Toward Intrinsic Room-Temperature Ferromagnetism in Two-Dimensional Semiconductors

Defect effects on the electronic, valley, and magnetic properties of the two-dimensional ferrovalley material VSi2N4

Due to their novel spin and valley properties, two-dimensional (2D) ferrovalley materials are expected to be promising candidates for next-generation spintronic and valleytronic devices. However, they are subject to various defects in practical applications. Therefore, the electronic, valley, and magnetic properties may be modified in the presence of the defects. In this work, utilizing first-principles calculations, we systematically studied the effects of defects on the electronic, valley, and magnetic properties of the 2D ferrovalley material VSi2N4. It has been found that C doping, O doping, and N vacancies result in the half-metallic feature, Si vacancies result in the metallic feature, and V vacancies result in a bipolar gapless semiconductor. These defect-induced electronic properties can be effectively tuned by changing defect concentration and layer thickness. Since the impurity bands do not affect the K and K' valleys, valley polarization is well maintained in O-doped and N-defective systems. Importantly, these defects play a crucial role in modifying the magnetic properties of the pristine VSi2N4, especially the magnitude of local magnetic moments and the magnetic anisotropy energy. Detailed analysis of the density of states demonstrates that the variations of the total magnetic moment and magnetic anisotropy energy with biaxial strain are determined by the electronic states near the Fermi level rather than the type of defect, which provides a new understanding of the effects of defects on the magnetic properties of 2D materials. Moreover, the layer thickness can affect the magnetic coupling between defects and surrounding V atoms. Our results offer insight into the electronic, valley, and magnetic properties of VSi2N4 in the presence of various point defects.

Strain-induced ferroelectric polarization reversal without undergoing geometric inversion in blue SiSe monolayer

Ferroelectricity in two-dimensional (2D) systems generally arises from phonons and has been widely investigated. On the contrary, electronic ferroelectricity in 2D systems has been rarely studied. Using first-principles calculations, the ferroelectric behavior of the buckled blue SiSe monolayer under strain are explored. It is found that the direction of the out-of-plane ferroelectric polarization can be reversed by applying an in-plane strain. And such polarization switching is realized without undergoing geometric inversion. Besides, the strain-triggered polarization reversal emerges in both biaxial and uniaxial strain cases, indicating it is an intrinsic feature of such a system. Further analysis shows that the polarization switching is the result of the reversal of the magnitudes of the positive and negative charge center vectors. And the variation of buckling is found to play an important role, which results in the switch. Moreover, a non-monotonic variation of band gap with strain is revealed. Our findings throws light on the investigation of novel electronic ferroelectric systems.

Laser-Induced Ultrafast Spin Injection in All-Semiconductor Magnetic CrI3/WSe2 Heterobilayer

Spin injection stands out as a crucial method employed for initializing, manipulating, and measuring the spin states of electrons, which are fundamental to the creation of qubits in quantum computing. However, ensuring efficient spin injection while maintaining compatibility with standard semiconductor processing techniques is a significant challenge. Herein, we demonstrate the capability of inducing an ultrafast spin injection into a WSe2 layer from a magnetic CrI3 layer on a femtosecond time scale, achieved through real-time time-dependent density functional theory calculations upon a laser pulse. Following the peak of the magnetic moment in the CrI3 sublayer, the magnetic moment of the WSe2 layer reaches a maximum of 0.89 μB (per unit cell containing 4 WSe2 and 1 CrI3 units). During the spin dynamics, spin-polarized excited electrons transfer from the WSe2 layer to the CrI3 layer via type-II band alignment. The large spin splitting in conduction bands and the difference in the number of spin-polarized local unoccupied states available in the CrI3 layer lead to a net spin in the WSe2 layer. Furthermore, we confirmed that the number of available states, the spin-flip process, and the laser pulse parameters play important roles during the spin injection process. This work highlights the dynamic and rapid nature of spin manipulation in layered all-semiconductor systems, offering significant implications for the development and enhancement of quantum information processing technologies.

