2D Ferromagnetic Semiconductors Research Articles

Giant tunnel magnetoresistance in in-plane magnetic tunnel junctions based on the heterointerface-induced half-metallic 2H-VS[formula omitted]

Magnetic tunnel junctions (MTJs) constructed from atomically thin two-dimensional (2D) magnetic materials have attracted great attention in recent years because it meets the requirements of miniaturization and high tunability of next-generation spintronic devices. In this work, we demonstrate that the ferromagnetic semiconductor VS2 is transformed into a half-metal in VS2/MoSSe vdW heterostructure. Based on the heterostructure, we design an in-plane MTJs that comprise a monolayer VS2 barrier sandwiched between two VS2/MoSSe heterostructure electrodes. Through density functional calculations combined with a nonequilibrium Green’s function technique, it is found that the tunnel magnetoresistance (TMR) ratio as high as 4.35 × 109% can be achieved. Moreover, the TMR ratio can be tuned by the barrier length, and the maximum value exceeds 1015%. These results not only provide a novel route for designing MTJs using 2D ferromagnetic semiconductor material, but also demonstrate the great importance of vdW heterostructures in the design of spintronic devices.

Out‐of‐Plane Superexchange Interaction Enhanced Ferromagnetism in Semiconducting Monolayer Cr2Se3

Abstract2D magnetic semiconductors exhibit great potential for next‐generation spintronics, but realizing their full capabilities has been hindered by the low Curie temperatures (Tc) below 50 K observed in current materials. Here, a new mechanism to substantially enhance the Tc of 2D semiconducting materials through incorporating both in‐plane and out‐of‐plane superexchange interactions enabled by structural design is demonstrated. Specifically, monolayer Cr2Se3 is synthesized with a five‐layer Se–Cr–Se–Cr–Se atomic structure using molecular beam epitaxy (MBE). This unique structure not only possesses optimized in‐plane superexchange interaction but also incorporates out‐of‐plane Cr–Se–Cr couplings. Scanning tunneling spectroscopy (STS) and angular‐resolved photoemission spectroscopy (ARPES) confirm its semiconducting nature. Remarkably, the ferromagnetic phase transition observed by ARPES and Magnetic Force Microscopy (MFM) indicated that its Tc is up to 230 K. This not only establishes a new record for Tc in 2D ferromagnetic semiconductor materials but also introduces a novel approach to modulating materials' properties by manipulating the vertical dimension in 2D materials.

Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor

Electric control of magnetization dynamics in two-dimensional (2D) magnetic materials is an essential step for the development of novel spintronic nanodevices. Electrostatic gating has been shown to greatly affect the static magnetic properties of some van der Waals magnets, but the control over their magnetization dynamics is still largely unexplored. Here we show that the optically-induced magnetization dynamics in the van der Waals ferromagnet Cr2Ge2Te6 can be effectively controlled by electrostatic gates, with a one order of magnitude change in the precession amplitude and over 10% change in the internal effective field. In contrast to the purely thermally-induced mechanisms previously reported for 2D magnets, we find that coherent opto-magnetic phenomena play a major role in the excitation of magnetization dynamics in Cr2Ge2Te6. Our work sets the first steps towards electric control over the magnetization dynamics in 2D ferromagnetic semiconductors, demonstrating their potential for applications in ultrafast opto-magnonic devices.

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

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 Heisenberg model with material-dependent superexchange interactions

The two-dimensional (2D) van der Waals ferromagnetic semiconductors, such as CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, and the 2D ferromagnetic metals, such as Fe$_3$GeTe$_2$ and MnSe$_2$, have been obtained in recent experiments and attracted a lot of attentions. The superexchange interaction has been suggested to dominate the magnetic interactions in these 2D magnetic systems. In the usual theoretical studies, the expression of the 2D Heisenberg models were fixed by hand due to experiences. Here, we propose a method to determine the expression of the 2D Heisenberg models by counting the possible superexchange paths with the density functional theory (DFT) and Wannier function calculations. With this method, we obtain a 2D Heisenberg model with six different nearest-neighbor exchange coupling constants for the 2D ferromagnetic metal Cr$_3$Te$_6$, which is very different for the crystal structure of Cr atoms in Cr$_3$Te$_6$. The calculated Curie temperature Tc = 328 K is close to the Tc = 344 K of 2D Cr$_3$Te$_6$ reported in recent experiment. In addition, we predict two stable 2D ferromagnetic semiconductors Cr$_3$O$_6$ and Mn$_3$O$_6$ sharing the same crystal structure of Cr$_3$Te$_6$. The similar Heisenberg models are obtained for 2D Cr$_3$O$_6$ and Mn$_3$O$_6$, where the calculated Tc is 218 K and 208 K, respectively. Our method offers a general approach to determine the expression of Heisenberg models for these 2D magnetic semiconductors and metals, and builds up a solid basis for further studies.

