Transition-Metal Dihydride Monolayers: A New Family of Two-Dimensional Ferromagnetic Materials with Intrinsic Room-Temperature Half-Metallicity

Room-temperature ferromagnetism, half-metallicity and spin transport in monolayer CrSc2Te4-based magnetic tunnel junction devices

The discovery of novel two-dimensional (2D) half-metallic materials with a robust ferromagnetic (FM) order and a high Curie temperature (Tc) is attractive for the advancement of next-generation spintronic devices. Here, we propose a monolayer with stable 2D intrinsic FM half-metallicity, i.e., the CrSc2Te4 monolayer, which was constructed by intercalating a monolayer of 1T-CrTe2-type sandwiched between two layers of 2H-ScTe2 monolayers. Our calculations reveal that it exhibits exceptional dynamical, thermal, and mechanical stabilities accompanied by a robust half-metallicity characterized by a wide bandgap of 1.02 eV and FM ordering with a high Tc of 326 K. Notably, these properties remain intact in almost the entire range of the biaxial strain from -5% to 5%. Furthermore, our investigations demonstrate excellent spin transport capabilities, including an outstanding spin-filtering effect, and a remarkably high tunneling magnetoresistance ratio peaking at 6087.07%. The remarkable magnetic features of the 2D CrSc2Te4 monolayer with room temperature FM, intrinsic half-metallicity, and 100% spin-polarization make it a promising candidate for the next-generation high-performance spintronic nanodevices as well as high-density magnetic recording and sensors.

Electronic structure and magnetothermal properties of two-dimensional ScCl

Two-dimensional ferromagnetic materials with intrinsic half-metallic properties have strong application advantages in nanoscale spintronics. Herein, density functional theory calculations show that monolayer ScCl is a ferromagnetic metallic material when undoped (n = 0), and the transition from metal to half-metal occurs with the continuous doping of holes. On the contrary, as the concentration of doped electrons increases, the system will exhibit metallic characteristics, which is particularly evident from a variation in spin polarizability. Furthermore, we have discussed how doped carriers affect the shape of the Fermi surface and the Fermi velocity of electrons. Most importantly, Monte Carlo simulations show that the ScCl monolayer is particularly regulated by carrier concentration (n) and magnetic field (h). Additionally, trends in energy and magnetic exchange coupling in different magnetic configurations (AFM phase and FM phase) with different doping concentrations are presented. When n < -0.16, the material is not only a half-metallic material that easily flips the magnetic axis, but also proves to be a candidate ferromagnetic material that works stably at room temperature in terms of dynamic stability. In addition, the origin of magnetocrystalline anisotropy is analyzed, and the contribution of different orbitals to spin-orbit coupling is presented. Moreover, we note that when magnetic field is small (h < 1 T), the change in size has a significant effect on ferromagnetic phase transition. However, when the system size is large (size >15 nm), TC is less sensitive to magnetic field. In addition, hole doping and size effect will greatly affect the hC of the system, but when the hole doping exceeds the critical value (n = -0.16), its influence on the hysteresis loop is no longer obvious. These interesting magnetic phenomena and easily adjustable physical properties show us that monolayer ScCl will be a promising functional material.

Electronic and magnetic properties of transition-metal-doped monolayer B2S2 within GGA + U framework

Considering the significant role of magnetism induction in two-dimensional (2D) semiconductor materials, we systematically investigate the effects of various dopants from the 3d and 4d transition metal (TM) series, including Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag and Cd, on the electronic and magnetic properties of monolayer B2S2 through first-principles calculations. The calculated formation energies indicate that substitutional doping at the B site with various TM atoms could be achieved under S-rich growth conditions. What matters is that with the exception of systems doped with Cu, Tc, and Ag elements, which exhibit non-magnetic semiconductor properties, all other doped systems demonstrate magnetism. Specifically, the Cr-, Ni- and Pd-doped monolayers are magnetic half-metals, while the rest are magnetic semiconductors. We have also performed calculations of magnetic couplings between two TM atoms with an impurity concentration of 3.12%, revealing the prevalence of weak magnetic coupling in the majority of the magnetic systems examined. Moreover, the monolayers doped with Cr, Zr and Pd atoms exhibit ferromagnetic ground states. These findings strongly support the high potential for inducing magnetism in the B2S2 monolayer through B-site doping.

Electric-field induced half-metallic properties in an experimentally synthesized CrSBr monolayer

Two-dimensional (2D) half-metallic materials are highly desirable for nanoscale spintronic applications. Here, we propose a new mechanism that can achieve half-metallicity in 2D ferromagnetic (FM) materials with two-layer magnetic atoms by electric field tuning. We use a concrete example of an experimentally synthesized CrSBr monolayer to illustrate our proposal through first-principles calculations. It is found that half-metallic properties can be achieved in CrSBr within an appropriate electric field range, and the corresponding amplitude of electric field intensity can be realized experimentally. Janus monolayer Cr2S2BrI is constructed, which possesses a built-in electric field due to broken horizontal mirror symmetry. However, Cr2S2BrI without and with an applied external electric field is always a FM semiconductor. A possible memory device is also proposed based on the CrSBr monolayer. Our work will stimulate the application of 2D FM CrSBr in future spintronic nanodevices.

Momentum matching induced giant magnetoresistance in two-dimensional magnetic tunnel junctions

Giant magnetoresistance was first experimentally discovered in three-dimensional magnetic tunnel junctions (MTJs) in the late 1980s and is of great importance in nonvolatile memory applications. How to achieve a magnetoresistance as large as possible is always a central task in the study of MTJs. However, it is normally only of the order of magnitude of tens of percent in traditional MTJs. The ideal situation is the metal-insulator transition together with the magnetization reversal of one magnetic lead. In this work, we will show that this can be achieved using a two-dimensional ferromagnetic zigzag SiC nanoribbon junction based on quantum transport calculations performed with a combination of density functional theory and non-equilibrium Green's function. Specifically, with the magnetization configuration switching of the two leads from parallel to anti-parallel, the junction will change abruptly from a conducting state to an insulating state, although the two leads are always metallic, with both spin up and spin down channels crossing the Fermi level simultaneously. Extensive analysis indicates that the insulating state in the anti-parallel magnetic configuration originates not from any present mechanisms that cause full suppression of electron transmission but from momentum direction mismatching. This finding suggests a fantastic mechanism for achieving magnetoresistance or electrical switching in nanoscale devices by manipulating band dispersion.