Prediction of intrinsic room-temperature ferromagnetism in two-dimensional CrInX2 (X = S, Se, Te) monolayers

Herein, using density functional theory, novel two-dimensional (2D) CrInX2 (X = S, Se, Te) structures are predicted to be practical ferromagnetic (FM) semiconductors. Phonon vibrations and molecular dynamics simulations verified their structural and thermodynamic stability. Sizable fully spin-polarized band gaps of 1.03 and 0.69 eV are found for CrInS2 and CrInSe2, while CrInTe2 exhibits half-metallic band nature (at 0 K with a perfect lattice). The high magnetic anisotropy energies are responsible for their long-range spin polarization. The Curie temperatures (Tc) are estimated to be 347, 397 and 447 K for CrInS2, CrInSe2 and CrInTe2, respectively, all well above the room-temperature. The high Tc originates from unusual FM direct exchange, the efficient super-exchange coupling between neighboring Cr eg-orbitals with zero virtual exchange gaps and the presence of dual Cr-X-Cr super-exchange channels. Our systematic study of the CrInX2 monolayer suggests that it could be a promising material for spintronics applications.

Two-Dimensional Asymmetric Multiferroics: Unique Way toward Strong Magnetoelectric Coupling and Multistate Memory

Two-dimensional (2D) materials have provided a fascinating platform for exploring novel multiferroics and emergent magnetoelectric coupling mechanisms. Here, a novel 2D asymmetric multiferroic based on Janus 2D multiferroic MXene-analogous oxynitrides (InTlNO2) is presented by using first-principles calculations. We find three inequivalent phases for InTlNO2, including two metallic phases (p1 and p2) and one semiconducting phase (p3) with a band gap of 0.88 eV. All phases are room-temperature multiferroics with different Curie temperatures, leading to tunability by phase transitions. We show that there is a 90° rotation of the magnetic anisotropy easy axis between p1 and p2, where p1 favors the in-plane and p2 the out-of-plane easy axis. Therefore, the magnetic anisotropy can be tuned by reversing the out-of-plane polarization. Our strategy provides a unique way toward strong magnetoelectric coupling and multistate memory.

High Curie temperature ferromagnetic monolayer T-CrSH and valley physics of T-CrSH/WS2 heterostructure

Two-dimensional magnetic materials are attracting widespread attention not only for their excellent applications in spintronic devices but also for their potential to regulate valley splitting, which is crucial for valleytronics. Herein, we design a monolayer Janus ferromagnetic semiconductor T-CrSH by using first-principles calculations. We reveal that monolayer T-CrSH has a magnetic moment of 3μB per unit cell, which is primarily contributed by the 3d orbitals of the Cr atom. Monte Carlo simulations suggest that the Curie temperature of T-CrSH is 193 K, and it can rise to 402 K when a 5% tensile strain is applied. Furthermore, the valley degeneracy of WS2 can be lifted when monolayer T-CrSH is used as a substrate. The obtained valley splitting in the conduction band is 13.7 meV and that in the valence band is 157.5 meV. In addition, the large valley polarization of 12.8 meV in the conduction band makes it easy to achieve an electron-doped valley Hall current and spin Hall current when performing in an in-plane electric field.

Toward intrinsic ultra-high-temperature ferromagnetism in a CrAuTe2/graphene heterosystem

Exploring intrinsic two-dimensional (2D) ferromagnetic (FM) materials with high Curie temperatures (TC) and large magnetic anisotropy energies (MAE) is one of the effective solutions to develop materials for high-performance spintronic applications. Using density functional theory calculations and high-throughput computations, we predict an intrinsic bimetallic FM monolayer, CrAuTe2, which has a large MAE and high TC. The results show that the value of the MAE can reach about 1.5 meV per Cr, and Monte Carlo simulations show that the TC of monolayer CrAuTe2 is about 840 K. Further analysis indicates that the joint effects of spin-orbit coupling (SOC) interaction and magnetic dipole-dipole interaction result in high in-plane magnetic anisotropy. In addition, this monolayer has good dynamic, thermal, and mechanical stabilities, which were confirmed by ab initio molecular dynamics simulations, phonon spectra, and elastic constants, respectively. In order to propose a practical synthesis approach, we built a CrAuTe2/graphene van der Waals heterostructure, and found that the heterostructure does not affect the magnetic properties of monolayer CrAuTe2. These findings appear promising for the future applications in nano-spintronics.

