Intrinsic ferromagnetism with high temperature, strong anisotropy and controllable magnetization in the CrX (X = P, As) monolayer

Enhancement of interlayer exchange coupling via intercalation in 2D magnetic bilayers: towards high Curie temperature

Two-dimensional magnetic materials are considered as promising candidates for developing next-generation spintronic devices by providing the possibility of scaling down to nanometers. However, a low Curie temperature is a crucial problem for practical applications, being intimately related to weak interlayer exchange coupling. Here, by using density functional theory calculations, we show that interlayer exchange coupling can be enhanced by intercalating 3d transition metals (Sc to Zn) into a bilayer of CrI3 and NiI2. It is found that intercalated Ni and Cr atoms exhibit strong antiferromagnetic coupling with the CrI3 and NiI2 host layers, respectively. This enhances the ferromagnetic interlayer exchange coupling between the host layers by many folds compared to pristine CrI3 and NiI2 bilayers. Moreover, both intercalated compounds show out-of-plane magnetic anisotropy with half metallic nature, which makes them ideal candidates for spintronics applications. Thereby our work provides a rational approach to raise the Curie temperature of non-metallic two-dimensional magnets by intercalation.

Enhanced Direct Exchange Interaction and Hybridization by Single-Atom Linkers for High Curie Temperature and Superior Visible-Light Harvesting in Cr3(CN3)2

Designing two-dimensional (2D) ferromagnetic (FM) semiconductors with elevated Curie temperature, high carrier mobility, and strong light harvesting is challenging but crucial to the development of spintronics with multifunctionalities. Herein, we show first-principles computation evidence of the 2D metal-organic framework Kagome ferromagnet Cr3(CN3)2. Monolayer Cr3(CN3)2 is predicted to be an FM semiconductor with a record-high Curie temperature of 943 K owing to the use of a single-atom linker (N), which results in strong direct d-p exchange interaction and hybridization between dyz/xz and pz of Cr and N, as well as excellent matching characteristics in energy and symmetry. The single-atom linker structural feature also leads to notable band dispersion and a relatively high carrier mobility of 420 cm2 V-1 s-1. Moreover, under the in-plane strain, 2D Cr3(CN3)2 can be tuned to possess a strong visible-light-harvesting functionality. These novel properties render monolayer Cr3(CN3)2 a distinct 2D ferromagnet with high potential for the development of multifunctional spintronics.

Two dimensional Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers: ferromagnetic semiconductors with spontaneous valley polarization and tunable magnetic anisotropy

Two-dimensional (2D) ferromagnetic (FM) materials with valley polarization are highly desirable for use in valleytronic devices. The 2D Janus materials have fascinating physical properties due to their asymmetrical structures. In this work, the electronic structure and magnetic properties of Janus RuXY (X, Y = Br, Cl, F, I, X ≠ Y) monolayers are systematically studied using first-principles calculations. RuBrCl, RuBrF, and RuClF monolayers are all FM semiconductors. The valley polarization is present in the band structure and this is determined by the spin orbit coupling (SOC). The valley splitting energy of the RuClF monolayer is as large as 204 meV, with a perpendicular magnetic anisotropy (PMA) energy of 1.918 mJ m-2 and a Curie temperature of 316 K. Therefore, spontaneous valley polarization at room temperature will be seen in the RuClF monolayer. The Curie temperature of the RuBrF monolayer is higher than that of the RuClF, but the magnetic anisotropy energy (MAE) is in-plane magnetic anisotropy (IMA). The valley splitting energy of the RuBrCl monolayer is higher and the PMA energy is lower than that of the RuClF monolayer. The Curie temperature was only 197 K. The valley polarization was modulated in the RuXY monolayers at different biaxial strains, during which the semiconductor properties are still maintained. The PMA of the RuClF and RuBrCl monolayers is enhanced by the biaxial compressive strains, which are mainly attributed to the variation of the (dyz, d2z) orbital matrix elements of the Ru atoms. The MAE of the RuBrF monolayer is tuned from IMA into PMA at a biaxial strain of -6%. These results show an example of a 2D Janus ferrovalley material.