Two-dimensional ferromagnetic semiconductors of monolayer BiXO3 (X = Ru, Os) with direct band gaps, high Curie temperatures, and large magnetic anisotropy

Two-dimensional (2D) ferromagnetic semiconductors are highly promising candidates for spintronics, but are rarely reported with direct band gaps, high Curie temperatures (T_c), and large magnetic anisotropy. Using first-principles calculations, we predict that two ferromagnetic monolayers, BiXO₃ (X = Ru, Os), are such materials with a direct band gap of 2.64 and 1.69 eV, respectively. Monte Carlo simulations reveal that the monolayers show high T_c beyond 400 K. Interestingly, both BiXO₃ monolayers exhibit out-of-plane magnetic anisotropy, with magnetic anisotropy energy (MAE) of 1.07 meV per Ru for BiRuO₃ and 5.79 meV per Os for BiOsO₃. The estimated MAE for the BiOsO₃ sheet is one order of magnitude larger than that for the CrI₃ monolayer (685 μeV per Cr). Based on the second-order perturbation theory, it is revealed that the large MAE of the monolayers BiRuO₃ and BiOsO₃ is mainly contributed by the matrix element differences between d_xy and d_x²-y² and d_yz and d_z² orbitals. Importantly, the ferromagnetism remains robust in 2D BiXO₃ under compressive strain, while undergoing a ferromagnetic to antiferromagnetic transition under tensile strain. The intriguing electronic and magnetic properties make BiXO₃ monolayers promising candidates for nanoscale electronics and spintronics.

Ferroelectricity and High Curie Temperature in a 2D Janus Magnet

The breaking of the out-of-plane symmetry makes a two-dimensional (2D) Janus monolayer a new platform to explore the coupling between ferroelectricity and ferromagnetism. Using density functional theory in combination with Monte Carlo simulations, we report a novel phase-switchable 2D multiferroic material VInSe3 with large intrinsic out-of-plane spontaneous electric polarization and a high Curie temperature (Tc). The structural transition energy barrier between the two phases is determined to be 0.4 eV, indicating the switchability of the electric polarizations and the potential ferroelectricity. Carrier doping can boost the Curie temperature above room temperature, attributing to the enhanced magnetic exchange interaction. A transition from the ferromagnetic (FM) state to the antiferromagnetic (AFM) state can be induced by carrier doping in octahedra-VInSe3, while FM coupling is well-preserved in tetrahedron-VInSe3, which can be regulated to be either an XY or Ising magnet at an appropriate carrier concentration. These findings not only enrich the family of high-Tc low-dimensional monolayers but also offer a new direction for the design and multifunctional application of multiferroic materials.

TMB12: a newly designed 2D transition-metal boride for spintronics and electrochemical catalyst applications

Exploring two-dimensional (2D) ferromagnetic materials with a high transition temperature and large magnetic anisotropy is extremely essential for highly efficient spintronic applications. With the density functional theory method, we predicted planar hypercoordinate transition-metal borides, TMB₁₂ (TM = Ti, V, Cr, Mn, Fe; B = boron), by the condensation of TM@B₈ and B₄ units. The results showed that these transition-metal borides possess superior thermal, dynamic and mechanical stabilities. Interestingly, the TMB₁₂ monolayer with TM = (V, Cr) is confirmed as a robust ferromagnetic metal with a high Curie temperature of ∼335 K and ∼221 K, respectively. In addition, the system with TM = (Mn, Fe) is found to be an antiferromagnetic metal with a Néel temperature of ∼173 K and ∼91 K, respectively. In particular, large perpendicular magnetic anisotropy is identified for CrB₁₂, MnB₁₂, and FeB₁₂ monolayers, around 198-623 μeV. Furthermore, four TMB₁₂ (TM = Ti, V, Cr, Mn) systems are determined to be candidate catalysts for the hydrogen evolution reaction, with nearly zero free energy of hydrogen adsorption (ΔG_H = -0.0003 to -0.03 eV). Our study highlighted potential 2D metal borides for spintronic devices and high efficiency electrochemical catalysts.

Multiconfiguration b-AsP-based doping systems with enriched elements (C and O): novel materials for spintronic devices

Black arsenical phosphorus (b-AsP), a derivative of black phosphorus, is a bimetallic alloy compound with the advantage of high carrier mobility, high stability, and tailorable configuration. However, lack of an effective tool to facilitate the application of AsP as a magnetic device. Herein, band gap modulation and the introduction of magnetism can be achieved by doping non-metallic atoms in three different AsP configurations. And the doping of the same atom will cause variation in the electronic structure depending on the configuration. Surprisingly, doping with both enriched elements C and O transforms AsP into a magnetic material. Furthermore, the source of the magnetic moment is explained by solving the wave function of the doped AsP, which is caused by the orbital coupling of the C and O atoms to AsP. To excavate the potentials of this magnetic AsP system for magnetic devices, field-effect transistors based on two doped armchair AsP3 nanoribbons are simulated. The devices show considerable negative differential conductivity effect and good spin filtering efficiency. These findings suggest that AsP doping with enriched elements C and O could be an excellent candidate for future spintronics applications.

Intrinsic Ferromagnetism in 2D Fe2H with a High Curie Temperature

The rational design of ferromagnetic materials is crucial for the development of spintronic devices. Using first-principles structural search calculations, we have identified 73 two-dimensional transition metal hydrides. Some of them show interesting magnetic properties, even when combined with the characteristics of the electrides. In particular, the P3̅m1 Fe₂H monolayer is stabilized in a 1T-MoS₂-type structure with a local magnetic moment of 3 μ_B per Fe atom, whose robust ferromagnetism is attributed to the exchange interaction between neighboring Fe atoms within and between sublayers, leading to a remarkably high Curie temperature of 340 K. On the other hand, it has a large magnetic anisotropic energy and spin-polarization ratio. Interestingly, the above room-temperature ferromagnetism of the Fe₂H monolayer is well preserved within a biaxial strain of 5%. The structure and electron property of surface-functionalized Fe₂H are also explored. All of these interesting properties make the Fe₂H monolayer an attractive candidate for spintronic nanodevices.

