Two-dimensional 4f magnetic EuSn2X2 (X = P, As) monolayers: A first-principles study

Two-dimensional ferromagnetic semiconductors of rare-earth monolayer GdX2 (X = Cl, Br, I) with large perpendicular magnetic anisotropy and high Curie temperature

In recent years, 2D ferromagnetic semiconductors have attracted much attention because of its potential application in spintronic devices. Using first‐principles calculations, the magnetic and optical properties of intrinsic and chalcogen‐doped VCl3 monolayers are investigated. In contrast to previous work, VCl3 monolayer is proved to be an antiferromagnetic semiconductor rather than a Dirac half‐metal after considering the electronic correlation effect. At a low S concentration x between and , S‐doped VCl3 monolayer forms a ferromagnetic semiconductor with a large bandgap and a strong exchange splitting in both valence and conduction bands. As the doping content x increases above , S‐doped VCl3 monolayer will change to be an anti‐ferromagnetic semiconductor and a non‐magnetic metal successively. Moreover, Se‐ and Te‐doped VCl3 monolayers can also form robust ferromagnetic semiconductors at low doping concentration. In particular, the Curie temperature of Se‐doped VCl3 monolayer can reach 170 K, higher than that of S‐ and Te‐doped VCl3 monolayers. At last, chalcogen‐doped VCl3 monolayers have enhanced optical absorption in the visible regions compared to intrinsic VCl3 monolayer. The results show that chalcogen‐doped VCl3 monolayers have promising potential applications in future spintronic and optoelectronic devices.

Two-dimensional (2D) ferromagnetic (FM) semiconductors with high Curie temperature (T c ) and large perpendicular magnetic anisotropy (PMA) are promising for developing next-generation magnetic storage devices. In this work, we investigated the structural, electronic, and magnetic properties of MoF3 and Janus Mo2F3 X 3 (X = Cl, Br, I) monolayers by first-principles methods. These materials are 2D FM semiconductors with large PMA and half-semiconducting character as both VBM and CBM belonging to the spin-up channel. Biaxial strain can modulate band gap, reverse easy magnetization axis, and induce magnetic phase transitions in MoF3 monolayer and its Janus structures. Compared to MoF3 monolayers, Janus Mo2F3 X 3 monolayers can preserve the structural ability and the FM ground state over a wider range of strain. The magnetic anisotropy energies (MAEs) of these 2D materials can be enhanced to greater than 1 meV/Mo by tensile strains. Intrinsic T c of MoF3 monolayer and its Janus structures are less than 110 K and are insensitive to strain. However, hole doping with a feasible concentration can achieve a room-temperature half-metallicity in these 2D materials. The required hole concentration is lower in Janus Mo2F3 X 3 monolayers than MoF3 monolayer. Our results indicate that MoF3 and Janus Mo2F3 X 3 (X = Cl, Br, I) monolayers are promising candidates for 2D spintronic applications and will stimulate experimental and theoretical broad studies.

Two-dimensional (2D) ferromagnetic semiconductors with a room-temperature Curie temperature (T c) are required for next-generation spintronic devices, but the current candidates suffer from a low T c and poor chemical stability. Here, a new layered compound RhI3 is discovered to be an above-room-temperature ferromagnetic semiconductor. This compound crystallizes in a monoclinic crystal system of space group C2/m, with the unit cell of a = 6.773(8) Å, b = 11.721(2) Å, c = 6.811(8) Å and β = 108.18(4) °. The structure consists of honeycomb rhodium layers separated by iodine–iodine van der Waals gap. Chemically stable RhI3 possesses an optical bandgap of 1.17 eV. Its robust ferromagnetism with a T c of above 400 K, which is far higher than 61 K for the well-known CrI3 and the highest among the bulk 2D ferromagnetic semiconductors. The robust intrinsic ferromagnetic response is attributed to the Rh2+ and exchange interactions between I-p and Rh-d electrons induced by iodine vacancies. This work reveals that RhI3 is a prime candidate for spintronic devices above room temperature and provides a strategy to obtain high temperature 2D ferromagnetic semiconductors by introducing vacancies.

