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

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

Two-dimensional (2D) ferromagnetic semiconductors (FMSs) hold exciting and promising potential for application in spintronic devices at the nanoscale. Currently, most 2D FMSs are based on 3d electrons; 4f electrons can provide nontrivial magnetism but have been much less studied to date. This paper presents a theoretical study, via first-principles calculations, of EuSn2X2 (X = P, As) monolayers based on rare-earth cations with f-electrons. The results show that EuSn2X2 monolayers possess a large magnetization (7 μB/Eu), a controllable magnetic anisotropy energy, and a unique d-electron-mediated f–f exchange mechanism. Both types of EuSn2X2 (X = P, As) monolayers are FMSs with indirect bandgaps of 1.00 and 0.99 eV, respectively, based on the Heyd–Scuseria–Ernzerhof (HSE06) method, which can be transform to direct bandgap semiconductors under biaxial strain. Interestingly, under the latter, spin–orbit coupling interaction gradually replaces the dipole–dipole interaction in the dominant position of magnetic anisotropy, resulting in the magnetic easy axis changing from in-plane to the more desirable out-of-plane. Considering their excellent dynamic, thermal, and mechanical stabilities and small cleavage energy, these EuSn2X2 monolayers can be exfoliated from their synthesized bulk. Our study not only helps to understand the properties of 2D 4f rare-earth magnets but also signposts a route toward improving the performance of EuSn2X2 monolayers in nano-electronic devices.

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.

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.

2D ferromagnetic semiconductors are recognized as the cornerstone of next-generation spintronics devices. However, their practical applications are severely hindered by the low Curie temperature, which originates from the weak d-p-d ferromagnetic superexchange interaction. H- anion with short ionic radius can effectively shorten the distance between magnetic centers and simultaneously induce a perfect 180° superexchange angle to strengthen the magnetic coupling, thus achieving high-temperature magnetic ordering. Here, by first-principles calculations, such a case in 2D Ruddlesden-Popper phase hydride double perovskite A4NiVH8 (A = Na, K, Rb) is demonstrated. These hydride monolayers possess quite good thermodynamic stability and can retain their structures under normal pressure at least at 500 K. Magnetic and electronic properties calculations reveal that they are all ferromagnetic semiconductors with high Curie temperatures (up to 429 K) and superior electron mobilities (up to 5522 cm2 V-1 s-1, based on the deformation potential theory). In addition, monolayer Na4NiVH8 exhibits the characteristics of a bipolar magnetic semiconductor with gate-tunable spin polarization.

Tunable magnetic coupling and high Curie temperature of two–dimensional PtBr3via van der waals heterostructures

Manipulating the interlayer magnetic coupling in van der Waals magnetic materials and heterostructures is the key to tailoring their magnetic and electronic properties for various electronic applications and fundamental studies in condensed matter physics. By utilizing the MnBi2Te4-family compounds and their heterostructures as a model system, we systematically studied the dependence of the sign and strength of interlayer magnetic coupling on constituent elements by using first-principles calculations. It was found that the coupling is a long-range superexchange interaction mediated by the chains of p orbitals between the magnetic atoms of neighboring septuple-layers. The interlayer exchange is always antiferromagnetic in the pure compounds, but can be tuned to ferromagnetic in some combinations of heterostructures, dictated by d orbital occupations. Strong interlayer magnetic coupling can be realized if the medial p electrons are delocalized and the d bands of magnetic atoms are near the Fermi level. The knowledge on the interlayer coupling mechanism enables us to engineer magnetic and topological properties of MnBi2Te4-family materials as well as many other insulating van der Waals magnetic materials and heterostructures.

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.

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.

The discovery of two-dimensional (2D) ferromagnetic semiconductors holds significant promise for advancing Moore's law and spintronics in-memory computing, sparking tremendous interest. However, the Curie temperature of explored 2D ferromagnetic semiconductors is much lower than room temperature. Although plenty of 2D room-temperature ferromagnetic semiconductors have been theoretically predicted, there have been formidable challenges in preparing such metastable materials with ordered structures and high stability. Here, utilizing a novel template-assisted chemical vapor deposition strategy, we synthesized layered MnS2 microstructures within a ReS2 template. The high-resolution atomic structure representation revealed that monolayer MnS2 microstructures well crystallize into a distorted T-phase. Room-temperature ferromagnetism was confirmed through vibrating sample magnetometer measurements, microzone magnetism imaging techniques, and transport characterization. Theoretical calculations indicated that the room-temperature ferromagnetism originates from the Mn-Mn short-range interaction. Our observation not only offered the experimental confirmation of the intrinsic room-temperature ferromagnetism in layered MnS2, but also provided an innovative strategy for the growth of 2D metastable functional materials.

