2, the AFM ground state of bilayer CrI3
would transform into FM. Under different gate voltages, the spin-flip field and
interlayer exchange constant are extracted in Fig. 19(c).427427. S. Jiang, L.
Li, Z. Wang, K. F. Mak, and J. Shan, Nat. Nanotechnol. 13(7), 549 (2018).
https://doi.org/10.1038/s41565-018-0135-x
FIG. 19. (a) Coercive force, saturation magnetization, and Curie temperature as
a function of gate voltage and gate induced doping density of bilayer CrI3. (b)
Doping density–magnetic field phase diagram of bilayer CrI3 at 4 K. (c)
Interlayer exchange constant and spin-flip transition field as a function of
gate voltage and gate induced doping density for bilayer CrI3. (d) Uniaxial
magnetic anisotropy fields and TC for different gate voltage of few-layer
Cr2Ge2Te6. (e) MAE of few-layer Cr2Ge2Te6 as a function of electron density. (f)
Phase diagram of trilayer Fe3GeTe2 sample varies with the gate voltage and
temperature. (g) The DOS of 0.5 hole and 0.5 electron doped CrI3 monolayer, and
the dashed vertical lines refer to the shifting of Fermi level. (h) Relative
energy of AFM and FM states under the variation of carrier concentration for
CrI3 monolayer. (i) Relative energy of AFM and FM states under the variation of
carrier concentration for 2D MnPSe3. Panels (a)–(c) reproduced with permission
from Jiang et al., Nat. Nanotechnol. 13, 549 (2018). Copyright 2018 Springer
Nature.427427. S. Jiang, L. Li, Z. Wang, K. F. Mak, and J. Shan, Nat.
Nanotechnol. 13(7), 549 (2018). https://doi.org/10.1038/s41565-018-0135-x Panels
(d) and (e) reproduced with permission from Verzhbitskiy et al., Nat. Electron.
3, 460 (2020). Copyright 2020 Springer Nature.431431. I. A. Verzhbitskiy, H.
Kurebayashi, H. Cheng, J. Zhou, S. Khan, Y. P. Feng, and G. Eda, Nat. Electron.
3(8), 460 (2020). https://doi.org/10.1038/s41928-020-0427-7 Panel (f) reproduced
with permission from Deng et al., Nature 563, 94 (2018). Copyright 2018 Springer
Nature.5050. Y. Deng, Y. Yu, Y. Song, J. Zhang, N. Z. Wang, Z. Sun, Y. Yi, Y. Z.
Wu, S. Wu, J. Zhu, J. Wang, X. H. Chen, and Y. Zhang, Nature 563(7729), 94
(2018). https://doi.org/10.1038/s41586-018-0626-9 Panels (g) and (h) reproduced
with permission from Wang et al., Europhys. Lett. 114, 47001 (2016). Copyright
2016 IOP.88. H. Wang, F. Fan, S. Zhu, and H. Wu, Europhys. Lett. 114(4), 47001
(2016). https://doi.org/10.1209/0295-5075/114/47001 Panel (i) reproduced with
permission from Li et al., J. Am. Chem. Soc. 136, 11065 (2014). Copyright 2014
American Chemical Society.307307. X. Li, X. Wu, and J. Yang, J. Am. Chem. Soc.
136(31), 11065 (2014). https://doi.org/10.1021/ja505097m
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* High-resolution
Two different gated bilayer CrI3 devices have been built by Huang et al.,428428.
B. Huang, G. Clark, D. R. Klein, D. MacNeill, E. Navarro-Moratalla, K. L.
Seyler, N. Wilson, M. A. McGuire, D. H. Cobden, D. Xiao, W. Yao, P.
Jarillo-Herrero, and X. Xu, Nat. Nanotechnol. 13(7), 544 (2018).
https://doi.org/10.1038/s41565-018-0121-3 and their magnetic behavior has been
probed by MOKE microscopy. Under fixed magnetic fields, voltage-controlled
switching between antiferromagnetic and ferromagnetic states was also found.
Under zero magnetic fields, two distinct layered AFM states (“↑↓” and “↓↑”)
exist on gate voltage, which are related to two initialization processes.
However, tunability of the net magnetization with gate voltage disappeared when
the bilayer device was warmed to about 40 K, revealing that the bilayer TN is
unaffected within the range of applied gate voltage. Parallel to this study,
dual-gate field-effect devices were also fabricated to investigate the electric
field effect on the magnetic order of bilayer CrI3. In the AFM bilayer CrI3, the
applied electric field creates an interlayer potential difference, which results
in a large linear magnetoelectric effect. In addition, a reversible electrical
switching of magnetic order has been observed near the AFM-FM spin-flip
transition point.429429. S. Jiang, J. Shan, and K. F. Mak, Nat. Mater. 17(5),
406 (2018). https://doi.org/10.1038/s41563-018-0040-6
Similar to bilayer CrI3, both gating and electronic field are able to modulate
the magnetic properties of few-layer Cr2Ge2Te6. Using ionic liquid and solid Si
gates, the carrier density below 103 cm−2 was obtained in Cr2Ge2Te6 FET
transistors.430430. Z. Wang, T. Zhang, M. Ding, B. Dong, Y. Li, M. Chen, X. Li,
J. Huang, H. Wang, X. Zhao, Y. Li, D. Li, C. Jia, L. Sun, H. Guo, Y. Ye, D. Sun,
Y. Chen, T. Yang, J. Zhang, S. Ono, Z. Han, and Z. Zhang, Nat. Nanotechnol.
13(7), 554 (2018). https://doi.org/10.1038/s41565-018-0186-z With such carrier
density, Cr2Ge2Te6 transistors show remarkable enhancement of saturation
magnetization and reduction of saturation field. However, Micro-area Kerr
measurements with different gate doping demonstrated bipolar tunable
magnetization loops below the Curie temperature. The unchanged TC may be
attributed to a rebalance of the spin-polarized band structure while tuning its
Fermi level. Using the electrical double layers transistors with a polymer gel
based on ionic liquid, a higher electron-doped concentration (∼1014 cm−2) was
achieved in few-layer Cr2Ge2Te6.431431. I. A. Verzhbitskiy, H. Kurebayashi, H.
Cheng, J. Zhou, S. Khan, Y. P. Feng, and G. Eda, Nat. Electron. 3(8), 460
(2020). https://doi.org/10.1038/s41928-020-0427-7 Compared to the case of low
carrier density, heavy electron doping not only enhances TC to 200 K, but also
turns the magnetic easy axis from out-of-plane to in-plane direction [Figs.
19(d)–19(e)]. First-principles calculations further clarified the origin of
doping induced magnetic ordering. The large carrier density would render
carrier-mediated indirect exchange mechanism as prevailing over the
superexchange mechanism in Cr2Ge2Te6. To understand the electric field effect,
Xing et al. fabricated Hall-bar devices using the nanofabrication technique. The
gate voltage dependence of channel resistances for 2D Cr2Ge2Te6 devices has been
investigated and a giant modulation of the channel resistance via electric field
effect has been revealed.9999. W. Xing, Y. Chen, P. M. Odenthal, X. Zhang, W.
Yuan, T. Su, Q. Song, T. Wang, J. Zhong, S. Jia, X. C. Xie, Y. Li, and W. Han,
2D Mater. 4(2), 024009 (2017). https://doi.org/10.1088/2053-1583/aa7034
In contrast to these FM/AFM insulators, the itinerant magnetism possibly allows
more effective tuning of TC by ionic gating. As we have discussed above, the
exchange mechanism of 2D Fe3GeTe2 is mediated through conduction electrons.
Thus, the extreme ionic gating induced electron doping could drastically
modulate DOS at the Fermi level. A trilayer Fe3GeTe2 ionic field-effect
transistor was set up by covering solid electrolyte (LiClO4) on both sides. The
charge transfer between lithium ions and the Fe3GeTe2 film induces electron
doping up to the order of 1014 cm−2 per layer. Such high doping level boosts TC
of Fe3GeTe2 to room temperature [Fig. 19(f)].5050. Y. Deng, Y. Yu, Y. Song, J.
Zhang, N. Z. Wang, Z. Sun, Y. Yi, Y. Z. Wu, S. Wu, J. Zhu, J. Wang, X. H. Chen,
and Y. Zhang, Nature 563(7729), 94 (2018).
https://doi.org/10.1038/s41586-018-0626-9 The gate-induced room-temperature FM
ordering was also observed in a four-layer sample.
On the theoretical side, one can mimic the gating effect of 2D materials by
adding carriers into the unit cell and calculate the electronic structures by
neutralizing with a homogeneous charge background.88. H. Wang, F. Fan, S. Zhu,
and H. Wu, Europhys. Lett. 114(4), 47001 (2016).
https://doi.org/10.1209/0295-5075/114/47001 For example, Wang et al.88. H. Wang,
F. Fan, S. Zhu, and H. Wu, Europhys. Lett. 114(4), 47001 (2016).
https://doi.org/10.1209/0295-5075/114/47001 have explained the correlation
between the Curie temperature and carrier concentration of CrI3 monolayer. As
discussed above, t2g valence bands and eg conduction bands of Cr atoms are
separated by the insulating gap in CrI3 monolayer. As seen in Fig. 19(g),
electron doping shifts the Fermi level into conduction bands, while hole doping
moves the Fermi level into valence bands. In both situations, the spin-up
channel in CrI3 monolayer becomes metallic, while the spin-down channel remains
insulating. This phenomenon renders the transition of exchange coupling
mechanism from weak superexchange semiconductor to itinerant FM half-metal,
which strongly enhances the FM stability [Fig. 19(h)] as well as the Curie
temperature. To be specific, the calculated TC increases from 75 K for pure CrI3
to 150 K (300 K) for half electron (hole) doped CrI3.88. H. Wang, F. Fan, S.