Hole-mediated ferromagnetic coupling in two-dimensional CrI3/VSe2 van der Waals heterostructures

Tunable electronic structure and magnetic anisotropy of two dimensional Mn2CFCl/MoSSe van der Waals heterostructures by electric field and biaxial strain

Prediction of high spin polarization and perpendicular magnetic anisotropy in two dimensional ferromagnetic Mn2CXX’ (X, X′=F, Cl, Br, I) Janus monolayers

Two-dimensional (2D) ferromagnetic semiconductors hold a great potential for nano-electronic and spintronic devices. Nevertheless, their experimental realization remains a big challenge. Through first-principles calculations, we here demonstrate the possibility of realizing 2D ferromagnetic semiconductors simply by exfoliating layered crystals of CrXTe3 (X = Si, Ge). The exfoliation of CrXTe3 is feasible due to its small cleavage energy, and CrXTe3 nanosheets can form free-standing membranes. Interestingly, upon exfoliation, the ferromagnetism and semiconducting character are well preserved from bulk to the nanosheet form. Long-range ferromagnetic order with a magnetization of 3 μB per Cr atom is confirmed in 2D CrXTe3 from classical Heisenberg model Monte Carlo simulations. Both bulk and 2D CrXTe3 are indirect-gap semiconductors with their valence and conduction bands fully spin-polarized in the same direction, which is promising for spin-polarized carrier injection and detection. We further demonstrate the tunability and enrichment of the properties of CrXTe3 nanosheets via external operations. Under moderate tensile strain, the 2D ferromagnetism can be largely enhanced. By pure electron doping or adsorbing nucleophilic molecules, CrXTe3 nanosheets become 2D half metals.

Two-dimensional (2D) van der Waals (vdW) heterostructures have potential applications in new low-dimensional spintronic devices owing to their unique electronic properties and magnetic anisotropy energies (MAEs). The electronic structures and magnetic properties of RuClF/WSe2 heterostructure are calculated using first-principles calculations. The most stable RuClF/WSe2 heterostructure is selected for property analysis. RuClF/WSe2 heterostructure has half-metallicity. Considering spin-orbit coupling (SOC), band inversion is present in the RuClF/WSe2 heterostructure, which is also demonstrated by the weight of the energy contributions. The local density of states (LDOS) of the edge states can provide strong evidence that the RuClF/WSe2 heterostructure has topological properties. The MAE of RuClF/WSe2 heterostructure is in-plane magnetic anisotropy (IMA), which mainly originates from the contribution of matrix element difference in Ru (dxy, dx2-y2) orbitals. The electronic properties and MAE of RuClF/WSe2 heterostructure can be regulated by biaxial strains and electric fields. The band inversion phenomenon is enhanced at electric fields in the opposite direction, which is also modified at different biaxial strains. However, the band inversion phenomenon disappears at the biaxial strains of 6% and an electric field of 0.5 V Å-1. The MAE of RuClF/WSe2 heterostructure is transformed from IMA into perpendicular magnetic anisotropy (PMA) at certain compressive strains and positively directed electric fields. The above results indicate that the RuClF/WSe2 heterostructure has potential applications in spintronic devices.

Prediction of electronic structure and magnetic anisotropy of two-dimensional MSi2N4 (M = 3d transition-metal) monolayers

The intrinsic ferromagnetism of two-dimensional transition metal carbide Co2C is remarkable. However, its practical application in spintronic devices is encumbered by a low Curie temperature (TC). To surmount this constraint, double transition-metal carbide CoMC (M = Ti, V, Cr, Mn, Fe, Ni) monolayers are constructed with the aim of improving the magnetic properties and Curie temperature of Co2C. The magnetic properties of CoMC monolayers are comprehensively investigated by first-principles calculations and the effects of hole doping and biaxial strain on the magnetic properties of CoMC (M = V, Cr, Mn) monolayers are also studied. The ground states of CoTiC, CoMnC and CoNiC monolayers all favor ferromagnetic ordering, whereas the CoVC and CoCrC monolayers favor antiferromagnetic ordering and the CoFeC monolayer is non-magnetic. Excitedly, the CoMnC monolayer displays a high total magnetic moment of 4.024μB and a TC of 1366 K. Moreover, the control of hole doping can effectively improve the TC of CoVC, CoCrC, and CoMnC monolayers to 680, 1317, 3044 K, respectively. Finally, applying the in-plain biaxial strain, the CoVC monolayer can be transformed into a ferromagnetic semiconductor under a tensile strain of 6%. The TC values of CoVC, CoCrC, and CoMnC monolayers are tuned by biaxial strain to 440, 1334 and 2390 K, respectively. Their TC above room temperature demonstrates that these monolayers have potential applications in spintronic devices. These theoretical investigations provide valuable insights into guiding experimental synthesis endeavors.