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

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

The Curie temperature: a key playmaker in self-regulated temperature hyperthermia

The Curie temperature is an important thermo-characteristic of magnetic materials, which causes a phase transition from ferromagnetic to paramagnetic by changing the spontaneous re-arrangement of their spins (intrinsic magnetic mechanism) due to an increase in temperature. The self-control-temperature (SCT) leads to the conversion of ferro/ferrimagnetic materials to paramagnetic materials, which can extend the temperature-based applications of these materials from industrial nanotechnology to the biomedical field. In this case, magnetic induction hyperthermia (MIH) with self-control-temperature has been proposed as a physical thermo-therapeutic method for killing cancer tumors in a biologically safe environment. Specifically, the thermal source of MIH is magnetic nanoparticles (MNPs), and thus their biocompatibility and Curie temperature are two important properties, where the former is required for their clinical application, while the latter acts as a switch to automatically control the temperature of MIH. In this review, we focus on the Curie temperature of magnetic materials and provide a complete overview beginning with basic magnetism and its inevitable relation with Curie's law, theoretical prediction and experimental measurement of the Curie temperature. Furthermore, we discuss the significance, evolution from different types of alloys to ferrites and impact of the shape, size, and concentration of particles on the Curie temperature considering the proposed SCT-based MIH together with their biocompatibility. Also, we highlight the thermal efficiency of MNPs in destroying tumor cells and the significance of a low Curie temperature. Finally, the challenges, concluding remarks, and future perspectives in promoting self-control-temperature based MIH to clinical application are discussed.

Hole-Doping-Induced Perpendicular Magnetic Anisotropy and High Curie Temperature in a CrSX (X = Cl, Br, I) Semiconductor Monolayer

A large perpendicular magnetic anisotropy and a high Curie temperature (TC) are crucial for the application of two-dimensional (2D) intrinsic ferromagnets to spintronic devices. Here, we investigated the electronic and magnetic properties of carrier-doped Van der Waals layered CrSX (X = Cl, Br, I) ferromagnets using first-principles calculations. It was found that hole doping can increase the magnitude of the magnetic anisotropy energy (MAE) and change the orientation of the easy magnetization axis at small doping amounts of 2.37 × 1013, 3.98 × 1012, and 3.33 × 1012/cm2 for CrSCl, CrSBr, and CrSI monolayers, respectively. The maximum values of the MAE reach 57, 133, and 1597 μeV/u.c. for the critical hole-doped CrSCl, CrSBr, and CrSI with spin orientation along the (001) direction, respectively. Furthermore, the Fermi energy level of lightly hole-doped CrSX (X = Cl, Br, I) moves into the spin-up valence band, leading to the CrSX (X = Cl, Br, I) magnetic semiconductor monolayer becoming first a half-metal and then a metal. In addition, the TC can also be increased up to 305, 317, and 345 K for CrSCl, CrSBr, and CrSI monolayers at doping amounts of 5.94 × 1014, 5.78 × 1014, and 5.55 × 1014/cm2, respectively. These properties suggest that the hole-doping process can render 2D CrSX (X = Cl, Br, I) monolayers remarkable materials for application to electrically controlled spintronic devices.

Strong Room-Temperature Ferromagnetism in Ultrathin NiOOH Nanosheets through Surfactant Manipulation

Two-dimensional (2D) ferromagnetic (FM) materials with nanoscale thickness and spontaneous net magnetization have emerged as a promising class of functional materials for applications in next-generation spintronics, quantum processing, and data storage devices. However, most 2D materials exhibit weak FM even at low temperatures, limiting their potential applications in many technological fields. The fabrication of strong room-temperature FM 2D materials is highly desirable for the development of practical applications. Here, we demonstrate an ionic layer epitaxy strategy to synthesize few-layered NiOOH nanosheets with strong room-temperature FM and a saturation magnetization up to 409.86 emu cm-3 at 300 K. The results are consistent with the ab initio predictions of a stable FM NiOOH nanolayer structure with an FM configuration. The FM strength of the NiOOH nanosheets can be tuned by controlling the surfactant monolayer density and annealing. This work offers a promising strategy for achieving strong high-temperature FM in 2D materials for spintronic applications.