Zhu, and H. Wu, Europhys. Lett. 114(4), 47001 (2016).
https://doi.org/10.1209/0295-5075/114/47001 In 2D AFM semiconductor MnPSe3,
doping concentrations of electron (hole) above 3 × 1013 (4 × 1013) carriers/cm2
would induce a transition from antiferromagnetic semiconductor to ferromagnetic
half-metal. MC simulations suggested the Curie temperature of doped 2D MnPSe3
crystal reaches up to 206 K [Fig. 19(i)].307307. X. Li, X. Wu, and J. Yang, J.
Am. Chem. Soc. 136(31), 11065 (2014). https://doi.org/10.1021/ja505097m In
pristine monolayer VCl3 and VI3 sheets, the Curie temperatures are only 80 and
98 K, respectively. They can be also be enhanced up to room temperature by
carrier doping.161161. J. He, S. Ma, P. Lyu, and P. Nachtigall, J. Mater. Chem.
C 4(13), 2518 (2016). https://doi.org/10.1039/C6TC00409A The carrier density
dependent total energy differences per MAX3 formula unit (M = V, Cr, Mn, Fe, Co,
Ni; A = Si, Ge, Sn; X = S, Se, Te) between the AFM and FM phases were obtained
from DFT-D2 calculations.290290. B. L. Chittari, D. Lee, N. Banerjee, A. H.
MacDonald, E. Hwang, and J. Jung, Phys. Rev. B 101(8), 085415 (2020).
https://doi.org/10.1103/PhysRevB.101.085415 In the case of FM semiconductors,
the competition between antiferromagnetic and ferromagnetic ordering can be
substantially altered for n doping in VSiTe3, VSnTe3, MnSiSe3, and FeSnTe3, and
p doping for NiSiSe3, NiSTe3, and MnSnTe3, respectively. Hence, the associate TC
can be also tailored. The investigated range of carrier density is 1013∼1014
carriers/cm2. Based on these experimental and theoretical results, we can infer
that a higher electron/hole doped concentration injected into 2D magnetic
semiconductors would modulate the magnetic exchange mechanism and enhance the TC
to room temperature.
Using first-principles quantum transport approach, Dolui et al.432432. K. Dolui,
M. D. Petrovic, K. Zollner, P. Plechac, J. Fabian, and B. K. Nikolic, Nano Lett.
20(4), 2288 (2020). https://doi.org/10.1021/acs.nanolett.9b04556 predicted that
injecting unpolarized charge current parallel to the interface of
bilayer-CrI3/monolayer-TaSe2 vdW heterostructure would induce a spin-orbit
torque. Such effect will convert the first CrI3 layer from AFM to FM ordering in
direct contact with TaSe2 without requiring any external magnetic field. This
reversible current-driven nonequilibrium AFM-FM phase transition is due to the
proximity effect, where evanescent wave functions from metallic TaSe2 is
injected to the first monolayer of CrI3. During this process, the second
monolayer of CrI3 remains insulating.
D. Magnetic field
It is natural to utilize external magnetic field to control the magnetic
properties of 2D magnets. As known, different types of magnetism are defined as
how the system respond to the external magnetic fields. On the one hand,
external magnetic field induced Zeeman spin splitting is used to control
magnetic ordering. On the other hand, the applied magnetic field will enforce
reorientation of the spin direction and thus determine their magnetic ground
state. The Curie temperature may depend strongly on the applied magnetic field
in terms of both strength and direction. If the external field is parallel to
the direction of spontaneous magnetization, TC typically increases with
increasing external magnetic field. On the contrary, the magnetic ordering can
be suppressed by applying an external magnetic field of opposite direction. For
the perpendicular magnetic field direction, a complicated variation may arise
because of the competition between the tendencies of both orderings.
For the recently reported 2D magnets, the effect of external magnetic field has
been firstly investigated in atomic layers of Cr2Ge2Te6. Gong et al.3737. C.
Gong, L. Li, Z. Li, H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang,
Z. Q. Qiu, R. J. Cava, S. G. Louie, J. Xia, and X. Zhang, Nature 546(7657), 265
(2017). https://doi.org/10.1038/nature22060 found that bilayer Cr2Ge2Te6 is a
Heisenberg-type ferromagnet and the Curie temperature is about 40 K. The
magnetic field control of the transition temperature of bilayer Cr2Ge2Te6 is
displayed in Fig. 20(a). Under a very small external magnetic field, magnetic
anisotropy would be induced, and the corresponding TC will increase rapidly. The
picture of spin-wave excitation can explain the weak magnetic fields effect. For
six-layer Cr2Ge2Te6 under a magnetic field of 0.3 T, the calculated Curie
temperature approaches the bulk value of 66 K. When ignoring the single ion
anisotropy, the magnetic field helps increase the magnetic stiffness
logarithmically in 2D materials, which is different from the exchange
interactions in 3D materials. In bulk Cr2Ge2Te6, the TC enhancement originates
from the effect of interlayer coupling.
FIG. 20. (a) The magnetic field controlled TC of bilayer Cr2Ge2Te6. (b) The 3D
plot of angle dependent magnetization under selected magnetic field. (c) The
band structures with magnetic moment along in-plane a axis and out-of-plane c
axis. Panel (a) reproduced with permission from Gong et al., Nature 546, 265
(2017). Copyright 2017 Springer Nature.3737. C. Gong, L. Li, Z. Li, H. Ji, A.
Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang, Z. Q. Qiu, R. J. Cava, S. G.
Louie, J. Xia, and X. Zhang, Nature 546(7657), 265 (2017).
https://doi.org/10.1038/nature22060 Panel (b) reproduced with permission from
Liu et al., Phys. Rev. B 100, 104403 (2019). Copyright 2019 American Physical
Society.433433. W. Liu, Y. Wang, J. Fan, L. Pi, M. Ge, L. Zhang, and Y. Zhang,
Phys. Rev. B 100(10), 104403 (2019). https://doi.org/10.1103/PhysRevB.100.104403
Panel (c) reproduced with permission from Klein et al., Science 360, 1218
(2018). Copyright 2018 American Association for the Advancement of
Science.435435. D. R. Klein, D. MacNeill, J. L. Lado, D. Soriano, E.
Navarro-Moratalla, K. Watanabe, T. Taniguchi, S. Manni, P. Canfield, J.
Fernández-Rossier, and P. Jarillo-Herrero, Science 360(6394), 1218 (2018).
https://doi.org/10.1126/science.aar3617
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* High-resolution
Large external magnetic field can directly tune the magnetic coupling of 2D
magnets. Liu et al.433433. W. Liu, Y. Wang, J. Fan, L. Pi, M. Ge, L. Zhang, and
Y. Zhang, Phys. Rev. B 100(10), 104403 (2019).
https://doi.org/10.1103/PhysRevB.100.104403 systematically investigated the
anisotropic behavior of Fe3-xGeTe2 in terms of anisotropic magnetization,
magnetic entropy change, and critical behavior. Figure 20(b) presents the
magnetization evolution between different magnetic field directions. The
magnetization exhibits the minimum when H‖ab, and it reaches the maximum when
H‖c, confirming that the easy axis of magnetization is along the c axis. The
anisotropic magnetic entropy changes further give two types of magnetic
couplings in Fe3-xGeTe2 for different magnetic field directions. Therefore, it
was inferred that the magnetic correlation around TC can be easily affected by
magnetic field. Jiang et al.434434. P. Jiang, L. Li, Z. Liao, Y. X. Zhao, and Z.
Zhong, Nano Lett. 18(6), 3844 (2018).
https://doi.org/10.1021/acs.nanolett.8b01125 suggested the spin direction of 2D
CrI3 can be switched from out-of-plane to in-plane by applying an in-plane
external magnetic field to overcome the MAE barrier. The change of spin
direction can significantly modify the electronic band structure, including
Fermi surface, topological states, and bandgap [Fig. 20(c)]. Moreover, the
impact of magnetic field is manifested by tunneling through the 2D magnets-based
junctions. For example, Klein et al.435435. D. R. Klein, D. MacNeill, J. L.
Lado, D. Soriano, E. Navarro-Moratalla, K. Watanabe, T. Taniguchi, S. Manni, P.
Canfield, J. Fernández-Rossier, and P. Jarillo-Herrero, Science 360(6394), 1218
(2018). https://doi.org/10.1126/science.aar3617 reported that metamagnetic
transition results in magnetoresistance values of 95%, 300%, and 550% for
bilayer, trilayer, and tetralayer CrI3 barriers, respectively.
E. Interfacial engineering
The emerging 2D intrinsic vdW magnets offer an immediate playground to engineer
them into various composites, which provide fruitful combinations for tailoring
the physical properties and exploring the related applications on spintronics
and quantum computing.436436. M. Gibertini, M. Koperski, A. F. Morpurgo, and K.