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.

CrGeTe3 recently emerges as a new two-dimensional (2D) ferromagnetic semiconductor that is promising for spintronic device applications. Unlike CrSiTe3 whose magnetism can be understood using the 2D-Ising model, CrGeTe3 exhibits a smaller van der Waals gap and larger cleavage energy, which could lead to a transition of magnetic mechanism from 2D to 3D. To confirm this speculation, we investigate the critical behavior CrGeTe3 around the second-order paramagnetic-ferromagnetic phase transition. We obtain the critical exponents estimated by several common experimental techniques including the modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis, which show that the magnetism of CrGeTe3 follows the tricritical mean-field model with the critical exponents \b{eta}, {\gamma}, and {\delta} of 0.240, 1.000, and 5.070, respectively, at the Curie temperature of 67.9 K. We therefore suggest that the magnetic phase transition from 2D to 3D for CrGeTe3 should locate near a tricritical point. Our experiment provides a direct demonstration of the applicability of the tricritical mean-field model to a 2D ferromagnetic semiconductor.

The large magneto-resistance (MR) effect produced by electric control of the magnetic state for van der Waals (vdW) heterostructures composed of vdW intrinsic magnets holds great significance for low-dissipation spintronic devices. Our first-principles calculations reveal that the proposed monolayer WV2N4 is a ferromagnetic (FM) metal with two magnetic V atomic layers, and the interlayer magnetic coupling between two V atomic layers can be switched from FM to antiferromagnetic coupling by applying a small compressive strain. Interestingly, a large MR ratio of 253% is achieved in the proposed graphite/monolayer WV2N4/graphite vdW heterostructure using a −1.5% compressive strain. Combining the strain-induced change in magnetism of monolayer WV2N4 and the graphite/monolayer WV2N4/graphite vdW heterostructure with the inverse piezoelectricity of piezoelectric materials, a feasible strategy is proposed to achieve electric control of the interlayer magnetic coupling of monolayer WV2N4 in the graphite/monolayer WV2N4/graphite vdW heterostructure clamped by piezoelectric materials by utilizing the inverse piezoelectricity, thereby generating a large MR ratio in the graphite/monolayer WV2N4/graphite vdW heterostructure clamped by the piezoelectric material. Our work presents a promising avenue for developing energy-efficient spintronic devices.

Two-dimensional (2D) ferromagnetic (FM) semiconductors with a high Curie temperature and tunable electronic properties are a long-term pursuing target for the development of high-performance spin-dependent optoelectronic devices. Herein, on the basis of density functional theory calculations, we report a new strategy to tune the Curie temperature and electronic structures of a ferromagnetic CrBr3 monolayer through the formation of CrBr3/GaN van der Waals heterostructures. Our calculated results demonstrate that the Curie temperature and band alignment of CrBr3/GaN heterostructures strongly depend on the thickness and polarization direction of the GaN layer. The combination of the CrBr3 monolayer with N-terminated GaN nanosheets leads to enhanced FM coupling via superexchange interactions between the Cr-t2g and Cr-eg orbitals, consequently resulting in a Curie temperature of CrBr3 of up to 67 K. Moreover, self-doped p-n junctions can be naturally formed in the heterostructures without additional modulation of external fields. The enhanced FM coupling and self-doping effect in the heterostructures are associated with the intrinsic polarization of the GaN layer that drives interfacial electron transfers from GaN to CrBr3. Therefore, this work not only offers an efficient scheme to boost the Curie temperature of the CrBr3 monolayer but also opens up a new route to realize nonvolatile van der Waals p-n junctions.

Strain and electric field dependent spin polarization in two-dimensional arsenene/CrI3 heterostructure