Toward Room-Temperature Electrical Control of Magnetic Order in Multiferroic van der Waals Materials

Electrical control of magnetic order in van der Waals (vdW) two-dimensional (2D) systems is appealing for high-efficiency and low-dissipation nanospintronic devices. For realistic applications, a vdW 2D material with ferromagnetic (FM) and ferroelectric (FE) orders coexisting and strongly coupling at room temperature is urgently needed. Here we present a potential candidate for nonvolatile electric-field control of magnetic orders at room temperature. Using first-principles calculations, we predict the coexistence of room-temperature FM and FE orders in a 2D transition metal carbide, where the spatial distribution of magnetic moments strongly couples with the orientation of out-of-plane electric polarization. Furthermore, an electric-field switching between interfacial FM and ferrimagnetic orders is realizable through constructing a multiferroic vdW heterostructure based on this material. These findings make a significant step toward realizing room-temperature multiferroicity and strong magnetoelectric coupling in 2D materials.

Robust Quantum Anomalous Hall States in Monolayer and Few-Layer TiTe

Quantum anomalous Hall (QAH) insulators possess exotic properties driven by novel topological physics, but related studies and potential applications have been hindered by the ultralow temperatures required to sustain the operating mechanisms dictated by key material parameters. Here, using first-principles calculations, we predict a robust QAH state in monolayer TiTe that exhibits a high ferromagnetic Curie temperature of 650 K and a sizable band gap of 261 meV. These outstanding benchmark properties stem from the Te atom's large size that favors ferromagnetic kinetic exchange with the neighboring Ti atoms and strong spin-orbit coupling that creates a QAH state by adding a mass term to the Dirac half-semimetal state. Remarkably, the ferromagnetic order remains robust against interlayer stacking via the d-p_z/p_y-p_z-d super-super exchange, generating unprecedented QAH states in few-layer configurations with enhanced Curie temperatures and higher Chern numbers. These results signify layered TiTe to be a prime template for exploring novel QAH physics at ambient and higher temperatures.

Alloying two-dimensional NbSi2N4: a new strategy to realize half-metallic antiferromagnets

Finding two-dimensional (2D) materials with both 100% spin polarization and zero net magnetic moment is essential for next-generation spintronics. Half-metallic antiferromagnets (HMAFs) are ideal materials to satisfy these exigent needs, but such a system has never been found among 2D inorganic materials. In this paper, we theoretically demonstrate that intrinsic 2D HMAFs can be realized by alloying Nb with Mn in 2D septuple-atomic-layer NbSi₂N₄. By continuously incorporating Mn, the stronger Mn-N hybridization relative to Nb-N induces a metal to half-metal to semiconductor transition. The competitive coupling between the Nb-d itinerant electron spin and the Nb-Mn d-d direct interaction drives the ferromagnetic to antiferromagnetic phase transition. For the first time in 2D inorganic materials, the exact cancellation of local magnetic moments and band gap opening in one spin channel is obtained simultaneously at a Nb/Mn ratio of 3 : 1, as demonstrated by our first-principles calculations. The present results would not only inspire materials design of more 2D HMAFs in the future but also impel the advancement of next-generation antiferromagnetic spintronic devices.

Robust Dirac spin gapless semiconductors in a two-dimensional oxalate based organic honeycomb-kagome lattice

Two-dimensional (2D) ferromagnetic materials with intrinsic and robust spin-polarized Dirac cones are of great interest in exploring exciting physics and in realizing spintronic devices. Using comprehensive ab initio calculations, herein we reveal a family of 2D oxalate-based metal-organic frameworks (MOFs) that possess the desired characteristics. We propose that these 2D oxalate-based MOFs may be assembled by oxalate ions (C₂O₄^2-) and two homo-transition metal atoms. We demonstrate that 2D MOFs of Ni₂(C₂O₄)₃ and Re₂(C₂O₄)₃ are intrinsic Dirac spin gapless semiconductors with linear band dispersion, low energy dissipation and high electron carrier velocity. As robust ferromagnets, they also possess large magnetic moments, large perpendicular magnetic anisotropy, and high Curie temperatures, e.g. 208 K for Ni₂(C₂O₄)₃. In particular, spin-orbit coupling triggers a topologically nontrivial band gap of 143 meV in Re₂(C₂O₄)₃, which is promising to realize the quantum anomalous Hall effect at high temperatures.

Transition Metal-Free Half-Metallicity in Two-Dimensional Gallium Nitride with a Quasi-Flat Band

Two-dimensional half-metallicity without a transition metal is an attractive attribute for spintronics applications. On the basis of first-principles calculation, we revealed that a two-dimensional gallium nitride (2D-GaN), which was recently synthesized between graphene and SiC or wurtzite GaN substrate, exhibits half-metallicity due to its half-filled quasi-flat band. We found that graphene plays a crucial role in stabilizing a local octahedral structure, whose unusually high density of states due to a flat band leads to a spontaneous phase transition to its half-metallic phase from normal metal. It was also found that its half-metallicity is strongly correlated to the in-plane lattice constants and thus subjected to substrate modification. To investigate the magnetic property, we simplified its magnetic structure with a two-dimensional Heisenberg model and performed Monte Carlo simulation. Our simulation estimated its Curie temperature (T_C) to be ∼165 K under a weak external magnetic field, suggesting that transition metal-free 2D-GaN exhibiting p orbital-based half-metallicity can be utilized in future spintronics.