S. Novoselov, Nat. Nanotechnol. 14(5), 408 (2019).
https://doi.org/10.1038/s41565-019-0438-6 Choosing a 2D intrinsic magnet as one
part of heterostructure, many different types of 2D materials can be stacked in
the vertical direction as the other part. For example, one can integrate
different 2D magnets together, stack the same kind of 2D magnets together, or
combine 2D magnets with some other non-magnetic 2D materials. No doubt, the
interlayer magnetic coupling in the resulting heterostructure is important,
which is susceptible to the contacting materials in terms of vdW force/chemical
bond, magnetic ordering, stacking modes, and the number of layers. Based on
different combinations, many important mechanisms to modulate the magnetic
properties have been proposed, including symmetry breaking, interlayer charge
transfer, build-in electric field, overlap of electron wave functions, band
alignment, orbital hybridization, dielectric screening, electron hopping paths,
and SOC proximity.9090. W. Zhang, P. K. J. Wong, R. Zhu, and A. T. S. Wee,
InfoMat 1(4), 479 (2019). https://doi.org/10.1002/inf2.12048 These strategies
for enhancing the TC by tuning their interfacial interaction are broadly
highlighted as interfacial engineering.
As we stated above, the magneto-optical Kerr effect firstly confirmed the
existence of magnetism in atomically thin CrI3 sheets.2828. B. Huang, G. Clark,
E. Navarro-Moratalla, D. R. Klein, R. Cheng, K. L. Seyler, D. Zhong, E.
Schmidgall, M. A. McGuire, D. H. Cobden, W. Yao, D. Xiao, P. Jarillo-Herrero,
and X. Xu, Nature 546(7657), 270 (2017). https://doi.org/10.1038/nature22391
Remarkably, 2D CrI3 systems also exhibit magnetic response dependent on the
number of layers. The magnetic ground states of monolayer, bilayer, and trilayer
CrI3 are FM, AFM, FM, respectively, while the magnetic ground state of 3D bulk
CrI3 crystal is FM. The AFM interlayer coupling in bilayer CrI3 is mainly due to
structural transition437437. L. Thiel, Z. Wang, M. A. Tschudin, D. Rohner, I.
Gutiérrez-Lezama, N. Ubrig, M. Gibertini, E. Giannini, A. F. Morpurgo, and P.
Maletinsky, Science 364(6444), 973 (2019).
https://doi.org/10.1126/science.aav6926 and super-superexchange
interaction.2323. N. Sivadas, S. Okamoto, X. Xu, C. J. Fennie, and D. Xiao, Nano
Lett. 18(12), 7658 (2018). https://doi.org/10.1021/acs.nanolett.8b03321 The TC
of monolayer and trilayer CrI3 is 45 and 61 K, respectively, meaning that the
magnetic coupling is strengthened as the thickness of CrI3 sheet increases. The
dependences of TC/TN on the number of layers have also been found in the other
experimentally fabricated 2D magnets, including Fe3GeTe2,41,5041. Z. Fei, B.
Huang, P. Malinowski, W. Wang, T. Song, J. Sanchez, W. Yao, D. Xiao, X. Zhu, A.
F. May, W. Wu, D. H. Cobden, J. H. Chu, and X. Xu, Nat. Mater. 17(9), 778
(2018). https://doi.org/10.1038/s41563-018-0149-750. Y. Deng, Y. Yu, Y. Song, J.
Zhang, N. Z. Wang, Z. Sun, Y. Yi, Y. Z. Wu, S. Wu, J. Zhu, J. Wang, X. H. Chen,
and Y. Zhang, Nature 563(7729), 94 (2018).
https://doi.org/10.1038/s41586-018-0626-9 CrGeTe3,3737. C. Gong, L. Li, Z. Li,
H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang, Z. Q. Qiu, R. J.
Cava, S. G. Louie, J. Xia, and X. Zhang, Nature 546(7657), 265 (2017).
https://doi.org/10.1038/nature22060 FePS3,4848. J.-U. Lee, S. Lee, J. H. Ryoo,
S. Kang, T. Y. Kim, P. Kim, C.-H. Park, J.-G. Park, and H. Cheong, Nano Lett.
16(12), 7433 (2016). https://doi.org/10.1021/acs.nanolett.6b03052 and
MnBi2Te4.319319. M. M. Otrokov, I. P. Rusinov, M. Blanco-Rey, M. Hoffmann, A. Y.
Vyazovskaya, S. V. Eremeev, A. Ernst, P. M. Echenique, A. Arnau, and E. V.
Chulkov, Phys. Rev. Lett. 122(10), 107202 (2019).
https://doi.org/10.1103/PhysRevLett.122.107202 Unlike CrI3, there is no
thickness-induced flipping of magnetic ground state in multilayer Fe3GeTe2,
CrGeTe3, and FePS3 systems. However, their increasing trend with the number of
layers behaves differently, which are more pronounced in Fe3GeTe2 and CrGeTe3.
The TC of four- or five-layer Fe3GeTe2 is 220 K, dropping to 180 K for bilayer,
and finally to 130 K for monolayer.4141. Z. Fei, B. Huang, P. Malinowski, W.
Wang, T. Song, J. Sanchez, W. Yao, D. Xiao, X. Zhu, A. F. May, W. Wu, D. H.
Cobden, J. H. Chu, and X. Xu, Nat. Mater. 17(9), 778 (2018).
https://doi.org/10.1038/s41563-018-0149-7 For CrGeTe3, the TC of bilayer and
bulk phase is 41 and 66 K, respectively. In contrast, the value of TN in AFM
FePS3 is not sensitive to the number of layers, with a fixed value of
118 K.4848. J.-U. Lee, S. Lee, J. H. Ryoo, S. Kang, T. Y. Kim, P. Kim, C.-H.
Park, J.-G. Park, and H. Cheong, Nano Lett. 16(12), 7433 (2016).
https://doi.org/10.1021/acs.nanolett.6b03052 In addition, the unique
layer-dependent magnetism has also been found in MnBi2Te4.319319. M. M. Otrokov,
I. P. Rusinov, M. Blanco-Rey, M. Hoffmann, A. Y. Vyazovskaya, S. V. Eremeev, A.
Ernst, P. M. Echenique, A. Arnau, and E. V. Chulkov, Phys. Rev. Lett. 122(10),
107202 (2019). https://doi.org/10.1103/PhysRevLett.122.107202 The monolayer
MnBi2Te4 is a topologically trivial ferromagnet, whereas the multilayer systems
made up of odd and even numbers of layers are interlayer ferromagnets and
antiferromagnets, respectively.
To further improve the Curie temperature, 2D heterostructures consisting of
different magnetic materials are highly anticipated. At the interface of vdW
heterostructures, more significant charge transfer and increasing external extra
spin-exchange paths can be introduced to enhance the interlayer coupling
strength.438–440438. K. Zollner, P. E. Faria, and J. Fabian, Phys. Rev. B
101(8), 085112 (2020). https://doi.org/10.1103/PhysRevB.101.085112439. H. X. Fu,
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J. F. Ding, P. Jena, and E. J. Kan, J. Phys. Chem. C 123(29), 17987 (2019).
https://doi.org/10.1021/acs.jpcc.9b04631 For example, Fu et al.439439. H. X. Fu,
C. X. Liu, and B. H. Yan, Sci. Adv. 6(10), eaaz0948 (2020).
https://doi.org/10.1126/sciadv.aaz0948 proposed to induce out-of-plane surface
magnetism in the AFM MnBi2Te4 films via magnetism proximity with magnetic CrI3
substrate. A strong exchange bias of ∼40 meV originates from the long Cr-eg
orbital tails that hybridize strongly with Te-p orbitals. Taking monolayer CrI3
on MoTe2 as a prototype system, Chen et al.440440. S. B. Chen, C. X. Huang, H.
S. Sun, J. F. Ding, P. Jena, and E. J. Kan, J. Phys. Chem. C 123(29), 17987
(2019). https://doi.org/10.1021/acs.jpcc.9b04631 revealed that CrI3/MoTe2
heterostructure is an intrinsic FM semiconductor with TC of ∼60 K, which is 15 K
higher than that of pure CrI3. The value of TC can be further enhanced to ∼85 K
by applying an out-of-plane pressure of 4.2 GPa, which corresponds to the
reduced interlayer distance of 1.2 Å. Such a doubling of TC comes from the
introduction of extra spin Cr–Te–Cr superexchange paths [Fig. 21(a)].
FIG. 21. (a) The Curie temperature and the additional superexchange interactions
in compressed and uncompressed CrI3/MoTe2 heterostructure. (b)–(d) The exchange
pathways for type I-I, type II-II, and type I-II bilayers, respectively. Δ and U
represent the t2g-eg crystal field splitting and on-site Hubbard energy,
respectively. π and σ represent the d-p atomic bonding type. Panel (a)
reproduced with permission from Chen et al., J. Phys. Chem. C 123, 17987 (2019).
Copyright 2019 American Chemical Society.440440. S. B. Chen, C. X. Huang, H. S.