Possible Room-Temperature Ferromagnetic Semiconductors

Magnetic semiconductors integrate the dual characteristics of magnets and semiconductors. It is difficult to manufacture magnetic semiconductors that function at room temperature. Here, we review a series of our recent theoretical predictions on room-temperature ferromagnetic semiconductors. Since the creation of two-dimensional (2D) magnetic semiconductors in 2017, there have been numerous developments in both experimental and theoretical investigations. By density functional theory calculations and model analysis, we recently predicted several 2D room-temperature magnetic semiconductors, including CrGeSe3 with strain, CrGeTe3/PtSe2 heterostructure, and technetium-based semiconductors (TcSiTe3, TcGeSe3, and TcGeTe3), as well as PdBr3 and PtBr3 with a potential room-temperature quantum anomalous Hall effect. Our findings demonstrated that the Curie temperature of these 2D ferromagnetic semiconductors can be dramatically enhanced by some external fields, such as strain, construction of heterostructure, and electric field. In addition, we proposed appropriate doping conditions for diluted magnetic semiconductors, and predicted the Cr doped GaSb and InSb as possible room-temperature magnetic semiconductors.

Large spontaneous valley polarization and high magnetic transition temperature in stable two-dimensional ferrovalley YX2(X=I, Br, and Cl )

Ferrovalley materials with spontaneous valley polarization are crucial to valleytronic application. Based on first-principles calculations, we demonstrate that two-dimensional (2D) $\mathrm{Y}{X}_{2}(X=\mathrm{I}, \mathrm{Br}, \text{and} \mathrm{Cl})$ in a 2H structure constitutes a series of promising ferrovalley semiconductors with large spontaneous valley polarization and high magnetic transition temperature. Our calculations reveal that $\mathrm{Y}{X}_{2}$ are dynamically, energetically, thermally and mechanically stable 2D ferromagnetic semiconductors with a magnetic transition temperature about 200 K. Due to the natural noncentrosymmetric structure, intrinsic ferromagnetic order and strong spin orbital coupling, the large spontaneous valley polarizations of 108.98, 57.70, and 22.35 meV are also predicted in single-layer $\mathrm{Y}{X}_{2}(X=\mathrm{I}, \mathrm{Br}, \text{and} \mathrm{Cl})$, respectively. The anomalous valley Hall effect is also proposed based on the valley contrasting Berry curvature. Moreover, the ferromagnetism and valley polarization are found to be effectively tuning by applying a biaxial strain. Interestingly, the suppressed valley physics of ${\mathrm{YBr}}_{2}$ and ${\mathrm{YCl}}_{2}$ can be switched on via applying a moderate compression strain. The present findings promise $\mathrm{Y}{X}_{2}$ as competitive candidates for the further experimental studies and practical applications in valleytronics.

Stress-driven structural and bond reconstruction in 2D ferromagnetic semiconductor VSe2

Two-dimensional (2D) semiconducting transition metal dichalcogenides can be used to make high-performance electronic, spintronic, and optoelectronic devices. Recently, room-temperature ferromagnetism and semiconduction in 2D VSe2 nanoflakes were attributed to the stable 2H-phase of VSe2 in the 2D limit. Here, our first-principles investigation shows that a metastable semiconducting H′ phase can be formed from the H VSe2 monolayer through uniaxial stress or uniaxial strain. The calculated phonon spectra indicate the dynamical stability of the metastable H′ VSe2 and the path of phase switching between the H and H′ VSe2 phases is calculated. For the uniaxial stress (or strain) scheme, the H′ phase can become lower in total energy than the H phase at a transition point. The H′ phase has stronger ferromagnetism and its Curier temperature can be enhanced by applying uniaxial stress or strain. Applying uniaxial stress or strain can substantially change spin-resolved electronic structures, energy band edges, and effective carrier masses for both of the H and H′ phases, and can cause some flat bands near the band edges in the strained H′ phase. Further analysis indicates that one of the Se–Se bonds in the H′ phase can be shortened by 19% and the related Se–V–Se bond angles are reduced by 23% with respect to those of the H phase, which is believed to increase the Se–Se covalence feature and reduce the valence of the nearby V atoms. Therefore, structural and bond reconstruction can be realized by applying uniaxial stress in such 2D ferromagnetic semiconductors for potential spintronic and optoelectronic applications.