High Curie Temperature and Intrinsic Ferromagnetic Half-Metallicity in Mn2X3 (X = S, Se, Te) Nanosheets

Two-dimensional (2D) intrinsic half-metallic materials with room-temperature ferromagnetism, sizable magnetic anisotropy energy (MAE), and wide half-metallic gap are excellent candidates for pure spin generation, injection, and transport in nanospintronic applications. However, until now, such 2D half metallicity has been rarely observed in experiment. In this work, by using first-principles calculations, we design a series of such materials, namely, Mn₂X₃ (X = S, Se, Te) nanosheets, which could be obtained by controlling the thickness of synthesized α-MnX(111) nanofilm to a quintuple X-Mn-X-Mn-X layer. All these nanosheets are dynamically and thermally stable. Electronic and magnetic studies reveal they are intrinsic half metals with high Curie temperatures between 718 and 820 K, sizable MAEs with -1.843 meV/Mn for Mn₂Te₃ nanosheet, and wide half-metallic gaps from 1.55 to 1.94 eV. Above all, the outstanding features of Mn₂X₃ nanosheets make them promising in fabricating nanospintronic devices working at room temperature.

Robust Intrinsic Multiferroicity in a FeHfSe3 Layer

As a consequence of the mutually exclusive origins of ferroelectricity and magnetism, multiferroic materials with electromagnetic coupling are rare. In this work, stable two-dimensional FeHfSe₃ with experimental accessibility is however demonstrated to harbor robust electromagnetic coupling. FeHfSe₃ illustrates spontaneous in-plane polarization of 1.29 × 10^-10 C/m, and the energy barrier of 116.54 meV ensures easy switching and a high Curie temperature. In addition, semiconducting FeHfSe₃ possesses a stable antiferromagnetic ground state with a Néel temperature of approximately 300 K. In the case of applying strain, ferroelectricity and magnetism coexist stably, and uniaxial tensile strain can effectively enhance the ferroelectricity.

Electric-Field Tunable Magnetism in van der Waals Bilayers with A-Type Antiferromagnetic Order: Unipolar versus Bipolar Magnetic Semiconductor

Uncovering the physics behind the electrical manipulation of low-dimensional magnetic materials remains a fundamental issue in practical application of nanoscale spintronics. Here, we propose a strategy to transform A-type antiferromagnetic (AFM) semiconductors into asymmetric AFM unipolar or bipolar magnetic semiconductors by applying perpendicular electric fields in van der Waals bilayer systems. Electric fields lifting energy levels of electrons within same spin channel from consistent layers in opposite direction enables unipolar magnetic semiconductor, whereas electrons within opposite spin channel enable bipolar magnetic semiconductor. A comprehensive study demonstrates this discrepancy originates from spatial distributions of spin density of valence band and conduction band edges in two layers of systems. The electric field induced unipolar or bipolar magnetic semiconducting behavior represents great potential of nanoscale AFM spintronics for information storage and processing.

Two-Dimensional Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) Monolayers: Half-Metallic Ferromagnets with Tunable Magnetic Properties under Strain

Two-dimensional (2D) ferromagnetic materials with high spin polarization are highly desirable for spintronic devices. 2D Janus materials exhibit novel properties due to their broken symmetry. However, the electronic structure and magnetic properties of 2D Janus magnetic materials with high spin polarization are still unclear. Inspired by the successful synthesis of a ferromagnetic FeCl₂ monolayer and 2D Janus MoSSe and WSSe, we systematically study the electronic structure and magnetic properties of Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) monolayers. Based on the Goodenough-Kanamori-Anderson theory, the ferromagnetism stems from the superexchange interaction mediated by Fe-X/Y-Fe bonds. The band gaps of spin-up channels are large enough (>4 eV) to prevent spin flipping, which is beneficial for spintronic devices. Additionally, the sizable magnetocrystalline anisotropy energy (MAE) indicates that Janus FeXY monolayers are suitable for information storage. More importantly, the half-metallic character is still kept in Janus FeXY monolayers, and their magnetic properties are enhanced by the biaxial compressive strain. The MAE of FeClI and FeBrI increases by 1 order of magnitude, and the Curie temperature of FeXY monolayers enhances by 100%. These results provide an example of the 2D Janus half-metallic materials and enrich the 2D magnetic material library.

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.

A first-principles study on the electronic property and magnetic anisotropy of ferromagnetic CrOF and CrOCl monolayers

Two-dimensional ferromagnetic materials with large perpendicular magnetic anisotropy (PMA) hold great potential in realizing low critical switching current, high thermal stability and high density nonvolatile storage in magnetic random-access memories. Our first-principles calculations reveal that CrOF and CrOCl monolayers (MLs) are two-dimensional (2D) ferromagnetic semiconductors with out-of-plane magnetic easy axis, and PMAs of CrOF and CrOCl MLs are mainly contributed by Cr atoms. The magnetic anisotropy of CrOF and CrOCl MLs can be controlled and enhanced by applying biaxial strain. Tensile strain can further enhance PMAs of CrOF and CrOCl MLs by 82.9% and 161.0% higher than those of unstrained systems, respectively. In addition, appropriate compressive strain can switch the magnetic easy axis of CrOF and CrOCl MLs from out-of-plane direction to in-plane direction. The semiconductor natures of CrOF and CrOCl MLs robust against biaxial strain, the band gaps of these systems under biaxial strain are in the range of 1.26 eV to 2.40 eV. By applying biaxial strain, the Curie temperatures of CrOF and CrOCl MLs increase up to 282 K and 163 K, respectively. These tunable properties suggest that CrOF and CrOCl MLs have great application potentials for magnetic data storage.

A high performance N-doped graphene nanoribbon based spintronic device applicable with a wide range of adatoms

Designing and fabricating nanosize spintronic devices is a crucial task to develop information technology of the future. However, most of the introduced spin filters suffer from several limitations including difficulty in manipulating the spin current, incapability in utilizing a wide range of dopants to provide magnetism, or obstacles in their experimental realization. Here, by employing first principles calculations, we introduce a structurally simple and functionally efficient spin filter device composed of a zigzag graphene nanoribbon (ZGNR) with an embedded nitrogenated divacancy. We show that the proposed system, possessing a robust ferromagnetic (FM) ordering, exhibits perfect half metallic behavior in the absence of frequently used transition metals (TMs). Our calculations also show that the suggested system is compatible with a wide range of adatoms including basic metals, metalloids, and TMs. It means that besides d electron magnetism originating from TMs, p electrons of incorporated elements of the main group can also cause half metallicity in the electronic structure of the introduced system. Our system exploiting the robustness of doping-induced FM ordering would be beneficial for promising multifunctional spin filter devices.