Two-dimensional (2D) ferromagnetic semiconductors (FM SCs) provide an ideal platform for the development of quantum information technology in nanoscale devices. However, many developed 2D FM materials present a very low Curie temperature (TC), greatly limiting their application in spintronic devices. In this work, we predict two stable 2D transition metal chalcogenides, V3Se3X2 (X = S, Te) monolayers, by using first-principles calculations. Our results show that the V3Se3Te2 monolayer is a robust bipolar magnetic SC with a moderate bandgap of 0.53 eV, while V3Se3S2 is a direct band-gap FM SC with a bandgap of 0.59 eV. Interestingly, the ferromagnetisms of both monolayers are robust due to the V-S/Se/Te-V superexchange interaction, and TCs are about 406 K and 301 K, respectively. Applying biaxial strains, the FM SC to antiferromagnetic (AFM) SC transition is revealed at 5% and 3% of biaxial tensile strain. In addition, their high mechanical, dynamical, and thermal stabilities are further verified by phonon dispersion calculations and ab initio molecular dynamics (AIMD) calculations. Their outstanding attributes render the V3Se3Y2 (Y = S, Te) monolayers promising candidates as 2D FM SCs for a wide range of applications.

Strain tailored electronic structure and magnetic properties of Fe-doped Zr8C4T8 (T = F, O) monolayers

Abstract2D van der Waals (vdW) multiferroic heterostructures have potential applications in novel 2D spintronic devices. Here, the electronic structure and magnetic anisotropy of 2D vdW multiferroic Mn2CFCl/CuBiP2Se6 heterostructures are investigated systematically by first‐principles calculations. Ferroelectric material CuBiP2Se6 has a built‐in electric field. Mn2CFCl is a half‐metallicity with ferromagnetic. The plane average potential difference of Mn2CFCl/CuBiP2Se6 heterostructures is 0.02 eV in P↓ state and 0.39 eV in P↑ state. The magnetic anisotropy of P↑ and P↓ states is shown perpendicular magnetic anisotropy (PMA). The P↑ state exhibits metallic character with the biaxial strain of 4%, −6%, while the P↓ state demonstrates half‐metallicity at a strain of 2%, 4%, 6%. The P↑ state shows PMA at the different biaxial strain. However, the P↓ state shows PMA at biaxial strain of −2%, −4%, and 6%, which shifts PMA to in‐plane magnetic anisotropy at −6%, 2%, and 4%. These tunable electronic structure and magnetic anisotropy suggest that the 2D multiferroic Mn2CFCl/CuBiP2Se6 vdW heterostructures have potential applications in spintronic devices.

CrI3, which belongs to a rare category of two-dimensional (2D) ferromagnetic semiconductors, is of great interest for spintronic device applications. Unlike CrCl3 whose magnetism presents a 2D-Heisenberg behavior, CrI3 exhibits a larger van der Waals gap, smaller cleavage energy, and stronger magnetic anisotropy which could lead to a 3D magnetic characteristic. Hence, we investigate the critical behavior of CrI3 in the vicinity of magnetic transition. We use the modified Arrott plot and Kouvel-Fisher method and conduct critical isotherm analysis to estimate the critical exponents near the ferromagnetic phase transition. This shows that the magnetism of CrI3 follows the crossover behavior of a 3D-Ising behavior with mean field type interactions where the critical exponents β, γ, and δ are 0.323 ± 0.006, 0.835 ± 0.005, and 3.585 ± 0.006, respectively, at the Curie temperature of 64 K. We propose that the crossover behavior can be attributed to the strong uniaxial anisotropy and inevitable interlayer coupling. Our experiment demonstrates the applicability of crossover behavior to a 2D ferromagnetic semiconductor.