Electrically and Magnetically Tunable Valley Polarization in Monolayer MoSe2 Proximitized by a 2D Ferromagnetic Semiconductor

AbstractThe emergence of atomically thin valleytronic semiconductors and 2D ferromagnetic materials is opening up new technological avenues for future information storage and processing. A key fundamental challenge is to identify physical knobs that may effectively manipulate the spin‐valley polarization, preferably in the device context. Here, a novel spin functional device that exhibits both electrical and magnetic tunability is fabricated, by contacting a monolayer MoSe2 with a 2D ferromagnetic semiconductor Cr2Ge2Te6. Remarkably, the valley‐polarization of MoSe2 is found to be controlled by a back‐gate voltage with an appreciably enlarged valley splitting rate. At fixed gate voltages, the valley‐polarization exhibits magnetic‐field and temperature dependence that corroborates well with the intrinsic magnetic properties of Cr2Ge2Te6, pointing to the impact of magnetic exchange interactions. Due to the interfacial arrangement, the charge‐carrying trion photoemission predominates in the devices, which may be exploited to enable drift‐based spin‐optoelectronic devices. These results provide new insights into valley‐polarization manipulation in transition metal dichalcogenides by means of ferromagnetic semiconductor proximitizing and represent an important step forward in devising field‐controlled 2D magneto‐optoelectronic devices.

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

Two-dimensional (2D) ferromagnetic (FM) semiconductors play imperative roles in the development of spintronic devices. However, the Curie temperature (Tc) of most 2D FM semiconductors are fairly low for practical applications. Herein, using the first-principles calculations, it revealed that the magnetic coupling strength within CrI3 can be modulated via the construction of CrI3/VSe2 van der Waals heterostructure. The system remains semiconducting with the band gap of 0.22 eV (GGA level). Its Tc can be boosted to ∼210 K via p-type doping, well above the liquid nitrogen temperature, which makes it a step closer to practical application. The greatly promoted FM coupling is originated from the additional super-exchange paths as well as the enhanced in-plane Cr-I-Cr super-exchange interaction. It is found that the interlayer magnetic coupling is tunable upon the stacking alignment, which is feasible to manipulate the magnetic orders using external mechanical motion.

Polaronic Conductivity in Cr2Ge2Te6 Single Crystals

AbstractIntrinsic 2D ferromagnetic semiconductors are an important class of materials for spin‐charge conversion applications. Cr2Ge2Te6 retains long‐range magnetic order in the bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X‐ray scattering, electronic, thermal transport and first‐principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr2Ge2Te6 are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr2Ge2Te6—based devices by electron–phonon‐ and spin–orbit coupling‐based tailoring of polaron properties.

Optical and magneto-optical properties of ferromagnetic monolayer CrBr3 : A first-principles GW and GW plus Bethe-Salpeter equation study