Electric polarization related Dirac half-metallicity in Mn-trihalide Janus monolayers

A two-dimensional Dirac half-metal system, in which the 100% spin polarization and massless Dirac fermions can coexist, will show more advantages on the efficient spin injection and high spin mobility in spintronic devices. Moreover, it is attractive to achieve out-of-plane electric polarization in addition to the Dirac half-metal behavior, because this will open a new horizon in the field of multifunctional devices. In this work, a systematic study is made of Janus monolayers of Mn2X3Y3 (X, Y = Cl, Br and I, X ≠ Y) with asymmetric out-of-plane structural configurations, based on first-principles calculations. We demonstrate that monolayer Mn2X3Y3 freestanding films will remain stable experimentally by using the stability analysis. All the Janus monolayers show a ferromagnetic ground state and maintain their original DHM behavior. However, due to the large electric polarization, the hybridization intensities of Mn and the halogen atoms on both sides of Mn2Cl3I3 are very different, resulting in an obvious distortion of the spin-polarized Dirac cone. The distorted Dirac cone is repaired by the compression, indicating that strain can improve the orbital distortion induced by the electric polarization. All Mn2X3Y3 monolayer have in-plane magnetization anisotropy, which is mainly contributed by heavy halogen elements (Br and I), and the polarized substitution and biaxial strain will not change the easy magnetization orientation of the system. Thus, the electrically polarized Dirac half-metal system has potential for application in multifunctional spintronic devices.

Unveiling the origin of room-temperature ferromagnetism in monolayer VSe2: the role of extrinsic effects

Room-temperature ferromagnetism in monolayer vanadium diselenide (VSe2) on graphite is the object of a controversial debate. Herein, we unveil the contribution from extrinsic factors to the magnetic properties of monolayer VSe2 by means of density functional theory. Specifically, we demonstrate that either intrinsic defects or the adsorption of molecules enhances ferromagnetic interactions. The expansion of the VSe2 lattice increases the magnetic moment on vanadium ions, whereas both compression and out-of-plane distortion withdraw magnetic moments. The exchange interactions between vanadium ions and magnetic defects (vacancies and impurities) in the surface and subsurface layers of the substrate are able to turn the unstable two-dimensional (2D) ferromagnetism into stable three-dimensional (3D) ferromagnetism. Definitely, the combination of effects related to chemisorption, substrate-induced distortion and magnetic defects of the substrate could enhance or suppress ferromagnetism in monolayer VSe2.

Quantum Anomalous Hall Effect in Magnetic Doped Topological Insulators and Ferromagnetic Spin-Gapless Semiconductors-A Perspective Review

Quantum anomalous Hall effect, with a trademark of dissipationless chiral edge states for electronics/spintronics transport applications, can be realized in materials with large spin-orbit coupling and strong intrinsic magnetization. After Haldane's seminal proposal, several models have been presented to control/enhance the spin-orbit coupling and intrinsic magnetic exchange interaction. After brief introduction of Haldane model for spineless fermions, following three fundamental quantum anomalous Hall models are discussed in this perspective review: i) low-energy effective four band model for magnetic-doped topological insulator (Bi,Sb)₂ Te₃ thin films, ii) four band tight-binding model for graphene with magnetic adatoms, and iii) two (three) band spinful tight-binding model for ferromagnetic spin-gapless semiconductors with honeycomb (kagome) lattice where ground state is intrinsically ferromagnetic. These models cover 2D Dirac materials hosting spinless, spinful, and spin-degenerate Dirac points where various mass terms open bandgap and lead to quantum anomalous Hall effect. With emphasis on the topological phase transition driven by ferromagnetic exchange interaction and its interplay with spin-orbit-coupling, various symmetry constraints on the nature of mass term and the materialization of these models are discussed. This study will shed light on the fundamental theoretical perspectives of quantum anomalous Hall materials.

Robust intrinsic half-metallic ferromagnetism in stable 2D single-layer MnAsS4

Two-dimensional (2D) intrinsic half-metallic materials are of great interest to explore the exciting physics and applications of nanoscale spintronic devices, but no such materials have been experimentally realized. Using first-principles calculations based on density-functional theory, we predicted that single-layer MnAsS4was a 2D intrinsic ferromagnetic (FM) half-metal. The half-metallic spin gap for single-layer MnAsS4is about 1.46 eV, and it has a large spin splitting of about 0.49 eV in the conduction band. Monte Carlo simulations predicted the Curie temperature (Tc) was about 740 K. Moreover, within the biaxial strain ranging from -5% to 5%, the FM half-metallic properties remain unchanged. Its ground-state with 100% spin-polarization ratio at Fermi level may be a promising candidate material for 2D spintronic applications.

High Curie temperature and carrier mobility of novel Fe, Co and Ni carbide MXenes

Two-dimensional (2D) magnets with room temperature ferromagnetism and semiconductors with moderate band gap and high carrier mobility are highly desired for applications in nanoscale electronics and spintronics. By performing the first-principles calculations, we investigate novel Fe, Co, Ni carbide based pristine (M₂C) and functionalized (M₂CT₂, T: F, O, OH) MXenes. Our calculations show that Fe₂C, Co₂C, Ni₂C, Fe₂CF₂, Fe₂CO₂, Fe₂C(OH)₂, Co₂CF₂, Co₂C(OH)₂ and Ni₂CF₂ are dynamically and mechanically stable. More importantly, Fe₂C, Co₂C, Fe₂CF₂ and Fe₂C(OH)₂ exhibit intrinsic ferromagnetism (magnetic moments 2-5μ_B per unit cell). Monte Carlo simulations suggest high Curie temperatures of 590 and 920 K for Fe₂C and Fe₂CF₂, respectively, at the HSE06 level owing to the large spin magnetic moments and strong ferromagnetic coupling. Based on the deformation potential theory, we predict high and anisotropic hole mobility (0.2-1.4 × 10⁴ cm² V^-1 s^-1) for semiconducting Fe₂CO₂ and Co₂C(OH)₂. Additionally, Ni₂CF₂ demonstrates highly anisotropic electron mobility together with a direct band gap. Our results further show the effectiveness of surface functionalization in modulating the electronic and magnetic properties and broadening the properties of MXenes to achieve long-range intrinsic ferromagnetism well above room temperature and high carrier mobility.