Two-dimensional Janus SVAN2 (A = Si, Ge) monolayers with intrinsic semiconductor character and room temperature ferromagnetism: tunable electronic properties via strain and an electric field

In the context of developing next-generation information technology, two-dimensional materials with inherent ferromagnetism, a Curie temperature above room temperature, and significant magnetic anisotropy hold great promise. In this work, we employed first-principles calculations to investigate a novel two-dimensional Janus structure, namely SVAN2 (A = Si, Ge). Our findings reveal that these structures are not only dynamically and thermally stable, but also exhibit semiconductor properties alongside their ferromagnetic states. The Janus SVSiN2 monolayer exhibits an in-plane easy axis, while the SVGeN2 monolayer shows an out-of-plane easy axis, both characterized by a significant magnetic anisotropy energy (129 and 172 μeV, respectively). Notably, through Monte Carlo simulation, we found that the Curie temperature of the SVSiN2 monolayer is 330 K, which is higher than room temperature. Finally, by applying biaxial strain and an external electric field, we successfully regulated the electronic properties of the SVAN2 (A = Si, Ge) monolayers, enabling a transition from semiconductor to half-metallic behavior. These remarkable electronic and magnetic properties make the Janus SVAN2 (A = Si, Ge) monolayers promising candidate materials for spin electron applications.

A novel two-dimensional NiCl2O8 lattice with negative Poisson's ratio and magnetic modulation

Two-dimensional (2D) materials with simultaneous magnetic semiconducting properties and a negative Poisson's ratio are crucial for fabricating multifunctional electronic devices. However, progress in this area has been generally constrained. Based on first-principles calculations, we engineered a 2D Ni-based oxyhalide with a honeycomb lattice structure. It was observed that the NiCl2O8 monolayer exhibits both high- and low-buckling states in its geometry, along with intrinsic magnetic semiconductor properties in its electronic structure. Importantly, we demonstrated that the magnetic ordering of the NiCl2O8 lattice is susceptible to applied strain, which resulted in a phase transition from paramagnetic to ferromagnetic under biaxial strain. The Curie temperature was also evaluated using Monte Carlo simulations within the Ising model. Additionally, our research uncovered that the 2D NiCl2O8 lattice chain displays a negative Poisson's ratio (NPR) along the z-direction. The triangular hinge structure in its centrosymmetric configuration was identified as the origin of this unique phenomenon. The coexistence of NPR and magnetic phase transition properties in the NiCl2O8 lattice makes it quite promising for applications in nanoelectronic and spintronic devices.

A Janus CrSSe monolayer with interesting ferromagnetism

The search for intrinsic half-metallic ferromagnetic (FM) monolayers with a high Curie temperature (TC), considerable magnetic anisotropy energy (MAE), and multiferroic coupling is key for the development of ultra-compact spintronics. Here, we have identified a new stable FM Janus monolayer, the tetrahedral CrSSe, through first-principles structural search calculations, which not only exhibits very interesting magnetoelectric properties with a high TC of 790 K, a large MAE of 0.622 meV per Cr, and robust half-metallicity, but also shows obvious ferroelasticity with a modest energy barrier of 0.31 eV per atom. Additionally, there appears to be interesting multiferroic coupling between in-plane magnetization and ferroelasticity. Furthermore, by replacing the Se/S atoms in the CrSSe monolayer with S/Se atoms, we obtained two new half-metallic FM CrS2 and CrSe2 monolayers, which also exhibit excellent magnetoelectric properties. Therefore, our findings provide a pathway to design novel multiferroic materials and enrich the understanding of 2D transition metal chalcogenides.