The discovery of atomically thin two-dimensional (2D) magnetic semiconductors has triggered enormous research interest recently. In this work, we use first-principles many-body perturbation theory to study a prototypical 2D ferromagnetic semiconductor, monolayer chromium tribromide (CrBr$_3$). With broken time-reversal symmetry, spin-orbit coupling, and excitonic effects included through the full-spinor $GW$ and $GW$ plus Bethe-Salpeter equation ($GW$-BSE) methods, we compute the frequency-dependent layer polarizability tensor and dielectric function tensor that govern the optical and magneto-optical properties. In addition, we provide a detailed theoretical formalism for simulating magnetic circular dichroism, magneto-optical Kerr effect, and Faraday effect, demonstrating the approach with monolayer CrBr$_3$. Due to reduced dielectric screening in 2D and the localized nature of the Cr $d$ orbitals, we find strong self-energy effects on the quasiparticle band structure of monolayer CrBr$_3$ that give a 3.8 eV indirect band gap. Also, excitonic effects dominate the low-energy optical and magneto-optical responses in monolayer CrBr$_3$ where a large exciton binding energy of 2.3 eV is found for the lowest bright exciton state with excitation energy at 1.5 eV. We further find that the magneto-optical signals demonstrate strong dependence on the excitation frequency and substrate refractive index. Our theoretical framework for modeling optical and magneto-optical effects could serve as a powerful theoretical tool for future study of optoelectronic and spintronics devices consisting of van der Waals 2D magnets.

A unique electronic state in a ferromagnetic semiconductor FeCl2 monolayer

The FeCl2 monolayer: an appealing 2D ferromagnetic semiconductor with strong strain tunability.

Tuning the magnetic anisotropy of ferromagnetic MnS2 monolayers via electron occupation of Mn d orbitals

Tuning magnetic anisotropy can facilitate the practical application of two-dimensional ferromagnets. Thus, understanding the relationship between magnetic anisotropy and electronic structures is desirable. Here, using the first-principles calculations and the second-order perturbation analyses, we investigate the microscopic mechanism (or electronic origin) underlying the change of the magnetic anisotropic energy (MAE) of the ${\mathrm{MnS}}_{2}$ monolayer. The calculated results show that strain enables the MAE to transform between an in-plane and off-plane orientation, and interlayer coupling (IC) from heterostuctures induces it to change positively. Furthermore, the strain-induced change in the MAE is greater than its IC-induced change. The further analyses indicate that although the microscopic mechanism underlying the strain- and IC-induced changes of the MAE is distinct, the change in electron occupation of $\mathrm{Mn}\text{\ensuremath{-}}d$ orbitals dominates the two changes. This work confirms the dominance of electron occupation on the MAE change and thus enriches the microscopic understanding about a MAE change, providing a theoretical reference to tuning the MAE of 2D ferromagnetic semiconductors in the future.

Interface‐Enhanced Ferromagnetism with Long‐Distance Effect in van der Waals Semiconductor

AbstractFerromagnetic semiconductors discovered in 2D materials open an avenue for highly integrated and multifunctional spintronics. The Curie temperature (TC) of existing 2D ferromagnetic semiconductors is extremely low and the modulation effect of their magnetism is limited compared with their 2D metallic counterparts. The interfacial effect is found to effectively manipulate the 3D magnetism, providing a unique opportunity for tailoring the 2D magnetism. Here, it is demonstrated that the TC of a 2D ferromagnetic semiconductor Cr2Ge2Te6 (CGT) can be enhanced by 130% (from ≈65 K to above 150 K) when adjacent to a tungsten layer. The interfacial W–Te bonding contributes to the TC enhancement with a strong perpendicular magnetic anisotropy, guaranteeing efficient magnetization switching by the spin‐orbit torque with a low current density at 150 K. Distinct from the rapid attenuation in conventional magnets, the interfacial effect exhibits a weak dependence on CGT thickness and a long‐distance effect (more than 10 nm) due to the weak interlayer coupling inherent to 2D magnets. This work not only reveals a unique interfacial behavior in 2D materials, but also advances the process toward practical 2D spintronics.

Highly-Tunable Intrinsic Room-Temperature Ferromagnetism in 2D van der Waals Semiconductor Crx Ga1- x Te.

The combination of semiconductivity and tunable ferromagnetism is pivotal for electrical control of ferromagnetism and next‐generation low‐power spintronic devices. However, Curie temperatures (T C) for most traditional intrinsic ferromagnetic semiconductors (≤200 K) and recently discovered two‐dimensional (2D) ones (<70 K) are far below room temperature. 2D van der Waals (vdW) semiconductors with intrinsic room‐temperature ferromagnetism remain elusive considering the unfavored 2D long‐range ferromagnetic order indicated by Mermin–Wagner theorem. Here, vdW semiconductor Cr x Ga1− x Te crystals exhibiting highly tunable above‐room‐temperature ferromagnetism with bandgap 1.62–1.66 eV are reported. The saturation magnetic moment (M sat) of Cr x Ga1− x Te crystals can be effectively regulated up to ≈5.4 times by tuning Cr content and ≈75.9 times by changing the thickness. vdW Cr x Ga1− x Te ultrathin semiconductor crystals show robust room‐temperature ferromagnetism with the 2D quantum confinement effect, enabling T C 314.9–329 K for nanosheets, record‐high for intrinsic vdW 2D ferromagnetic semiconductors. This work opens an avenue to room‐temperature 2D vdW ferromagnetic semiconductor for 2D electronic and spintronic devices.