Sun, J. F. Ding, P. Jena, and E. J. Kan, J. Phys. Chem. C 123(29), 17987 (2019).
https://doi.org/10.1021/acs.jpcc.9b04631 Panels (b)–(d) reproduced with
permission from Xiao et al., 2D Mater. 7, 045010 (2020). Copyright 2020
IOP.2424. J. Xiao and B. Yan, 2D Mater. 7(4), 045010 (2020).
https://doi.org/10.1088/2053-1583/ab9ea4
* PPT
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* High-resolution
Motivated by these discussions, Liu et al.441441. N. Liu, S. Zhou, and J. Zhao,
Phys. Rev. Mater. 4(9), 094003 (2020).
https://doi.org/10.1103/PhysRevMaterials.4.094003 recently investigated the
magnetic coupling between selected bulk semiconducting substrates and bilayer
CrI3, such as CdSe (0001), ZnS (0001), ZnO (0001), Si (111), SnS, MoS2, WSe2,
GaSe, h-BN, black phosphorus, and InSe. They found that the relatively strong
covalent interaction, i.e., CdSe (0001), ZnS (0001), and ZnO (0001), would
significantly enhance the interlayer exchange. Meanwhile, the stability of
intralayer FM state has also been improved. Hence, AFM to FM transition was
observed on bilayer CrI3. DFT calculations further confirmed that such strong
proximity effect originates from the prominent charge transfer from substrates.
The interlayer coupling between semiconducting substrates and bilayer CrI3 leads
to charge doping from the semiconducting substrates to the band structures of
CrI3, which in turn enhances the occupation of eg orbitals of Cr atom and
significantly increases the t2g-eg hybridization. Consequently, the FM exchange
coupling is strengthened.441441. N. Liu, S. Zhou, and J. Zhao, Phys. Rev. Mater.
4(9), 094003 (2020). https://doi.org/10.1103/PhysRevMaterials.4.094003 On the
experimental side, Xiu et al. also observed interfacial proximity effect at
Fe3GeTe2/CrSb interface by performing elemental-specific XMCD measurements,
which would enhance TC of four-layer Fe3GeTe2 from 140 to 230 K. The inverse
proximity effect drives the interfacial antiferromagnetic CrSb into
ferromagnetic state, and the Fe-Te/Cr-Sb interface is strongly ferromagnetic
coupled. Meanwhile, doping of spin-polarized electrons by the interfacial Cr
layer gives rise to enhancement of TC of the Fe3GeTe2 films.442442. F. Xiu, H.
Wu, Y. Xu, J. Zou, X. Kou, S. A. Morton, A. T. N'Diaye, A. T. S. Wee, P. K. J.
Wong, L. Ai, C. Huang, H. Gao, Y. Yang, J. Sun, W. Zhang, Z. Liao, X. Zhang, Z.
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Within the framework of octahedral crystal field, a more direct
electron-counting rule has been proposed to describe the interlayer magnetic
coupling of magnetic bilayers.443443. J. Xiao and B. Yan, arXiv:2003.09942
(2020). The components include MX2 (M=V, Cr, Mn; X = S, Se), MX2 (M = Mn, Fe,
Co, Ni; X = Cl, Br), MI3 (M =V, Cr), and CrGeTe3 monolayers. The exchange
interaction in all these 2D magnetic monolayers has been well demonstrated by
the superexchange mechanism. Based on the occupation number of d electrons,
their electronic configurations can be denoted as type-I t2gxegy (x + y < 5) and
type-II t2gxegy (x + y ≥ 5). In other words, there is no empty low-energy d
orbital in the type-II layer. Therefore, three types of bilayer heterostructures
can be built by these two kinds of monolayer components, i.e., type I-I, II-II,
and I-II. By considering the hopping pathways, one can figure out that the
exchange interaction between the occupied and occupied orbitals (type II)
belongs to AFM ordering. However, the hopping pathways through the
occupied-to-occupied orbitals and the occupied-to-empty orbitals coexist; thus
type I-I and I-II bilayer structures exhibit competing FM and AFM orderings.
Moreover, FM coupling is usually more favorable for I-II bilayer because of the
orbital orientation and large on-site U value. All these discussions are
illustrated in Figs. 21(b)–21(d). This exchange mechanism can be also applied to
explain most of the magnetic metallic bilayers well (VSe2, CrS2, MnSe2, FeCl2,
and FeBr2). Beyond this mechanism, the intralayer itinerant carriers should be
introduced, which would bring about additional FM exchange effect.443443. J.
Xiao and B. Yan, arXiv:2003.09942 (2020).
For the heterojunctions consisting of magnetic and non-magnetic monolayers, the
non-magnetic part can experience strong magnetic exchange field (MEF), which is
termed as magnetic proximity effect. This effect can break the time reversal
symmetry either to introduce FM ordering into a topological insulator
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Wee, ACS Nano 13(8), 8997 (2019). https://doi.org/10.1021/acsnano.9b02996 When a
TI is in contact with a ferromagnet, both time-reversal and inversion symmetries
are broken at the interface. An energy gap is thus formed at the TI surface, and
its electrons gain a net magnetic moment through short-range exchange
interactions. For example, the integration of monolayer nonmagnetic WSe2 and
ultrathin FM semiconductor CrI3 provides unprecedented control of spin and
valley pseudospin in WSe2, where a large MEF of nearly 13 T and rapid switching
of the WSe2 valley splitting and polarization via flipping of the CrI3 magnetism
are detected.451451. D. Zhong, K. L. Seyler, X. Y. Linpeng, R. Cheng, N.
Sivadas, B. Huang, E. Schmidgall, T. Taniguchi, K. Watanabe, M. A. McGuire, W.
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https://doi.org/10.1126/sciadv.1603113 Moreover, the WSe2 photoluminescence
intensity strongly depends on the relative alignment between photoexcited spins
in WSe2 and CrI3 magnetization, because of ultrafast spin-dependent charge
hopping across the heterostructure interface. Zhang et al.456456. W. Zhang, L.
Zhang, P. K. J. Wong, J. Yuan, G. Vinai, P. Torelli, G. van der Laan, Y. P.
Feng, and A. T. S. Wee, ACS Nano 13(8), 8997 (2019).
https://doi.org/10.1021/acsnano.9b02996 believe that the monolayer VSe2 is on
the verge of magnetic transition. Though interfacial hybridization with Co
atomic overlayer, a magnetic moment of ∼0.4 μB per V atom has been detected
experimentally. Such magnetic interface is free from extrinsic contamination and
could be useful for many spin-based effects.
For the heterojunctions consisting of magnetic and non-magnetic monolayers, the
magnetic part can also experience a magnetic phase transition. For example,
heterostructures of α-RuCl3 on graphene, WSe2, and EuS have been recently
synthesized.457457. Y. Wang, J. Balgley, E. Gerber, M. Gray, N. Kumar, X. Lu,
J.-Q. Yan, A. Fereidouni, R. Basnet, S. J. Yun, D. Suri, H. Kitadai, T.
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them, the insulating α-RuCl3 layer is thought to be in proximity to a quantum
spin liquid. The heterostructure undergoes hole doping of the graphene, WSe2,
and EuS layers with low work function and electron doping of α-RuCl3
layer,457457. Y. Wang, J. Balgley, E. Gerber, M. Gray, N. Kumar, X. Lu, J.-Q.
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phenomenon mainly originates from charge transfer between the correlated
insulating layer and itinerant layer.458458. S. Biswas, Y. Li, S. M. Winter, J.
Knolle, and R. Valentí, Phys. Rev. Lett. 123(23), 237201 (2019).
https://doi.org/10.1103/PhysRevLett.123.237201 In addition, magnetic tunnel
junction is also very important to develop the spintronic device. The large
magnetoresistance amounting was found in graphite-CrI3-graphite and
Fe3GeTe2-BN-Fe3GeTe2 multilayer structures.154,435,459,460154. T. Song, X. Cai,
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F. Defect engineering
Defects, such as vacancies, adatoms, and substitutional impurities, inevitably
exist in 2D materials. The presence of these defects leads to significant
impacts on the magnetic properties. For example, many experimental studies have
revealed that room-temperature ferromagnetism can be introduced into nonmagnetic
TMDs like MoS2, PtSe2, SnS2, WS2, MoTe2, and WSe2 via doping of 3d/4d transition
metal atoms (i.e., Ti, V, Cr, Mn, Fe, Co, Ta, and Ni).461–464461. A. Avsar, A.
Ciarrocchi, M. Pizzochero, D. Unuchek, O. V. Yazyev, and A. Kis, Nat.
Nanotechnol. 14(7), 674 (2019). https://doi.org/10.1038/s41565-019-0467-1462. B.
Li, T. Xing, M. Zhong, L. Huang, N. Lei, J. Zhang, J. Li, and Z. Wei, Nat.
Commun. 8(1), 1958 (2017). https://doi.org/10.1038/s41467-017-02077-z463. P. M.
Coelho, H.-P. Komsa, K. Lasek, V. Kalappattil, J. Karthikeyan, M.-H. Phan, A. V.
Krasheninnikov, and M. Batzill, Adv. Electron. Mater. 5(5), 1900044 (2019).
https://doi.org/10.1002/aelm.201900044464. L. Yang, H. Wu, W. Zhang, X. Lou, Z.
Xie, X. Yu, Y. Liu, and H. Chang, Adv. Electron. Mater. 5(10), 1900552 (2019).
https://doi.org/10.1002/aelm.201900552 Therefore, various strategies of defect
engineering can be developed for realization of the 2D intrinsic magnets with
high Curie temperature. Rational design of defects is able to optimize their
types, concentration, and spatial distribution. Hence, understanding the role of
defects on magnetic exchange interaction as well as developing effective defect
engineering schemes are of great significance for the future development of 2D
spintronics.