Ferromagnetic half-metal with high Curie temperature in Cr P nanoribbons: good material for spintronic applications

In this work, the stability, and electronic, magnetic, and transport characteristics of chromium phosphorus nanoribbons (CrPNRs) have been investigated. The results, obtained from the non-equilibrium Green's function and density functional theory, indicate that the nanoribbon is stable in terms of binding energies as well as dynamically. The calculated electronic structure shows that this nanoribbon has an indirect band gap of about 1.67 eV for the spin-down channel and is metallic for the spin-up channel. The metallicity of the spin-up channel originates from the P-3p orbitals and the magnetic properties are due to the Cr-3d orbitals. The obtained transport properties indicate the value of the current for the spin-up state is about 50 μA. The negative differential resistance (NDR) phenomenon and also a 100% spin filtering effect are seen between voltages of 0.4 and 0.5 V. The results showed that CrPNRs have a high Curie temperature of more than 690 K, indicating that this nanoribbon is a useful ferromagnetic material for nanoelectronic devices and spintronic applications at ambient temperature.

Distinct ferrovalley characteristics of the Janus RuClX (X = F, Br) monolayer

Two-dimensional ferrovalley materials should simultaneously possess three characteristics, that is, a Curie temperature beyond atmospheric temperature, perpendicular magnetic anisotropy, and large valley polarization for potential commercial applications. In this report, we predict two ferrovalley Janus RuClX (X = F, Br) monolayers by first-principles calculations and Monte Carlo simulations. The RuClF monolayer exhibited a valley-splitting energy as large as 194 meV, perpendicular magnetic anisotropy energy of 187 μeV per f.u., and Curie temperature of 320 K. Thus, spontaneous valley polarization at room temperature will be present in the RuClF monolayer, which is nonvolatile for spintronic and valleytronic devices. Although the valley-splitting energy of the RuClBr monolayer was as high as 226 meV with magnetic anisotropy energy of 1.852 meV per f.u., the magnetic anisotropy of the RuClBr monolayer was in-plane, and its Curie temperature was only 179 K. The orbital-resolved magnetic anisotropy energy revealed that the interaction between the occupied spin-up states of d_yz and the unoccupied spin-down states of d_z² dominated the out-of-plane magnetic anisotropy in the RuClF monolayer, but the in-plane magnetic anisotropy of the RuClBr monolayer was mostly contributed by the coupling of the d_xy and d_x²-y² orbitals. Interestingly, the valley polarizations in the Janus RuClF and RuClBr monolayers appeared in their valence band and conduction band, respectively. Thus, two anomalous valley Hall devices are proposed using the present Janus RuClF and RuClBr monolayers with hole and electron doping, respectively. This study provides interesting and alternative candidate materials for the development of valleytronic devices.

Two-dimensional XY ferromagnetism above room temperature in Janus monolayer V2XN (X = P, As)

Two-dimensional (2D) XY magnets with easy magnetization planes support the nontrivial topological spin textures whose dissipationless transport is highly desirable for 2D spintronic devices. Here, we predicted that Janus monolayer V₂XN (X = P, As) with a square lattice is a 2D-XY ferromagnet using first-principles calculations. Both magnetocrystalline anisotropy and magnetic shape anisotropy favor an in-plane magnetization, leading to an easy magnetization xy-plane in Janus monolayer V₂XN. With the help of the Monte Carlo simulations, we observed the Berezinskii-Kosterlitz-Thouless (BKT) phase transition in monolayer V₂XN with the transition temperature T_BKT being above room temperature. In particular, monolayer V₂AsN has a magnetic anisotropy energy (MAE) of 292.0 μeV per V atom and a T_BKT of 434 K, which is larger than that of monolayer V₂PN. Moreover, a tensile strain of 5% can further improve the T_BKT of monolayer V₂XN to be above 500 K. Our results indicated that Janus monolayer V₂XN (X = P, As) can be candidate materials to realize high-temperature 2D-XY ferromagnetism for spintronics applications.

Enhanced Curie temperature and conductivity of van der Waals ferromagnet MgV2S4via electrostatic doping

A van der Waals intrinsic ferromagnet with double magnetic atom layers is of great interest for both revealing fundamental physics and exploring promising applications in low-dimensional spintronics. Here, the magnetic and electronic properties of the van der Waals ferromagnet MgV₂S₄ monolayer are studied under electrostatic doping using first-principles calculations. A MgV₂S₄ monolayer presents the desired physical properties such as that of being a half-semiconductor with a direct bandgap of 1.21 eV and a ferromagnetic ground state, and having a high Curie temperature of 462 K. Unlike the robust ferromagnetic ground state, magnetic anisotropy and Curie temperature are sensitive to electrostatic doping. Meanwhile, the transition from a semiconductor to a half-metal and the significant improvement in conductivity under electrostatic doping make the MgV₂S₄ monolayer a promising candidate for low-dimensional spintronic field-effect transistors.

