Recent progress on 2D magnets: Fundamental mechanism, structural design and modification

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.

Covalent Mixing in the 2D Ferromagnet CrSiTe3 Evidenced by Magnetic X‐Ray Circular Dichroism

“…[ 10 ] From the observed change in resistivity at 80−120 K, the Curie temperature of those flakes was estimated to be ≈80 K. [ 10 ] This remarkable increase from the bulk value of 33 K [ 6,7,11 ] is in stark contrast to the behavior of other 2D ferromagnets and has been proposed to stem from the increased intralayer superexchange interaction via Te in the few‐layer limit. [ 9,12 ] Recently, in the ultrathin layer limit, the thickness‐dependent magnetic transition from 3D (Heisenberg) to 2D Ising behavior has been observed for a thickness of ≈7 nm, which goes hand in hand with a drop in Curie temperature from 33 to 17 K. [ 13 ] This behavior is often tied to the strong dependence of the electronic structure on the reduced dimensionality in the ultrathin layer limit, as it has been experimentally observed for . [ 1 ]…”

“…[ 5 ] It is important to note that despite their close similarities such as their ferromagnetic bulk properties (in contrast to the prediction of being an antiferromagnet), [ 6–8 ] the very different magnetic properties of compared with monolayers are due to minute differences in their second‐ and third‐nearest neighbor exchange interactions, [ 5 ] the larger van der Waals (vdW) gap, and the smaller Cr−Cr distance. [ 9 ] Also, while the interlayer coupling in is antiferromagnetic, the intralayer coupling is ferromagnetic, and the overall properties of the material therefore strongly depend on the c ‐axis lattice constant. [ 5 ] The thickness dependence of the magnetic properties of , down to single‐ and few‐monolayer flakes, has been investigated via Hall measurements.…”

Recent Progress of Atomic Layer Technology in Spintronics: Mechanism, Materials and Prospects

“…Firstly, the intrinsic ferromagnetism of 2D magnets was only detected in low-temperature conditions, which was a significant step forward. In 2018, O'Hara et al observed ferromagnetism in MnSe x films grown by MBE at room temperature [6,7]. A representative metal 2D magnet is represented by the monolayerr VSe 2 , which exhibits charge density wave and magnetic phenomena at the same time.…”

Structure and Magnetism of Iron- and Chromium-Intercalated Niobium and Tantalum Disulfides

“…38,39 Additional terms, such as the DM interaction and the magnetocrystalline anisotropy energy (MCAE), arise from both spin-orbit coupling and the underlying symmetry of the compound. 40,41 In this section, we introduce the terms thought to be most relevant to magnetic ordering in Fe-and Cr-intercalated NbS 2 and TaS 2 , namely the RKKY interaction, the DM interaction, and MCAE. Excellent general introductions to magnetism in extended solids can be found elsewhere.…”