Based on DFT calculations, Zhao et al.465465. Y. Zhao, L. Lin, Q. Zhou, Y. Li,
S. Yuan, Q. Chen, S. Dong, and J. Wang, Nano Lett. 18(5), 2943 (2018).
https://doi.org/10.1021/acs.nanolett.8b00314 investigated the electronic and
magnetic properties of CrI3 monolayer with surface I vacancies (denoted as
IV-CrI3). First, the incorporation of I vacancies would break the symmetry. The
two Cr atoms located close to I vacancy (denoted as I-Cr atoms) are different
from the other Cr atoms. The magnetic moment of the I-Cr atom increases from 3
to 3.5 μB due to about ∼0.5 electrons gained from each I vacancy. Second, the
introduction of I vacancies will reduce the bandgap. With vacancy concentration
of 1.96 × 10−6 mol/m2, the bandgap will decrease from 1.13 eV for pristine CrI3
to 0.16 eV for IV-CrI3. More importantly, incorporation of I vacancies is able
to improve TC. The TC of defective IV-CrI3 monolayers with vacancy
concentrations of 0.65, 0.98, and 1.96 × 10−6 mol/m2 are 38, 38, and 44 K,
respectively, in comparison with TC = 35 K for pristine CrI3. To reveal the
origin of such increments of Curie temperature, the exchange mechanisms of CrI3
and IV-CrI3 are schematically displayed in Figs. 22(a) and 22(b), respectively.
The virtual hopping of two pathways, i.e., t2g and eg levels, and eg and eg
levels, are affected by I vacancies. With I vacancy, the former hopping pathway
enhances the FM superexchange interaction by shortening the separation of t2g
and eg levels. The latter one also strengthens FM coupling around the I vacancy,
which is dominated by double exchange interaction. As a result, the effective
ferromagnetic exchange is enhanced.465465. Y. Zhao, L. Lin, Q. Zhou, Y. Li, S.
Yuan, Q. Chen, S. Dong, and J. Wang, Nano Lett. 18(5), 2943 (2018).
https://doi.org/10.1021/acs.nanolett.8b00314
FIG. 22. (a) The magnetic exchange mechanism of pristine CrI3. (b) The magnetic
exchange mechanism of CrI3 with I vacancy. (c) The energy difference of FM and
AFM states (ΔE) from DFT calculations (blue stars) and Heisenberg model
calculations (red circles). (d) Relationship between ΔE and the Cr–Cr bond
length near the point defect. Panels (a) and (b) reproduced with permission from
Zhao et al., Nano Lett. 18, 2943 (2018). Copyright 2018 American Chemical
Society.465465. Y. Zhao, L. Lin, Q. Zhou, Y. Li, S. Yuan, Q. Chen, S. Dong, and
J. Wang, Nano Lett. 18(5), 2943 (2018).
https://doi.org/10.1021/acs.nanolett.8b00314 Panels (c) and (d) reproduced with
permission from Wang et al., Chem. Mater. 32, 1545 (2020). Copyright 2020,
American Chemical Society.466466. R. Wang, Y. Su, G. Yang, J. Zhang, and S.
Zhang, Chem. Mater. 32(4), 1545 (2020).
https://doi.org/10.1021/acs.chemmater.9b04645
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* High-resolution
The effects of different defect types on the magnetic performance of CrI3
monolayer have been discussed by Wang et al. based on defect theory and
first-principles calculations.466466. R. Wang, Y. Su, G. Yang, J. Zhang, and S.
Zhang, Chem. Mater. 32(4), 1545 (2020).
https://doi.org/10.1021/acs.chemmater.9b04645 In total, 20 types of defects have
been constructed, including vacancy, interstitial, substitution, and
bond-rotation defects. The calculated energy differences between FM and AFM
states (ΔE) are summarized in Fig. 22(c). It was found that most point defects,
such as VCr, VI, VCr2-2, VCr2-4, and CrI, can increase the energy difference ΔE.
Hence, the resulted TC will be also enhanced, and the highest TC reaches 210 K
in CrI3 system with CrI defect. In addition, the relationship between Cr–Cr
distance and ΔE are discussed and shown in Fig. 22(d). One can see that AFM (FM)
appears when Cr−Cr distance is less (larger) than 3.70 Å (4.04 Å). This means
that point defects in 2D CrI3 can trigger the FM-AFM transition by distorting
their local configuration. Recently, similar results have also been discussed by
Pizzochero et al.467467. M. Pizzochero, J. Phys. D: Appl. Phys. 53(24), 244003
(2020). https://doi.org/10.1088/1361-6463/ab7ca3 The magnetic properties of CrI3
monolayer embedded with transition metal atoms of most 3d and 4d elements have
been evaluated using DFT calculations.468468. A.-M. Hu, X.-H. Zhang, H.-J. Luo,
and W.-Z. Xiao, Mater. Today Commun. 25, 101438 (2020).
https://doi.org/10.1016/j.mtcomm.2020.101438 Among them, Ti implantation
elevates the magnetic moment from 6.0 to 10 μB per unit cell and renders the
CrI3 monolayer transition from semiconducting to half-metallic. The estimated TC
of Ti-embedded CrI3 monolayer reaches up to 282 K.
Since 2D materials have large surface-to-volume ratio, they are favorable for
adsorption of other atoms or gas molecules. As a representative model, Guo et
al.469469. Y. Guo, S. Yuan, B. Wang, L. Shi, and J. Wang, J. Mater. Chem. C
6(21), 5716 (2018). https://doi.org/10.1039/C8TC01302K found that ultrathin CrI3
nanosheets can be tailored from FM insulator to FM half-metal by adsorption of
Li atoms. The total magnetic moment and energy difference between FM and AFM
phase (EAFM–FM) of Li adsorbed CrI3 sheets increase linearly with the increase
of Li concentration. For 100% Li coverage, the EAFM–FM can reach up to 125 meV,
which is 50% higher than that of pure CrI3 monolayer and suggests a much higher
TC. Bader charge analysis showed that each adsorbed Li atom donates about 0.83
electrons to the six surrounding I atoms. In Sec. IV C, we have already
discussed that charge doping can significantly improve the magnetic stability.
Considering the exposed atmosphere, the magnetic properties of Cr2Ge2Te6
monolayer adsorbed with various gas molecules (i.e., CO, CO2, H2O, N2, NH3, NO,
NO2, O2, and SO2) have been systematically investigated.470470. J. He, G. Ding,
C. Zhong, S. Li, D. Li, and G. Zhang, J. Mater. Chem. C 7(17), 5084 (2019).
https://doi.org/10.1039/C8TC05530K The former five molecules behave as donors,
whereas the latter four molecules act as acceptors. All these gas molecules can
effectively enhance the ferromagnetism and Curie temperature of Cr2Ge2Te6
monolayer, which are ascribed to the considerable charge transfer as well as the
alignment of the frontier molecular orbital. Among them, the TC of NO and NO2
adsorbed Cr2Ge2Te6 systems are 213 and 205 K, respectively, which are
substantially higher than 156 K of pristine Cr2Ge2Te6.
Besides FM semiconductor, one should also note that the defects in itinerant FM
metal may either suppress or retain the FM coupling. In 2D metal VSe2,
theoretical studies suggested that the weak localization effect introduced by
some specific defects would lead to a FM-PM transformation.471471. Q. Cao, F. F.
Yun, L. Sang, F. Xiang, G. Liu, and X. Wang, Nanotechnology 28(47), 475703
(2017). https://doi.org/10.1088/1361-6528/aa8f6c Based on the magnetic data, the
paramagnetism of 2D VSe2 is possibly attributed to edge related defects,
interstitial atoms or dislocations. Recent STM/XMCD/MFM experiments by Chua's
group have also demonstrated this phenomenon.192192. R. Chua, J. Yang, X. He, X.
Yu, W. Yu, F. Bussolotti, P. K. J. Wong, K. P. Loh, M. B. H. Breese, K. E. J.
Goh, Y. L. Huang, and A. T. S. Wee, Adv. Mater. 32(24), 2000693 (2020).
https://doi.org/10.1002/adma.202000693 They observed the formation of
Se‐deficient line epitaxially grown on MoS2 via thermal annealing and these
reconstructed VSe2 monolayers displayed room temperature ferromagnetism. The
corresponding DFT calculations have also indicated the enhancement of
magnetization after reconstruction.
To confirm the intrinsic ferromagnetism, Chua et al. inferred that defect-free
sample is the key in 2D VSe2.192192. R. Chua, J. Yang, X. He, X. Yu, W. Yu, F.
Bussolotti, P. K. J. Wong, K. P. Loh, M. B. H. Breese, K. E. J. Goh, Y. L.
Huang, and A. T. S. Wee, Adv. Mater. 32(24), 2000693 (2020).
https://doi.org/10.1002/adma.202000693 For 2D MnSe2, the presence of
high-density Se vacancies in single layer MnSe2 can stay as a dynamically stable
ferromagnetic 2D crystal.144144. I. Eren, F. Iyikanat, and H. Sahin, Phys. Chem.
Chem. Phys. 21(30), 16718 (2019). https://doi.org/10.1039/C9CP03112J Considering
the total energies for two defective magnetic phases, the FM state still yields
15 meV more per formula than the AFM state.