Proximity-Induced Interfacial Room-Temperature Ferromagnetism in Semiconducting Fe3GeTe2

The discoveries of two-dimensional ferromagnetism and magnetic semiconductors highly enrich the magnetic material family for constructing spin-based electronic devices, but with an acknowledged challenge that the Curie temperature (Tc) is usually far below room temperature. Many efforts such as voltage control and magnetic ion doping are currently underway to enhance the functional temperature, in which the involvement of additional electrodes or extra magnetic ions limits their application in practical devices. Here we demonstrate that the magnetic proximity, a robust effect but with elusive mechanisms, can induce room-temperature ferromagnetism at the interface between sputtered Pt and semiconducting Fe3GeTe2, both of which do not show ferromagnetism at 300 K. The independent electrical and magnetization measurements, structure analysis, and control samples with Ta highlighting the role of Pt confirm that the ferromagnetism with the Tc of above 400 K arises from the Fe3GeTe2/Pt interfaces, rather than Fe aggregation or other artificial effects. Moreover, contrary to conventional ferromagnet/Pt structures, the spin current generated by the Pt layer is enhanced more than two times at the Fe3GeTe2/Pt interfaces, indicating the potential applications of the unique proximity effect in building highly efficient spintronic devices. These results may pave a new avenue to create room-temperature functional spin devices based on low-Tc materials and provide clear evidence of magnetic proximity effects by using nonferromagnetic materials.

Intrinsic half-metallicity in two-dimensional Cr2TeX2 (X = I, Br, Cl) monolayers

Two-dimensional (2D) materials with intrinsic half-metallicity at or above room temperature are important in spin nanodevices. Nevertheless, such 2D materials in experiment are still rarely realized. In this work, a new family of 2D Cr2TeX2 (X = I, Br, Cl) monolayers has been predicted using first-principles calculations. The monolayer is made of five atomic sublayers with ABCAB-type stacking along the perpendicular direction. It is found that the energies for all the ferromagnetic (FM) half-metallic states are the lowest. The phonon spectrum calculations and molecular dynamics simulations both demonstrate that the FM states are stable, indicating the possibility of experimentally obtaining the 2D Cr2TeX2 monolayers with half-metallicity. The Curie temperatures from Monte Carlo simulations are 486, 445, and 451 K for Cr2TeI2, Cr2TeBr2, and Cr2TeCl2 monolayers, respectively, and their half-metallic bandgaps are 1.72, 1.86 and 1.90 eV. The corresponding magnetocrystalline anisotropy energies (MAEs) are about 1185, 502, 899 μeV per Cr atom for Cr2TeX2 monolayers, in which the easy axes are along the plane for the Cr2TeBr2 and Cr2TeCl2 monolayers, but being out of the plane in the Cr2TeI2. Our study implies the potential application of the 2D Cr2TeX2 (X = I, Br, Cl) monolayers in spin nanodevices.

Intrinsic Ferromagnetic Semiconductors with High Saturation Magnetization from Hybrid Perovskites

Ferromagnetic semiconductors (FMS) enable simultaneous control of both charge and spin transport of charge carriers, and they have emerged as a class of highly desirable but rare materials for applications in spin field-effect transistors and quantum computing. Organic-inorganic hybrid perovskites with high compositional adjustability and structural versatility can offer unique benefits in the design of FMS but has not been fully explored. Here, a series of molecular FMSs based on the 2D organic-inorganic hybrid perovskite structure, namely (2ampy)CuCl4 , (3ampy)CuCl4 , and (4ampy)CuCl4 , is demonstrated, which exhibits high saturation magnetization, dramatic temperature-dependent conductivity change, and tunable ferromagnetic resonance. Magnetic measurements reveal a high saturation magnetization up to 18.56 emu g-1 for (4ampy)CuCl4 , which is one of the highest value among reported hybrid FMSs to date. Conductivity studies of the three FMSs demonstrate that the smaller adjacent octahedron distance in the 2D layer results in higher conductivity. Systematic ferromagnetic resonance investigation shows that the gyromagnetic ratio and Landau factor values are strongly dependent on the types of organic cations used. This work demonstrates that 2D hybrid perovskite materials can simultaneously possess both tunable long-range ferromagnetic ordering and semiconductivity, providing a straightforward strategy for designing and synthesizing high-performance intrinsic FMSs.