Quantum anomalous Hall effect in germanene by proximity coupling to a semiconducting ferromagnetic substrate NiI2

Interfaces between materials are ubiquitous in materials science, especially in devices. As device dimensions continue to be reduced, understanding the physical characteristics that appear at interfaces is crucial to exploit them for applications, spintronics in this case. Here, based on first-principles calculations, we propose a general and tunable platform to realize an exotic quantum anomalous Hall effect (QAHE) with the germanene monolayer by proximity coupling to a semiconducting ferromagnetic NiI₂ (Ge/NiI₂). Through analysis of the Berry curvature and band structure with spin-orbit coupling, the QAHE phase with an integer Chern number (C = -1), which is induced by band inversion between Ge-p orbitals, can achieve complete spin polarization for low-dissipation electronic devices. Also, the proximity coupling between germanene and the NiI₂ substrate makes the non-trivial bandgap reach up to 85 meV, and the Curie temperature of the Ge/NiI₂ heterostructure (HTS) is enhanced to 238 K, which is much higher than that of pristine NiI₂. An effective k·p model is proposed to clarify the quantum phenomena in the Ge/NiI₂ HTS. These findings shed light on the possible role of magnetic proximity effects on condensed matter physics in germanene and open new perspectives for multifunctional spin quantum devices in spintronics.

The Magnetic Genome of Two-Dimensional van der Waals Materials

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

Magnetic anisotropy and ferroelectric-driven magnetic phase transition in monolayer Cr2Ge2Te6

Monolayer Cr₂Ge₂Te₆ (ML-CGT) has attracted broad interest due to its novel electronic and magnetic properties. However, there are still controversies on the origin of its intrinsic magnetism. Here, by exploring the electronic and magnetic properties of ML-CGT, we find that the magnetic shape anisotropy (MSA) is vital for establishing the long-range ferromagnetism, except for the contribution from magnetocrystalline anisotropy energy (MCA). Electronic band analysis, combined with atomic- and orbital-resolved magnetic anisotropy from a second-order perturbation theory, further reveals that the MCA of ML-CGT is mainly originated from hybridized Te-p_y and -p_z orbitals. The MSA from magnetic Cr atoms in ML-CGT is larger than MCA, resulting in an in-plane magnetic anisotropy. Noticeably, by constructing a heterostructure (HTS) with ferroelectric Sc₂CO₂, CGT undergoes an in-plane to out-of-plane spin reorientation via ferroelectric polarization switching, accompanied with an electronic property transition from semiconductor to half-metal. The Curie temperature of CGT/Sc₂CO₂ HTS can be enhanced to 92.4 K under the ferroelectric polarization, which is much higher than that of pristine ML-CGT (34.7 K). These results not only clarify the contradiction of magnetic mechanism of ML-CGT in previous experimental and theoretical works, but also open the door for realizing nonvolatile magnetic memory devices based on a multifunctional ferromagnetic/ferroelectric HTS.

Easy-axis rotation in ferromagnetic monolayer CrN induced by fluorine and chlorine functionalization

The schematic energy diagram with crystal-field splitting of the d states before and after functionalization of CrN is reported.

Enhancing Ferromagnetism and Tuning Electronic Properties of CrI3 Monolayers by Adsorption of Transition-Metal Atoms

Among first experimentally discovered two-dimensional (2D) ferromagnetic materials, chromium triiodide (CrI₃) monolayers have attracted particular attention due to their potential applications in electronics and spintronics. However, the Curie temperature T_c of the CrI₃ monolayer is below room temperature, which greatly limits practical development of the devices. Herein, using density functional theory calculation, we explore how the electronic and magnetic properties of CrI₃ monolayers change upon adsorption of 3d transition-metal (TM) atoms (from Sc to Zn). Our results indicate that the electronic properties of the TM-CrI₃ system can be tuned from semiconductor to metal/half-metal/spin gapless semiconductor depending on the choice of the adsorbed TM atoms. Moreover, the adsorption can improve the ferromagnetic stability of CrI₃ monolayers by increasing both magnetic moments and T_c. Notably, T_c of CrI₃ with Sc and V adatoms can be increased by nearly a factor of 3. We suggest postsynthesis doping of 2D CrI₃ by deposition of TM atoms as a new route toward potential applications of TM-CrI₃ systems in nanoelectronic and spintronic devices.