G. Janus engineering
In the above contents, we have already discussed many feasible strategies to
modify the magnetic properties of 2D materials, including intercalations,
stacking the 2D van der Waals heterojunction, and applying an external electric
field and strain. In fact, they all indicated the decisive role of symmetry
breaking of atomic structures. Two-dimensional Janus materials, naturally
possessing the out-of-plane mirror asymmetry, are expected to modify the
magnetic exchange interaction. In experiments, such 2D Janus materials have been
successfully fabricated. For example, Janus MoSSe monolayer materials have been
prepared by selenizing MoS2 monolayer or substituted sulfurization reaction for
MoSe2 monolayer.472,473472. A.-Y. Lu, H. Zhu, J. Xiao, C.-P. Chuu, Y. Han, M.-H.
Chiu, C.-C. Cheng, C.-W. Yang, K.-H. Wei, Y. Yang, Y. Wang, D. Sokaras, D.
Nordlund, P. Yang, D. A. Muller, M.-Y. Chou, X. Zhang, and L.-J. Li, Nat.
Nanotechnol. 12(8), 744 (2017). https://doi.org/10.1038/nnano.2017.100473. J.
Zhang, S. Jia, I. Kholmanov, L. Dong, D. Er, W. Chen, H. Guo, Z. Jin, V. B.
Shenoy, L. Shi, and J. Lou, ACS Nano 11(8), 8192 (2017).
https://doi.org/10.1021/acsnano.7b03186
According to previous experiments, the transition metal trihalides CrI3, VI3,
and TMD VSe2 were reported to be FM materials. Starting from the structures of
transition metal trihalides (MX3) and TMD (MX2), the corresponding Janus
structures of MX3 and MX2 can be constructed by replacing all X atoms in one
plane by another halide/chalcogenide Y atoms, labeled as M2X3Y3 and MXY,
respectively. The point group symmetry of both MX3 and MX2 monolayers is D3v.
Accordingly, the 3d orbitals of transition metal atom M would split into
two-fold degenerate dz2 and dx2-y2 orbitals and three-fold degenerate dxz, dyz,
and dxy orbitals. Due to the breaking of mirror symmetry, the point group
symmetry of Janus M2X3Y3 and MXY reduces to C3v. The different
halide/chalcogenide atoms in the top and bottom layers would induce
non-degenerated 3d states; that is to say, the five dxz, dyz, dxy, dz2, and
dx2-y2 orbitals have distinct features. Based on different electronegativities
of the halide/chalcogenide atoms in the top and bottom layers, the strength of
FM coupling in M2X3Y3 and MXY monolayers is mediated by X/Y atoms through a
superexchange mechanism or a double exchange mechanism.25,47425. F. Zhang, Y.-C.
Kong, R. Pang, L. Hu, P.-L. Gong, X.-Q. Shi, and Z.-K. Tang, New J. Phys. 21(5),
053033 (2019). https://doi.org/10.1088/1367-2630/ab1ee4474. F. Zhang, W. Mi, and
X. Wang, Adv. Electron. Mater. 6(1), 1900778 (2020).
https://doi.org/10.1002/aelm.201900778 Therefore, high TC could be obtained by
Janus MX3 and MX2 monolayers.474–477474. F. Zhang, W. Mi, and X. Wang, Adv.
Electron. Mater. 6(1), 1900778 (2020).
https://doi.org/10.1002/aelm.201900778475. J. He and S. Li, Comput. Mater. Sci.
152, 151 (2018). https://doi.org/10.1016/j.commatsci.2018.05.049476. C. Zhang,
Y. Nie, S. Sanvito, and A. Du, Nano Lett. 19(2), 1366 (2019).
https://doi.org/10.1021/acs.nanolett.8b05050477. Y. Ren, Q. Li, W. Wan, Y. Liu,
and Y. Ge, Phys. Rev. B 101(13), 134421 (2020).
https://doi.org/10.1103/PhysRevB.101.134421
As a specific example, DFT calculations by He et al.475475. J. He and S. Li,
Comput. Mater. Sci. 152, 151 (2018).
https://doi.org/10.1016/j.commatsci.2018.05.049 predicted that the Janus TMD
monolayers have large spin polarization and high Curie temperature. The
room-temperature Curie temperatures were found for 2D VSSe and VSeTe among Janus
systems of MXY (M = V, Cr, Mn; X, Y = S, Se, Te; X ≠ Y). The V atoms and
nearest-neighboring S/Se/Te atoms keep an AFM spin arrangement. Hence, double
exchange interaction induced FM ordering is most favorable. Zhang et al.476476.
C. Zhang, Y. Nie, S. Sanvito, and A. Du, Nano Lett. 19(2), 1366 (2019).
https://doi.org/10.1021/acs.nanolett.8b05050 have also found that VSSe monolayer
is a highly stable room-temperature ferromagnet (TC = 346∼1079 K) by
particle-swarm search and DFT calculations.
Stimulated by the low TC of CrI3, the magnetic properties of 2D Janus Cr2I3X3 (X
= Br, Cl) monolayers have been studied by DFT calculations.474474. F. Zhang, W.
Mi, and X. Wang, Adv. Electron. Mater. 6(1), 1900778 (2020).
https://doi.org/10.1002/aelm.201900778 The exchange energies in CrI3, Cr2I3Br3,
and Cr2I3Cl3 were 42.7 meV, 25.7 meV, and 20.5 meV, respectively, indicating
their FM ground states. Based on MFT theory and Heisenberg model, the TC of
CrI3, Cr2I3Br3, and Cr2I3Cl3 were predicted to be 55, 33, and 26 K,
respectively. Moaied et al. systemically explored the electronic and magnetic
properties of CrX3 (X = Cl, Br, I) and their Janus monolayers X3-Cr2-Y3 (X, Y =
Cl, Br, I, X; X ≠ Y). They found both electronic gap and Curie temperature are
sensitive to the halide atoms. Unfortunately, the TC of Janus systems is lower
than that of CrI3. Unlike in CrI3, the Janus strategy shows a positive effect on
TC in VI3.477477. Y. Ren, Q. Li, W. Wan, Y. Liu, and Y. Ge, Phys. Rev. B
101(13), 134421 (2020). https://doi.org/10.1103/PhysRevB.101.134421 From
first-principles calculations, the Curie temperatures of VI3-derived Janus
monolayers, i.e., V2Cl3I3, V2Br3I3, and V2Cl3Br3, were determined to be 240,
224, and 232 K, respectively, which is at least 170 K higher than the value of
VI3 (50 K). Such dramatic increment on the TC of Janus VI3 monolayers was
ascribed to the reduced virtual exchange gap Gex from one occupied t2g state to
empty eg states, which mainly determines the strength of FM coupling. The
corresponding Gex values were 1.73, 2.156, 0.395, 0.397, and 0.445 for VI3,
CrI3, V2Cl3I3, V2Br3I3, and V2Cl3Br3, respectively. Thus, the superexchange
interactions in V2Cl3I3, V2Br3I3, and V2Cl3Br3 are enhanced relative to VI3 and
CrI3, owing to the reduced virtual exchange gap, Gex.
H. Optical controlling
Beyond the above modification strategies, optical controlling magnetic
properties are also highly anticipated, since no contact is involved. To
revolutionize the future technology of magnetic storage and spintronics, optical
induced manipulation of spin has advantages of fast speed and low power
dissipative.478478. J. K. Dewhurst, P. Elliott, S. Shallcross, E. K. U. Gross,
and S. Sharma, Nano Lett. 18(3), 1842 (2018).
https://doi.org/10.1021/acs.nanolett.7b05118 Recent breakthrough in 2D magnets
highlights the research of optically tuning of magnetism. Several pioneer works
have already been carried out to investigate the interaction of light and
magnetism.
Based on first-principles calculations, Tian et al.479479. Y. Tian, W. Gao, E.
A. Henriksen, J. R. Chelikowsky, and L. Yang, Nano Lett. 19(11), 7673 (2019).
https://doi.org/10.1021/acs.nanolett.9b02523 studied the ground state of
monolayer RuCl3 under optical doping. They found that optical doping can cause a
phase transition from spin-liquid phase to FM ordering with a moderate
electron-hole (e-h) density. The optically tunable 2D magnetism in RuCl3 and the
calculated exchange coupling constants under different doping densities are
schematically plotted in Figs. 23(a)–23(c). We can see both ferromagnetism and
Curie temperature are significantly increased with increasing optical doping e-h
pair density. As e-h pair reaches up to 3× 1013 cm−2, TC is close to room
temperature. Such enhancement originates from the itinerant electron mechanism,
which is demonstrated by PDOS analysis. The Van Hove singularities in the PDOS
appear right above and below the Fermi energy, which have the characteristics of
localized Ru t2g orbital.479479. Y. Tian, W. Gao, E. A. Henriksen, J. R.
Chelikowsky, and L. Yang, Nano Lett. 19(11), 7673 (2019).
https://doi.org/10.1021/acs.nanolett.9b02523
FIG. 23. (a) Schematic plot of the optically tunable magnetism in 2D RuCl3. (b)
The exchange coupling constants according to optical doping in RuCl3. (c)
Variation of TC for RuCl3 under optical e-h doping. (d) The time evolution of
local magnetic moment in Cr2VC2F2. (e) The relative magnetization dynamics for
Cr1, Cr2, and V atom in Cr2VC2F2. (f)–(h) The snapshots of magnetization density
at different time. Panels (a)–(c) reproduced with permission from Tian et al.,
Nano Lett. 19, 7673 (2019). Copyright 2019 American Chemical Society.479479. Y.
Tian, W. Gao, E. A. Henriksen, J. R. Chelikowsky, and L. Yang, Nano Lett.