Non-volatile control of topological phase transition in an asymmetric ferroelectric In2Te2S monolayer

The coupling of topological electronic states and ferroelectricity is highly desired due to their abundant physical phenomenon and potential applications in multifunctional devices. However, it is difficult to achieve such a phenomenon in a single ferroelectric (FE) monolayer because the two polarized states are topologically equivalent. Here, we demonstrate that the symmetry of polarized states can be broken by constructing a Janus structure in a FE monolayer. We illustrate such a general idea by replacing a layer of Te atoms in the In2Te3 monolayer with S atoms. Using first-principles calculations, we show that the In2Te2S monolayer has two asymmetric polarized states, which are characterized by a metal and semiconductor, respectively. Importantly, as the spin-orbit coupling is included, a band gap (50.4 meV) is created in the metallic state, resulting in a non-trivial topological phase. Thus, it proves to be a feasible method to engineer non-volatile FE control of topological order in a single-phase system. We also demonstrate the underlying physical mechanism of topological phase transition, which is unveiled to be related to the weakened intrinsic electric field resulting from charge transfer. These interesting results provide a general way to design asymmetric FE materials and shed light on their potential application in non-volatile multifunctional devices.

Recent advances in 2D van der Waals magnets: Detection, modulation, and applications

The emergence of two-dimensional (2D) van der Waals magnets provides an exciting platform for exploring magnetism in the monolayer limit. Exotic quantum phenomena and significant potential for spintronic applications are demonstrated in 2D magnetic crystals and heterostructures, which offer unprecedented possibilities in advanced formation technology with low power and high efficiency. In this review, we summarize recent advances in 2D van der Waals magnetic crystals. We focus mainly on van der Waals materials of truly 2D nature with intrinsic magnetism. The detection methods of 2D magnetic materials are first introduced in detail. Subsequently, the effective strategies to modulate the magnetic behavior of 2D magnets (e.g., Curie temperature, magnetic anisotropy, magnetic exchange interaction) are presented. Then, we list the applications of 2D magnets in the spintronic devices. We also highlight current challenges and broad space for the development of 2D magnets in further studies.

Origin and regulation of triaxial magnetic anisotropy in the ferromagnetic semiconductor CrSBr monolayer

Magnetic anisotropy plays a vital role in stabilizing the long-range magnetic order of two-dimensional ferromagnetic systems. In this work, using the first-principles method, we systematically explored the triaxial magnetic anisotropic properties of a ferromagnetic semiconductor CrSBr monolayer, which is recently exfoliated from its bulk. Further analysis shows that the triaxial magnetic anisotropic properties originate from the coexistence of the magnetic dipole-dipole interaction (shape anisotropy) and the spin-orbit coupling interaction (magnetocrystalline anisotropy). Interestingly, the shape anisotropy, which has been neglected in most previous works, dominates over the magnetocrystalline anisotropy. Besides, the experimental Curie temperature of the CrSBr monolayer is well reproduced using Monte Carlo simulations. What is more, the easy magnetic axes and ferromagnetism in the CrSBr monolayer can be manipulated by strains and are relatively more susceptible to the uniaxial strain in the x direction. Our study not only explains the mechanism of triaxial magnetic anisotropy of the CrSBr monolayer, but also sheds light on how to tune the magnetic anisotropy and Curie temperature in ferromagnetic monolayers.

Spin-splitting and switchable half-metallicity in a van der Waals multiferroic CuBiP2Se6/GdClBr heterojunction

Multiferroic van der Waals (vdW) heterojunctions have a strong and nonvolatile magnetoelectric coupling effect, which is of great significance in spintronic devices. The electronic structure and magnetic properties of a GdClBr/CuBiP₂Se₆ vdW multiferroic heterojunction have been calculated using first-principles methods. Due to the spin-up charge transfer and Zeeman field, the ferroelectric CuBiP₂Se₆ exhibits spin splitting at the gamma point. It is found that the electronic structure and magnetic properties of the GdClBr/CuBiP₂Se₆ vdW multiferroic heterojunction have been significantly modulated by the electric polarization of CuBiP₂Se₆. During the reversal of the ferroelectric polarization of CuBiP₂Se₆, the ferromagnetic GdClBr monolayer transforms from a semiconductor to a half-metal. Meanwhile, in both upward and downward ferroelectric polarization, the GdClBr/CuBiP₂Se₆ heterojunction exhibits perpendicular magnetic anisotropy with a Curie temperature of 239 K. As the strain changes from -6% to 6%, the band structure of GdClBr shifts upward, and the band structure of CuBiP₂Se₆ shifts downward. Compressive strain can increase the Curie temperature of the GdClBr/CuBiP₂Se₆ heterojunction. The magnetic anisotropy of heterojunctions highly depends on biaxial strain, where the perpendicular (in-plane) magnetic anisotropy increases with the increased compressive (tensile) strain. The vdW multiferroic GdClBr/CuBiP₂Se₆ heterojunction has potential applications in spintronic devices.