Electric Field-Modulated Magnetic Phase Transition in van der Waals CrI3 Bilayers

Two-dimensional van der Waals (vdW) magnetic materials are well-recognized milestones toward nanostructured spintronics. An interesting example is CrI₃; its magnetic states can be modulated electrically, allowing spintronics applications that are highly compatible with electronics technologies. Here, we report the electric field alone induces the interlayer antiferromagnetic-to-ferromagnetic (AFM-to-FM) phase transition in CrI₃ bilayers with critical field as low as 0.12 V/Å. The AFM-FM energy difference ΔE increases with electric field and is closely related to the field-induced on-site energy difference defined as the splitting between the electronic states of the two vdW layers. Our tight-binding model fits closely with ΔE as a function of electric field and gives a consistent estimation for orbital hopping, exchange splitting, and crystal field splitting. Furthermore, a CrI₃-based spin field-effect device is suggested with the spin current switched on and off solely by the electric field. These findings not only reveal the physics underlying the transition but also provide guidelines for future discovery and design.

Stacking-Independent Ferromagnetism in Bilayer VI3 with Half-Metallic Characteristic

Two-dimensional (2D) ferromagnetic (FM) materials have been identified as the foundation for next-generation electronic devices. However, there are still rare materials that show intrinsic ferromagnetism, which makes searching for new ferromagnetic materials a very important task. In this work, we systematically investigate the electronic and magnetic properties of bilayer VI₃ based on density functional theory (DFT). Our results show that bilayer VI₃ shows a half-metallic characteristic. Furthermore, we observe a robust ferromagnetic ground state in bilayer VI₃, which is independent of the stacking configurations. A model including intralayer through-bond and interlayer super-superexchange (SSE) effects is employed to analyze the mechanism of the ferromagnetic coupling. On account of the half-metallic character, an application as a spin valve is then explored using quantum transport simulations, which show 100% magnetoresistance. Our results provide guidelines for the application of the bilayer VI₃ for the next-generation spintronic devices.

Switchable metal-to-half-metal transition at the semi-hydrogenated graphene/ferroelectric interface

Tuning the half-metallicity of low-dimensional materials using an electric field is particularly appealing for spintronic applications but typically requires an ultra-high field, hampering practical applications. Interface engineering has been suggested as an alternative practical means to overcome this limitation and control the metal-to-half-metal transition. Here, we show from first-principles calculations that the polarization switching at the interface of semi-hydrogenated graphene (i.e., graphone) and a ferroelectric PbTiO3 layer can reversibly tune a metal to half-metal transition in graphone. Using a simple Hubbard model, this is rationalized using interface atomic orbital hybridization, which also reveals the origin of the high-quality screening of metallic graphone, preserving bulk-like stable ferroelectric polarization in the PbTiO3 film down to a thickness of two unit cells. These findings do not only open a new perspective on engineering half-metallicity at the interface of two-dimensional materials and ferroelectrics, but also identify graphone as a powerful atomically thin electrode, which holds great promise for the design of ultrafast and high integration density information-storage devices.

Recent advances in two-dimensional ferromagnetism: materials synthesis, physical properties and device applications

Two-dimensional (2D) ferromagnetism is critical for both scientific investigation and technological development owing to its low-dimensionality that brings in quantization of electronic states as well as free axes for device modulation. However, the scarcity of high-temperature 2D ferromagnets has been the obstacle of many research studies, such as the quantum anomalous Hall effect (QAHE) and thin-film spintronics. Indeed, in the case of the isotropic Heisenberg model with finite-range exchange interactions as an example, low-dimensionality is shown to be contraindicated with ferromagnetism. However, the advantages of low-dimensionality for micro-scale patterning could enhance the Curie temperature (T_C) of 2D ferromagnets beyond the T_C of bulk materials, opening the door for designing high-temperature ferromagnets in the 2D limit. In this paper, we review the recent advances in the field of 2D ferromagnets, including their material systems, physical properties, and potential device applications.

Two-Dimensional (001) LaAlO3/SrTiO3 Heterostructures with Adjustable Band Gap and Magnetic Properties

Very recently, freestanding crystalline perovskite films as thin as a single unit cell have been successfully synthesized, which expands the opportunities for research and applications of low-dimensional materials with novel functionalities. In this work, we constructed a series of two-dimensional (2D) (001) LaAlO₃/SrTiO₃ heterostructures and systematically investigated their atomic and electronic properties by means of first-principles calculations. Our results show that (1) nonstoichiometry leads to ferromagnetism at the interfaces of the systems; (2) half-metallicity can be realized by introducing slight hole-doping; and (3) a semiconductor-to-metal transition can be triggered by applying a moderate (within 3%) out-of-plane strain. Besides, based on in-depth analysis of the electronic structures, we propose that the orbital hybridization of interfacial O and Ti atoms may play a crucial role in determining the above interesting phenomena. Our findings are expected to stimulate further experimental researches on the related 2D perovskite heterostructures and to be beneficial for the design of new multifunctional electronic devices.

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.

Two-Dimensional Ferroics and Multiferroics: Platforms for New Physics and Applications

Two-dimensional (2D) ferroics, including ferromagnets, ferroelectrics, ferroelastics, and multiferroics, recently have been theoretically proposed or experimentally revealed. The research has attracted tremendous attention because of the novel physics and promising applications for nanoelectronics, revealing ferroics in the 2D limit. In the present Perspective, we comprehensively review the recent research progress and also the proposed applications of 2D ferromagnetic, ferroelectric, and ferroelastic materials from theoretical and experimental viewpoints. We then introduce the coupling between ferroic orders and highlight the latest research on 2D multiferroic materials. The promising research directions and outlooks are discussed at the end of the Perspective. It is expected that the comprehensive overview of 2D ferroic materials can provide guidelines for researchers in the area and inspire further explorations of new physics and ferroic devices.