Conversation from antiferromagnetic MnBr2 to ferromagnetic Mn3Br8 monolayer with large MAE

A pressing need in low energy spintronics is two-dimensional (2D) ferromagnets with Curie temperature above the liquid-nitrogen temperature (77 K), and sizeable magnetic anisotropy. We studied Mn3Br8 monolayer which is obtained via inducing Mn vacancy at 1/4 population in MnBr2 monolayer. Such defective configuration is designed to change the coordination structure of the Mn-d5 and achieve ferromagnetism with sizeable magnetic anisotropy energy (MAE). Our calculations show that Mn3Br8 monolayer is a ferromagnetic (FM) half-metal with Curie temperature of 130 K, large MAE of - 2.33 meV per formula unit, and atomic magnetic moment of 13/3μB for the Mn atom. Additionally, Mn3Br8 monolayer maintains to be FM under small biaxial strain, whose Curie temperature under 5% compressive strain is 160 K. Additionally, both biaxial strain and carrier doping make the MAE increases, which mainly contributed by the magneto-crystalline anisotropy energy (MCE). Our designed defective structure of MnBr2 monolayer provides a simple but effective way to achieve ferromagnetism with large MAE in 2D materials.

Anisotropic Thermoelectric Materials: Pentagonal PtM2 (M = S, Se, Te)

We here report a new pentagonal network structure of the PtM₂ (M = S, Se, Te) monolayers with the P2₁/c (no. 14) space group. The electronic structure and thermoelectric properties of the pentagonal PtM₂ monolayers are calculated through the VASP and BoltzTraP codes. We verify their dynamic and thermodynamic stabilities by calculating their phonon spectra and simulating ab initio molecular dynamics. It is found that the new material belongs to the medium-wide indirect band gap semiconductors from the PBE and HSE06 methods. At 300 K, the lattice thermal conductivities (K_l) of the pentagonal PtTe₂ in the x and y directions are the smallest among these three materials, being 1.77 and 5.17 W/m K, respectively. The anisotropic zT values (2.60/1.14) in the x/y direction of the pentagonal PtTe₂ at 300 K are much greater than those of the pentagonal PtSe₂ (1.75/0.82) and the pentagonal PtS₂ (0.58/0.16) at 300 K. Importantly, the p-type pentagonal PtTe₂ also has excellent thermoelectric properties at 600 K, with a zT value of 5.03 in the x direction, indicating that the p-type pentagonal PtTe₂ has a good application potential in the thermoelectric field.

Emergence of 2D high-temperature nodal-line half-metal in monolayer AgN

Nodal-line half-metals (NLHMs) are highly desirable for future spintronic devices due to their exotic quantum properties. However, the experimental realization in spin-polarized materials is nontrivial to date. Herein we perform first-principles calculations to demonstrate a 2D honeycomb, AgN, as a promising candidate of NLHMs, which is thermodynamically and dynamically stable. Band structure analysis reveals that two concentric NLs coexist centered at a Γ point near EF, accompanied by the electron and hole pockets that touch each other linearly with single-spin components. Inclusion of SOC can enrich the electronic structures of AgN, sensitive to the protection of mirror reflection symmetry: the NLHM survives if the spin is perpendicular to the Mz mirror plane, while it tunes into Wyle nodal-points by rotating spins from the out-of-plane to the in-plane direction. The characteristics of HM and NL can be well maintained on semiconducting h-BN and is immune to mechanical strains. These tunable magnetic properties render 2D AgN suitable for exotic quantum transports in nodal fermions as well as related spintronic devices.

High-temperature and multichannel quantum anomalous Hall effect in pristine and alkali-metal-doped CrBr3 monolayers

The realization of the high-temperature and multichannel quantum anomalous Hall effect (QAHE) has been a central research area in the development of low-power-consumption electronics and quantum computing. Recently discovered two-dimensional (2D) ferromagnetic (FM) materials provide unprecedented opportunities for the exploration of the high-temperature QAHE. Based on first-principles approaches, we first reveal that a FM CrBr₃ monolayer harbors topologically nontrivial conduction bands with a high Chern number of C = 2. Then, we reveal that the interesting conduction bands can be moved downwards to the Fermi levels by electron and alkali-metal-doping; meanwhile, the QAHE characteristics can be preserved. Most strikingly, the Na-doped CrBr₃ system possesses a higher Chern number of C = -4 with a transition temperature of ∼54 K, which is attributed to the constructive coupling effect of the quadratic non-Dirac and linear Dirac band dispersions. The present study, together with recent achievements in the field of 2D FM materials, provides an experimentally achievable guide for realizing the high-temperature and multichannel QAHE based purely on 2D FM systems.

Two-dimensional stable Fe-based ferromagnetic semiconductors: FeI3 and FeI1.5Cl1.5 monolayers

Two kinds of novel ferromagnetic semiconductors FeI₃ and FeI_1.5Cl_1.5 have high Curie temperature (>77 K) and sizable MAE.