19(11), 7673 (2019). https://doi.org/10.1021/acs.nanolett.9b02523 Panels (d)–(h)
reproduced with permission from He et al., J. Phys. Chem. Lett. 11, 6219 (2020).
Copyright 2020 American Chemical Society.480480. J. He and T. Frauenheim, J.
Phys. Chem. Lett. 11(15), 6219 (2020).
https://doi.org/10.1021/acs.jpclett.0c02007
* PPT
|
* High-resolution
The optically manipulating magnetic order transition was also predicted in
MXenes.480480. J. He and T. Frauenheim, J. Phys. Chem. Lett. 11(15), 6219
(2020). https://doi.org/10.1021/acs.jpclett.0c02007 DFT calculations revealed
that the initial ground states of Cr2VC2F2, Mo2VC2F2, Mo2VN2F2, Mo3C2F2, and
Mo3N2F2 are ferrimagnetic. The real time time-dependent DFT simulations on these
MXenes showed that the FiM state will transform to FM state at the early stage
of simulation. Figures 23(d)–23(h) display the spin dynamics of Cr2VC2F2 MXene
under a laser pulse. The local magnetic moments on both Cr and V atoms change
dramatically. However, the total magnetic moment in the MXene is retained. At
5.8 fs, the spin direction of V atom is reversed from –0.4 to 0.9 μB, which
results in a magnetic ordering transition from FiM to FM. The microscopic
mechanism underpinning this ultrafast switching of magnetic ordering is governed
by optically induced inter-site spin transfer effect. In addition, other
critical magnetic parameters, such as MAE and magnetic exchange field, can be
optically controlled in WSe2/CrI3 heterostructures and CrI3
monolayer.455,481455. K. L. Seyler, D. Zhong, B. Huang, X. Linpeng, N. P.
Wilson, T. Taniguchi, K. Watanabe, W. Yao, D. Xiao, M. A. McGuire, K.-M. C. Fu,
and X. Xu, Nano Lett. 18(6), 3823 (2018).
https://doi.org/10.1021/acs.nanolett.8b01105481. J. Kim, K.-W. Kim, B. Kim,
C.-J. Kang, D. Shin, S.-H. Lee, B.-C. Min, and N. Park, Nano Lett. 20(2), 929
(2020). https://doi.org/10.1021/acs.nanolett.9b03815
V. CONCLUSION AND OUTLOOK
Section:
ChooseTop of pageABSTRACTI. INTRODUCTIONII. THE ORIGIN OF MAGNETI...III. THE 2D
VDW MAGNETS D...IV. MODIFICATIONSV. CONCLUSION AND OUTLOOK <<AUTHORS'
CONTRIBUTIONSCITING ARTICLESChoose
In this paper, we have comprehensively reviewed recent experimental and
theoretical progress on the 2D intrinsic magnets. Their fundamental physical
parameters, magnetic origin, and underlying mechanism of exchange interaction
have been discussed. Despite the great achievements during the past decade, the
research of 2D intrinsic magnets is still in its early stage and calls for
continuous efforts. Going forward, there are four rapidly expanding fields in
the foreseeable future. The first one is the discovery of “unknown”
room-temperature 2D magnets. The second one is to deeply understand the spin
coupling mechanism and provide some feasible ways to manipulate the spin. The
third one is to further explore the complex spin-based effects, especially on
systems with both magnetism and superconductivity, FM and ferroelectrics (FE),
FM and ferroelastics (FA), magnetism and TI, and magnetism and
thermoelectricity. Based on the newly found 2D intrinsic magnets, the fourth one
is to integrate them into practical devices. We will further elaborate on these
issues in this section.
From the theoretical point of view, many hypothetical 2D magnetic materials have
been proposed. However, their structural stability, environmental stability at
ambient air, and experimental feasibility for synthesis still need to be further
confirmed.31,9231. Z. Zhang, J. Shang, C. Jiang, A. Rasmita, W. Gao, and T. Yu,
Nano Lett. 19(5), 3138 (2019). https://doi.org/10.1021/acs.nanolett.9b0055392.
X. Zhang and C. Gong, Science 363(6428), 4450 (2019).
https://doi.org/10.1126/science.aav4450 Moreover, their magnetic ground states
and Curie temperatures sometimes depend on the theoretical methodology, e.g.,
exchange-correlation functionals, the choice of U term and the details of
Heisenberg model. Some are poor functionals and/or don't properly include
correlation effect, which may give inaccurate results. In this regard, the
electronic and magnetic properties of these 2D materials should be calculated
with high-level accuracy. Meanwhile, these reliable results will provide a
starting point for further data analysis and screening.68,48268. Y. Zhu, X.
Kong, T. D. Rhone, and H. Guo, Phys. Rev. Mater. 2(8), 081001 (2018).
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Costa, K. A. Persson, S. P. Ong, P. Huck, Y. Lu, X. Ma, Y. Chen, H. Tang, and Y.
P. Feng, Sci. Data 6(1), 86 (2019). https://doi.org/10.1038/s41597-019-0097-3
Then high-throughput computation and machine learning can greatly accelerate the
discovery of 2D magnetic materials and help understand the magnetic exchange
interaction as well as the origin of magnetic ordering thoroughly.483483. J.
Nelson and S. Sanvito, Phys. Rev. Mater. 3(10), 104405 (2019).
https://doi.org/10.1103/PhysRevMaterials.3.104405
On the experimental side, besides exfoliation of 2D magnets from the
corresponding 3D layered materials, other approaches for directly growing
high-quality 2D magnets should be developed. For example, MBE allows us to
create novel 2D ultrathin films from those that are not inherently layered
materials. This method has demonstrated its success in preparing 2D magnetic,
CrI3,484484. P. Li, C. Wang, J. Zhang, S. Chen, D. Guo, W. Ji, and D. Zhong,
Sci. Bull. 65(13), 1064 (2020). https://doi.org/10.1016/j.scib.2020.03.031 TMDs
(VSe2, MnSe2, VTe2, CrTe2), and MnSn at high temperature.34,35,192,48534. M.
Bonilla, S. Kolekar, Y. Ma, H. C. Diaz, V. Kalappattil, R. Das, T. Eggers, H. R.
Gutierrez, M.-H. Phan, and M. Batzill, Nature Nanotechnol. 13(4), 289 (2018).
https://doi.org/10.1038/s41565-018-0063-935. J. Li, B. Zhao, P. Chen, R. Wu, B.
Li, Q. Xia, G. Guo, J. Luo, K. Zang, Z. Zhang, H. Ma, G. Sun, X. Duan, and X.
Duan, Adv. Mater. 30(36), 1801043 (2018).
https://doi.org/10.1002/adma.201801043192. R. Chua, J. Yang, X. He, X. Yu, W.
Yu, F. Bussolotti, P. K. J. Wong, K. P. Loh, M. B. H. Breese, K. E. J. Goh, Y.
L. Huang, and A. T. S. Wee, Adv. Mater. 32(24), 2000693 (2020).
https://doi.org/10.1002/adma.202000693485. K. Lasek, P. M. Coelho, K. Zberecki,
Y. Xin, S. K. Kolekar, J. Li, and M. Batzill, ACS Nano 14(7), 8473 (2020).
https://doi.org/10.1021/acsnano.0c02712 By carefully controlling the temperature
of CVD method, Kang et al.209209. L. Kang, C. Ye, X. Zhao, X. Zhou, J. Hu, Q.
Li, D. Liu, C. M. Das, J. Yang, D. Hu, J. Chen, X. Cao, Y. Zhang, M. Xu, J. Di,
D. Tian, P. Song, G. Kutty, Q. Zeng, Q. Fu, Y. Deng, J. Zhou, A. Ariando, F.
Miao, G. Hong, Y. Huang, S. J. Pennycook, K.-T. Yong, W. Ji, X. R. Wang, and Z.
Liu, Nat. Commun. 11(1), 3729 (2020). https://doi.org/10.1038/s41467-020-17253-x
have synthesized antiferromagnetic ultrathin 2D layered tetragonal FeTe
nanoplates and ferromagnetic non-layered hexagonal FeTe nanoplates with
thickness down to 2.8 and 3.6 nm on SiO2/Si substrates, respectively. The
observed TN of the former phase is 71.8 K, while the TC of the latter phase is
220 K. In addition, chemical vapor transport is also an efficient strategy for
producing highly crystalline 2D magnets in industry, such as CrI3
nanolayer.486486. M. Grönke, B. Buschbeck, P. Schmidt, M. Valldor, S. Oswald, Q.
Hao, A. Lubk, D. Wolf, U. Steiner, B. Büchner, and S. Hampel, Adv. Mater. Inter.
6(24), 1901410 (2019). https://doi.org/10.1002/admi.201901410 Han et al.487487.
N. Han, D. Yang, C. Zhang, X. Zhang, J. Shao, Y. Cheng, and W. Huang, J. Phys.
Chem. Lett. 11(21), 9453 (2020). https://doi.org/10.1021/acs.jpclett.0c02717
further demonstrated the important role of iodine buffer layer and the role of
CrI2 clusters as building units, which pointed out the reason of achieving 2D
CrI3 sheets with nanolayer. In addition, syntheses of 2D magnetic films using
techniques like electrochemical deposition, metal-organic chemical vapor
deposition, pulsed laser deposition, sputtering, and thermal evaporation are in
their infancy and need further investigation. All these methods provide valuable
insights for the preparation of 2D magnets, which could certainly extend the
experimental database of emerging 2D magnetic materials. Moreover, direct growth
of 2D magnets on Si substrates is beneficial for compatibility with the mature
Si technology.