Room temperature ferromagnetism and antiferromagnetism in two-dimensional iron arsenides

The discovery of two-dimensional (2D) magnetic materials with high critical temperature and intrinsic magnetic properties has attracted significant research interest. By using swarm-intelligence structure search and first-principles calculations, we predict three 2D iron arsenide monolayers (denoted as FeAs-I, II and III) with good energetic and dynamical stabilities. We find that FeAs-I and II are ferromagnets, while FeAs-III is an antiferromagnet. FeAs-I and III have sizable magnetic anisotropy comparable to the magnetic recording materials such as the FeCo alloy. Importantly, we show that FeAs-I and III have critical temperatures of 645 K and 350 K, respectively, which are above room temperature. In addition, FeAs-I and II are metallic, while FeAs-III is semiconducting with a gap comparable to Si. For FeAs-III, there exist two pairs of 2D antiferromagnetic Dirac points below the Fermi level, and it displays a giant magneto band-structure effect. The superior magnetic and electronic properties of the FeAs monolayers make them promising candidates for spintronics applications.

Strain-induced N-N bonding and magnetic changes in monolayer intrinsic ferromagnetic TmN2 (Tm = Tc and Nb)

The modulation of magnetic property in two-dimensional (2D) intrinsic ferromagnets is important for their future application in spintronic devices at the nanoscale. In this work, using first-principles calculation, we investigate the effects of strain on the structures, electronic structures and magnetic properties of monolayer 1H-NbN₂ and 1H-TcN₂. The results show that both the unstrained monolayer 1H-NbN₂ and 1H-TcN₂ are 2D intrinsically ferromagnetic (FM) metal, in which the magnetic moment of the 1H-NbN₂ and 1H-TcN₂ comes mainly from the d orbitals of Nb atom and the p orbitals of N atom, respectively. Remarkably, two neighboring N atoms in the unstrained 1H-NbN₂ form N-N bond, while those in the 1H-TcN₂ do not. When lattice constant a increases to 3.17 Å, monolayer 1H-TcN₂ undergoes N-N nonbonding-bonding transition at which the distance between the N atoms d _N-N suddenly drops by almost 25%. In particular, due to the bonding between two neighboring N atoms, the magnetic moment of N atoms in 1H-TcN₂ are quenched and the ground state transfers to non-magnetic. In contrast, when a decreases to 3.18 Å, monolayer 1H-NbN₂ undergoes N  -  N bonding-nonbonding transition at which the d _N-N suddenly increases from 1.79 Å to 1.97 Å. The N-N bonding-nonbonding transition induces the magnetic moments to transfer from the d orbitals of Nb atom to the p orbitals of N atom, while ground state of monolayer 1H-NbN₂ remains FM metal.

Room-Temperature Ferromagnetism in Transition Metal Embedded Borophene Nanosheets

Exploring two-dimensional (2D) materials with room-temperature ferromagnetism and large perpendicular magnetic anisotropy is highly desirable but challenging. Here, through first-principles calculations, we propose a viable strategy to achieve such materials based on transition metal (TM) embedded borophene nanosheets. Due to electron deficiency, the commonly existent hexagon boron vacancies in various borophene phases serve as intrinsic anchor points for electron-rich transition metals, which not only adsorb strongly upon the vacancies but also favor to be embedded into the vacancies, forming 2D planar hybrid nanosheets. The adsorption-to-embedding transition is feasible thermodynamically and kinetically, owing to its exothermic nature and relatively small kinetic barriers. After embedding, phase transition is further proposed to obtain diverse structures of TM embedded borophenes with versatile magnetic properties. Based on the example of χ₃ phase borophene, several ferromagnetic TM embedded borophene nanosheets with high Curie temperature and large perpendicular magnetic anisotropy have been predicted.

2D Magnetic Janus Semiconductors with Exotic Structural and Quantum-Phase Transitions

2D magnetic semiconductors are intriguing for their great potential applications in spintronic nanodevices. Despite intensive research for decades, intrinsically 2D magnetic Janus semiconductors are scarce, and their design guidelines remain elusive. Herein we propose new 2D Janus Cr₂O₂XY (X = Cl, Y = Br/I) ferromagnets with asymmetric out-of-plane structural configurations from ab initio calculations. Abnormally, 2D Janus Cr₂O₂XY crystals with Pmm2 structures derived from pristine CrOX compounds are dynamically metastable. By introducing novel structural phase transitions, we generated new Pma2 phases with lower total energy and great dynamical stability. These new Janus Cr₂O₂XY monolayers are intrinsically ferromagnetic semiconductors and could be easily synthesized from experiment. Most interestingly, exotic quantum-phase transitions from the ferromagnetic semiconductor to the antiferromagnetic metal/semiconductor could be achieved in the Cr₂O₂ClI monolayer by applying compressive strains. Our study provides an alternative strategy to design new Janus Cr₂O₂XY monolayers and will inspire further investigations on relevant materials for electronic and spintronic applications.

Two-Dimensional Gold Sulfide Monolayers with Direct Band Gap and Ultrahigh Electron Mobility

It remains a pressing task to search for new two-dimensional (2D) semiconducting materials for future-generation electronic applications. By using density functional theory computations and global structure prediction methods, we demonstrate two new gold sulfide monolayers (2D Au₂S and AuS), both exhibiting excellent electronic properties and high stabilities. All the gold sulfide monolayers are semiconductors with band gaps in the range 1.0-3.6 eV. In particular, the α-Au₂S monolayer is predicted to possess a direct band gap of 1.0 eV and extremely high electron and hole mobilities of 8.45 × 10⁴ and 0.40 × 10⁴ cm² V^-1 S^-1, respectively. The phonon dispersion calculations and ab initio molecular dynamics simulations indicate that the gold sulfide monolayers exhibit robust dynamical and thermal stabilities. Moreover, the α-Au₂S monolayer appears to show strong oxidation resistibility. The novel electronic properties, coupled with structural and chemical stabilities, endow the new gold sulfide monolayers to be highly promising for future applications in nanoelectronics.