Ferromagnetism in 2D Vanadium Diselenide.

Two-dimensional (2D) van der Waals ferromagnets carry the promise of ultimately miniature spintronics and information storage devices. Among the 2D ferromagnets, all inherit the magnetic ordering from their bulk ancestors. Here we report a 2D ferromagnetic semiconductor at room temperature, 2H-phase vanadium diselenide (VSe2), which shows ferromagnetic at the 2D form only. This 2D ferromagnetic semiconductor manifests an enhanced magnetic ordering owing to structural anisotropy at the 2D limit.

Large-Area Epitaxial Film Growth of van der Waals Ferromagnetic Ternary Chalcogenides.

Following the first experimental realization of intrinsic ferromagnetism in 2D van der Waals (vdW) crystals, several ternary metal chalcogenides with unprecedented long-range ferromagnetic order have been explored. However, the synthesis of large-area 2D ternary metal chalcogenide thin films is a great challenge, and a generalized synthesis has not been demonstrated yet. Here, a quick and scalable synthesis of epitaxially aligned ferromagnetic ternary metal chalcogenide thin films (Cr2 Ge2 Te6 , Cr2 Si2 Te6 , Mn3 Si2 Te6 ) is reported. The synthesis is based on the flux-controlled surface diffusion of Te on metal (Cr, Mn)-deposited wafer (Ge, Si) substrates. Magnetic anisotropy study of the epitaxial ternary thin films reveals the intrinsic magnetic easy axis; out-of-plane direction for Cr2 Ge2 Te6 and Cr2 Si2 Te6 , and in-plane direction for Mn3 Si2 Te6 . In addition to the synthesis, this work creates an opportunity for transfer-free device fabrication for realizing magnetoelectronics based on the electrical control of both charge and spin degrees of freedom in 2D ferromagnetic semiconductors.

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 are considered to be one of the most promising candidates for spintronics. Nonetheless, the 2D ferromagnetic semiconductors with large perpendicular magnetic anisotropy and high Curie temperature (Tc) are rarely reported. Here, through first-principles calculations, we predict a series of ferromagnetic semiconductors monolayer GdX2 (X = Cl, Br, I) by utilizing the rare-earth Gd with large spin-orbit coupling effect. Monte Carlo simulations manifest that the monolayer GdX2 show high Tc beyond 220 K, showing the promising applications in spintronics devices. Most interesting, three monolayer GdX2 show different magnetic anisotropy, among which the easy axis of monolayer GdCl2 is along the [001] direction, while monolayer GdBr2 and GdI2 possess in-plane magnetic anisotropy. Based on the analysis of the contributions from different atomic orbitals to magnetic anisotropy energy (MAE), it is found that the competition between the contributions of Gd-p orbitals, Gd-d orbitals, and p orbitals of halogen atoms to MAE transfers easy axis from out-of-plane to in-plane from Cl to I. The lattice mismatch between 2D materials and substrate definitely affects the properties of 2D materials, −5 % ∼ +5 % strain have been applied on the monolayer GdX2. The MAE of monolayer GdCl2 can increase from 111 to 190 μeV/f.u. under −5 % compressive strain. Besides, the easy axis of monolayer GdBr2 transfers from in-plane to out-of-plane over −3 % compressive strain. Furthermore, all the monolayer GdX2 have excellent thermal and dynamic stabilities. The present study is useful to design functional 2D materials with large perpendicular magnetic anisotropy and high Tc for the next generation spintronics.

Prediction of Chalcogen‐Doped VCl3 Monolayers as 2D Ferromagnetic Semiconductors with Enhanced Optical Absorption

AbstractIn 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.