Ferromagnetism usually only occurs in materials containing elements that form covalent 3d and 4f bonds. Its occurrence in pure carbon is therefore surprising, even controversial. A systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries.

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Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects

Based on first-principles calculations, we investigate a novel class of 2D materials – MX2 metal dihalides (X = Cl, Br, I). Our results show that single-layer dihalides are energetically and dynamically stable and can be potentially exfoliated from their bulk layered forms. We found that 2D FeX2, NiX2, CoCl2 and CoBr2 monolayers are ferromagnetic (FM), while VX2, CrX2, MnX2 and CoI2 are antiferromagnetic (AFM). The magnetic properties of 2D dihalides originate from the competition between AFM direct nearest-neighbor d–d exchange and FM superexchange via halogen p states, which leads to a variety of magnetic states. The thermal dependence of magnetic properties and the Curie temperature of magnetic transition are evaluated using statistical Monte Carlo simulations based on the Ising model with classical Heisenberg Hamiltonian. The magnetic properties of single-layer dihalides can be further tuned by strain and carrier doping. Our study broadens the family of existing 2D materials with promising applications in nanospintronics.

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Single-layer metal halides MX2 (X = Cl, Br, I): stability and tunable magnetism from first principles and Monte Carlo simulations

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.

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

We extend the 2D MXene family into the boride world, namely, MBenes High-throughput calculations screen twelve MBenes with excellent stability Among them, 2D MnB MBene exhibits robust metallic ferromagnetism (∼32 μB per Mn atom) and high Curie temperature (345 K) After functionalization with the –F and –OH groups, the ferromagnetic ground state of 2D MnB is well preserved The Curie temperature is increased to 405 and 600 K, respectively, providing a novel and feasible strategy to tailor the TC of 2D magnetic materials

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MBene (MnB): a new type of 2D metallic ferromagnet with high Curie temperature

Two-dimensional (2D) intrinsic ferromagnets with high Curie temperature (TC) are desirable for spintronic applications. Using systematic first-principles calculations, we investigate the electronic and magnetic properties of 22 monolayer 2D materials with layered bulk phases. From these candidates, we screen out five ferromagnetic monolayer materials belonging to three types of structures: type i (ScCl, YCl, LaCl), type ii (LaBr2), and type iii (CrSBr). Type i is a kind of metallic ferromagnetic material, whereas LaBr2 and CrSBr of type ii and iii are small-bandgap ferromagnetic semiconductors with TC near room temperature. Moreover, the ferromagnetic CrSBr monolayer possesses a large magnetic moment of ∼3 μB per Cr atom, originating from its distorted octahedron coordination. The robust ferromagnetism of the CrSBr monolayer is ascribed to the halogen-mediated (Cr–Br–Cr) and chalcogen-mediated (Cr–S–Cr) superexchange interactions; then, an isoelectronic substitution strategy is proposed to tailor the magn...

Screening and Design of Novel 2D Ferromagnetic Materials with High Curie Temperature above Room Temperature

In spintronics, it is highly desirable to find new materials that can simultaneously possess complete spin-polarization, high-speed conduction electrons, large Curie temperature, and robust ferromagnetic ground states. Using first-principles calculations, we demonstrate that the stable YN2 monolayer with octahedral coordination is a novel p-state Dirac half metal (DHM), which not only has a fully spin-polarized Dirac state, but also the highest Fermi velocity (3.74 × 105 m/s) of the DHMs reported to date. In addition, its half-metallic gap of 1.53 eV is large enough to prevent the spin-flip transition. Because of the strong nonlocal p orbitals of N atoms (N-p) direct exchange interaction, the Curie temperature reaches over 332 K. Moreover, its ferromagnetic ground state can be well preserved under carrier doping or external strain. Therefore, the YN2 monolayer is a promising DHM for high-speed spintronic devices and would lead to new opportunities in designing other p-state DHMs.

YN2 monolayer: Novel p-state Dirac half metal for high-speed spintronics

For next-generation nanospintronic devices, it is crucial to develop intrinsic half-metallic nanomaterials with a robust ferromagnetic (FM) ground state. Here, two kinds of two-dimensional (2D) intrinsic FM half-metals (HMs), i.e., p-state HMs (1T-TaN2 and 1T-NbN2) and d-state HM (1T-MnN2), have been characterized by a first-principles computational search of thirty possible structures of transition-metal (TM) dinitride monolayers, a new emerging class of 2D materials. Our comprehensive calculations of stability and magnetic properties show that the octahedral coordinated 1T-TaN2 monolayer is not only dynamically, thermally (500 K) and mechanically stable, but also possesses a robust FM ground state with a Curie temperature of ∼339 K due to the strong N–N direct exchange interaction. Moreover, its half-metallic gap of 0.72 eV obtained from the HSE06 method is large enough to efficiently prevent the thermally agitated spin-flip transition. Unlike the reported 2D half-metallic TM compounds, the half-metallicity and magnetic moments of 1T-TaN2 are mainly attributed to the p orbitals of non-metal atoms (N) instead of d orbitals of TM atoms (Ta), which are beneficial for overcoming the issue of the short spin-relaxation-time caused by large spin coupling of TM atoms. Based on these results, it is reasonable to believe that the selected 1T-TaN2 monolayer is one of the most promising 2D materials for nanospintronic applications.

Junyan Liu

Papers

YN2 monolayer: Novel p-state Dirac half metal for high-speed spintronics

Computational search for two-dimensional intrinsic half-metals in transition-metal dinitrides