With respect to the specific category of 2D intrinsic magnets, 2D binary
transition halides have received the most attention so far. However, the
dominated d-p-d superexchange interaction usually leads to very low TC. Hence,
the effective methods (such as electron doping and phase controlling) to improve
the TC of these semiconductor need to be further explored. To date, few 2D TMD
materials have been demonstrated as high-temperature FM metals. However, there
are still some disagreements on their magnetic behavior. The effects of CDW and
defects on the magnetic ground states need to be carefully examined. Both
effects contribute to the ongoing controversial debates regarding the magnetic
properties of 2D VSe2.192192. R. Chua, J. Yang, X. He, X. Yu, W. Yu, F.
Bussolotti, P. K. J. Wong, K. P. Loh, M. B. H. Breese, K. E. J. Goh, Y. L.
Huang, and A. T. S. Wee, Adv. Mater. 32(24), 2000693 (2020).
https://doi.org/10.1002/adma.202000693 Moreover, new FM semiconductor phases are
still highly expected in the 2D TMD families. We surprisingly noticed that
plenty of MXene phases are predicted to possess high TC. However, their magnetic
properties are sensitive to the functional groups terminated on the outer
surface. Owing to the difficulty of attaining MXene sheets with ordered
functional groups, it becomes a rather challenging task to prepare the robust
magnetic MXenes in laboratory. Among ternary transition metal compounds M-X-Y,
the relationship between magnetism and composition is an important topic. Among
this big family of 2D materials, many promising systems stand out, such as
CrXTe3 (X = Si, Ge, Sn), MPX3 (X = S, Se, Te), Fe-Ge-Te, MnBi2Te4, and MXY (X =
O, S, Se, Te, N; Y = Cl, Br, I) compounds. Some of them have been successfully
synthesized in laboratory and found to exhibit high TC, novel layer-dependent
AFM behavior, interesting topological quantum states, and high carrier mobility.
Therefore, we expect to find more 2D analogues in this family. Moreover, the
random combinations of M, X, and Y atoms in ternary transition metal compounds
may further cause more complicated magnetic exchange interactions, which in turn
would be beneficial to achieve high magnetic transition temperature.
The long-range p-electron and f-electron based 2D magnets were reported in a
series nonstoichiometric compounds and Gd-based compounds, respectively. The
competition between the localized and delocalized behavior of electrons may
bring into new magnetic exchange interaction mechanism. Hence, relevant efforts
toward high-temperature magnetism are promising. Moreover, a series of ultrathin
films and non-vdW materials, which exhibit room temperature FM ordering, were
also reported.34,35,172,209,485,48834. M. Bonilla, S. Kolekar, Y. Ma, H. C.
Diaz, V. Kalappattil, R. Das, T. Eggers, H. R. Gutierrez, M.-H. Phan, and M.
Batzill, Nature Nanotechnol. 13(4), 289 (2018).
https://doi.org/10.1038/s41565-018-0063-935. J. Li, B. Zhao, P. Chen, R. Wu, B.
Li, Q. Xia, G. Guo, J. Luo, K. Zang, Z. Zhang, H. Ma, G. Sun, X. Duan, and X.
Duan, Adv. Mater. 30(36), 1801043 (2018).
https://doi.org/10.1002/adma.201801043172. J.-Y. You, C. Chen, Z. Zhang, X.-L.
Sheng, S. A. Yang, and G. Su, Phys. Rev. B 100(6), 064408 (2019).
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Zhou, J. Hu, Q. Li, D. Liu, C. M. Das, J. Yang, D. Hu, J. Chen, X. Cao, Y.
Zhang, M. Xu, J. Di, D. Tian, P. Song, G. Kutty, Q. Zeng, Q. Fu, Y. Deng, J.
Zhou, A. Ariando, F. Miao, G. Hong, Y. Huang, S. J. Pennycook, K.-T. Yong, W.
Ji, X. R. Wang, and Z. Liu, Nat. Commun. 11(1), 3729 (2020).
https://doi.org/10.1038/s41467-020-17253-x485. K. Lasek, P. M. Coelho, K.
Zberecki, Y. Xin, S. K. Kolekar, J. Li, and M. Batzill, ACS Nano 14(7), 8473
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Radhakrishnan, C. F. Woellner, S. K. Sinha, L. Deng, C. L. Reyes, B. M. Rao, M.
Paulose, R. Neupane, A. Apte, V. Kochat, R. Vajtai, A. R. Harutyunyan, C. W.
Chu, G. Costin, D. S. Galvao, A. A. Martí, P. A. van Aken, O. K. Varghese, C. S.
Tiwary, A. Malie Madom Ramaswamy Iyer, and P. M. Ajayan, Nat. Nanotechnol.
13(7), 602 (2018). https://doi.org/10.1038/s41565-018-0134-y It is therefore
anticipated that large amounts of 2D magnetic materials beyond vdW family will
be unveiled soon. There is also plenty of room in the 2D organic magnets, as the
combination of coordination chemistry and crystal engineering enables
intentional design of organic molecule based 2D frameworks with good magnetic
stability and highly tunable magnetic properties. So far, the general
performance of 2D organic magnets is still modest compared to that of 2D
inorganic magnets. Room-temperature FM semiconductors are expected by rational
design of molecular assemblies and organic ligands.
Generally speaking, AFM ordering in 3D materials is more common than FM
ordering. However, 2D antiferromagnetic materials are rarely reported in
comparison with the widely investigated 2D ferromagnets. In fact, many unique
properties make 2D AFM materials even superior to 2D ferromagnets for practical
device implementations. 2D antiferromagnets may allow the continuous
miniaturization of spintronic devices. They have the advantage of being
insensitive to the parasitic external magnetic fields and high read/write
memory, and are thus poise to become an integral part of the next-generation
logical devices and memory.489489. C. Ó. Coileáin and H. C. Wu, Spin 7(3),
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Recently, 2D magnets with complex spin configurations are also of great
interest, which are most relevant to quantum Hall effect,490,491490. M. Lee, W.
Kang, Y. Onose, Y. Tokura, and N. P. Ong, Phys. Rev. Lett. 102(18), 186601
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high-temperature superconductivity,492492. P. Liu, X. Luo, Y. Cheng, X.-W. Wang,
W. Wang, H. Liu, K. Cho, W.-H. Wang, and F. Lu, J. Phys.: Condens. Matter
30(32), 325801 (2018). https://doi.org/10.1088/1361-648X/aad0d1 and data
storage.493493. D. Foster, C. Kind, P. J. Ackerman, J.-S. B. Tai, M. R. Dennis,
and I. I. Smalyukh, Nat. Phys. 15(7), 655 (2019).
https://doi.org/10.1038/s41567-019-0476-x Therefore, their identification and
interpretation are the important issues. For example, the concept of quantum
spin liquid has been first proposed in 1973 as the ground state for a system of
spin on a triangular lattice.494494. P. W. Anderson, Mater. Res. Bull. 8(2), 153
(1973). https://doi.org/10.1016/0025-5408(73)90167-0 One intuitive description
of the quantum spin liquid state is as a liquid of disordered spins. This state
is not like the other disordered states, which will preserve its disorder to
very low temperatures. Until now, no real 2D magnetic material with quantum spin
liquid state has been identified. Based on previous investigations,495495. L.
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Cardoso et al.524524. C. Cardoso, D. Soriano, N. A. García-Martínez, and J.
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exists at the Dirac point in the antiparallel configuration, whereas the
graphene bilayer remains conducting in the parallel configuration. These
exciting innovative devices are all related to magnetic heterostructures.
Stacking more potential room-temperature 2D magnets as the building blocks and
investigating their performances in spintronic and quantum devices are just
beginning.
All these exciting progresses will shed light on the design and synthesis of
room-temperature 2D magnets with intrinsic FM/AFM ordering. Compared to the bulk
phases, the atomic thinness of the layers also leads to strong tunability via
strain, intercalation, external fields (electronic, optical, and magnetic),
interfacial interaction, defects, and functional groups, as discussed in Sec.
IV. The successive research of 2D magnets with novel properties would not only
provide more abundant platform to explore the fundamental physics but also move
forward to develop efficient non-volatile memory, spin-based logic devices,
spin-dependent optoelectronics, and so on. In this regard, we believe that the
development of 2D magnets is still in its infancy but has already shown its huge
potential.
AUTHORS' CONTRIBUTIONS
Section:
ChooseTop of pageABSTRACTI. INTRODUCTIONII. THE ORIGIN OF MAGNETI...III. THE 2D
VDW MAGNETS D...IV. MODIFICATIONSV. CONCLUSION AND OUTLOOKAUTHORS' CONTRIBUTIONS
<<CITING ARTICLESChoose
All authors have contributed equally to this manuscript. All authors have
reviewed final version of this manuscript.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China
(Grant Nos. 11874097, 91961204, 12004065, 11964023) and the Fundamental Research
Funds for the Central Universities of China (Grant No. DUT19LK12). We
acknowledge the Xinghai Scholar project of Dalian University of Technology and
the project of Dalian Youth Science and Technology Star (Grant No. 2017RQ012).
DATA AVAILABLITY
Data sharing is not applicable to this article as no new data were created or
analyzed in this study.
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