High-Temperature Ferromagnetism in an Fe3P Monolayer with a Large Magnetic Anisotropy

For the development of high-performance spintronic nanodevices, one of the most urgent and challenging tasks is the preparation of two-dimensional materials with room-temperature ferromagnetism and a large magnetic anisotropic energy (MAE). Through first-principles swarm-intelligence structural search calculations, we identify an ideal ferromagnetic Fe₃P monolayer, in which Fe atoms show a perfect Kagome lattice, leading to strong in-plane Fe-Fe coupling. The predicted Curie temperature of Fe₃P reaches ∼420 K, and its MAE is comparable to those of ferromagnetic materials, such as Fe and Fe₂Si. Moreover, the Fe₃P monolayer remains as an above room-temperature ferromagnet under biaxial strains as large as 10%. Its lattice can be retained at temperatures of ≤1000 K, exhibiting a high thermodynamic stability. All of these desirable properties make the Fe₃P monolayer a promising candidate for applications in spintronic nanodevices.

Toward Room-Temperature Magnetic Semiconductors in Two-Dimensional Ferrimagnetic Organometallic Lattices

Obtaining room-temperature magnetically ordered two-dimensional (2D) semiconductors is urgently needed for high-speed nanospintronic devices but remains a big challenge. Here, we propose a potential route to solve this issue by constructing ferrimagnetic semiconductors in 2D metal organic frameworks, taking advantage of the high Curie temperature of ferrimagnetic semiconductors and easy tunability of metal organic frameworks. The proposal is confirmed by first-principles design of 2D metal organic frameworks with conjugated electron acceptors diketopyrrolopyrrole (DPP) as organic linkers and transition metal Cr (V) as nodes. The robust ferrimagnetic ordering comes from the strong direct exchange interaction between d-electron magnetic moments on transition metals and charge transfer-induced p-electron magnetic moments on DPPs, which can be modulated facilely by reducing the d-p orbital interaction distance via moderate compressive strain or increasing the d-p orbital charge transfer through introducing electron-withdrawing groups into the DPP moiety.

Engineering the magnetic properties of PtSe2 monolayer through transition metal doping

Using first-principles calculations, we have studied the energetic feasibility and magnetic properties of transition metal (TM) doped PtSe₂ monolayers. Our study shows that TM doped PtSe₂ layers with 6.25% doping exhibit versatile spintronic behaviour depending on the nature of the dopant TM atoms. Groups IVB and VIII10 TM doped PtSe₂ layers are non magnetic semiconductors, while groups IIIB, VB, VIII8, VIII9, IB TM doped PtSe₂ layers are half-metals and finally, groups VIB, VIIB and IIB TM doped PtSe₂ layers are spin polarized semiconductors. The presence of half-metallic and magnetic semiconducting characteristics suggest that TM doped PtSe₂ layers can be considered as a new kind of dilute magnetic semiconductor and thus have the promise to be used in spintronics. By studying the magnetic interactions between two TM dopants in PtSe₂ monolayers for dopant concentration of 12.5% and dopant distance of 12.85 [Formula: see text], we have found that in particular, Fe and Ru doped PtSe₂ systems are ferromagnetic half-metal having above-room-temperature Curie point of 422 and 379.9 K, respectively. By varying the dopant distance and concentration we have shown that the magnetic interaction is strongly dependent on dopant distance and concentration. Interestingly, the Curie temperature of TM doped PtSe₂ layers is affected by the correlation effects on the TM d states and also spin-orbit coupling. We have also studied the magnetic properties of defect complex composed of one TM dopant and one Pt vacancy (TM_Pt  +  V_Pt) which shows novel magnetism.

MnX (X = P, As) monolayers: a new type of two-dimensional intrinsic room temperature ferromagnetic half-metallic material with large magnetic anisotropy

Recent experimentally demonstrated intrinsic two-dimensional (2D) magnetism has sparked intense interest for advanced spintronic applications. However, the rather low Curie temperature and small magnetic anisotropic energy (MAE) greatly limit their application scope. Here, by using density functional theory calculations, we predict a series of stable 2D MnX (X = P, As, Sb) monolayers, among which MnP and MnAs monolayers exhibit intrinsic ferromagnetic (FM) ordering and considerably large MAEs of 166 and 281 μeV per Mn atom, respectively. More interestingly, the 2D MnP and MnAs monolayers exhibit highly desired half-metallicity with wide spin gaps of about 3 eV. Monte Carlo simulations suggest markedly high Curie temperatures of MnP and MnAs monolayers, ∼495 K and 711 K, respectively. Besides, these monolayers are the lowest energy structures in the 2D search space with excellent dynamic and thermal stabilities. A viable experimental synthesis route is also proposed to produce MnX monolayers via the selective chemical etching method. The outstanding attributes of MnP and MnAs monolayers would substantially broaden the applicability of 2D magnetism for a wide range of applications.

Structural phase transitions in VSe2: energetics, electronic structure and magnetism

First principles calculations of the magnetic and electronic properties of VSe₂ describing the transition between two structural phases (H,T) were performed.

Strain-tunable magnetic anisotropy in two-dimensional Dirac half-metals: nickel trihalides

Combining complete spin-polarization, high-speed conduction electrons, high T_C, robust ferromagnetic state and strain-tunable magnetic anisotropy in the monolayer NiX₃.

Cr2TiC2-based double MXenes: novel 2D bipolar antiferromagnetic semiconductor with gate-controllable spin orientation toward antiferromagnetic spintronics

Antiferromagnetic (AF) spin devices could be one of the representative components for applications of spintronics thanks to the numerous advantages such as resistance to magnetic field perturbation, stray field-free operation, and ultrahigh device operation speed. However, detecting and manipulating the spin of AF materials is still a major challenge due to the absence of a net magnetic moment and spin degeneracy in the band structure. Bipolar antiferromagnetic semiconductors are promising solutions to these problems. Herein, using density functional theory calculations, we present asymmetrical functionalized double MXenes (Cr2TiC2FCl) that behave as a novel bipolar antiferromagnetic semiconductor (BAFS) with vanishing magnetism, in which the valence band and conduction band around the Fermi level exhibit opposite spin directions. Remarkably, gate voltage can manipulate the spin orientation of the AF Cr2TiC2FCl and lead to a transition from BAFS to half-metal antiferromagnets (HMAF). Moreover, the mixed functionalized double MXenes with various F/Cl concentrations show the BAFS feature due to the different chemical environment for the Cr atom. Our results presented herein open a new strategy towards AF spintronics and the realization of the AF spin field effect transistor.