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.

Room-temperature half-metals induced via chemical surface modification: 2D Mn2Se2 monolayer

Compared with various antiferromagnetic (AFM) materials, two-dimensional (2D) room-temperature ferromagnetic (FM) materials are rarely discovered because of the geometrically determined spin interactions. Since 2D FM materials have shown great potential in the next-generational information devices, it is quite important to design new FM materials based on the reported AFM materials. Here, in this study, we found that the Mn₂Se₂ monolayer can be converted to half-metal from AFM semiconductor at room temperature by edge modification of certain chemical groups (such as -Cl, -Br, -I, and -S) based on systematical first-principles calculations. Our results show that the adsorbed chemical groups significantly modify the electronic states of Mn ions and the resulting spin interactions. Moreover, our results indicate that the Curie temperatures (T_c) of some Mn₂Se₂ monolayer derivatives approach or even exceed room temperature, among which Curie temperatures after chemical modification by -Cl, -Br, -I, -S are 290 K, 320 K, 400 K, and 1050 K, respectively. Thus, chemical modifications can be one of the effective methods to construct 2D FM materials in experiments.

Transmutation Engineering Makes a Large Class of Stable and Exfoliable A3BX2 Compounds with Exceptional High Magnetic Critical Temperatures and Exotic Electronic Properties

We establish a robust protocol for materials innovation based on our proposed transmutation engineering strategy combined with combinatorial chemistry and hierarchical high-throughput screening to make a large class of layered 2D A₃BX₂ materials. After several rounds of efficient screening, 60 types of easily exfoliable and highly stable A₃BX₂ monolayers have been obtained. Excitingly, four representative monolayers (ferromagnetic Fe₃SiS₂ and Fe₃GeS₂, antiferromagnetic Mn₃PbTe₂ and Co₃GeSe₂) demonstrate quite high magnetic critical temperatures of 600 (T_C), 630 (T_C), 770 (T_N), and 510 K (T_N), respectively. Through electronic fingerprint identification, the magnetic exchange mechanism is fundamentally unveiled at the atomic level in combination with a local chemical topology environment and crystal/exchange field. Furthermore, two simple and effective unified descriptors are proposed to perfectly explain the origin of magnetic strain regulation. Some intriguing materials (featuring double Dirac cones, node-loops, and ultrahigh Fermi velocities) are expected to be used in high-speed and low-dissipation nanodevices. This material family forms a dataset, which establishes a platform to discover and explore unexpected physicochemcial properties and develop promising applications under different circumstances. The chemical trends of diverse properties for this class of materials are revealed, which offers guiding insights for the development of spintronics and nanoelectronics with the target of exploiting both spin and charge degrees of freedom directed functional materials design and screening.

Sliding Phase Transition in Ferroelectric van der Waals Bilayers

We address the sliding thermodynamics of van der Waals-bonded bilayers by continuum electromechanics. We attribute the robustness of the ferroelectricity recently observed in h-BN and WTe_{2} bilayers to large in-plane stiffness of the monolayers. We compute the electric susceptibility and specific heat in a mean-field self-consistent phonon approximation. We compare critical temperatures and electric switching fields with the observed values.

Strain-tunable skyrmions in two-dimensional monolayer Janus magnets

The Dzyaloshinskii-Moriya interaction (DMI), which only exists in noncentrosymmetric systems, plays an important role in the formation of exotic chiral magnetic states. However, the absence of the DMI occurs in most two-dimensional (2D) magnetic materials due to their intrinsic inversion symmetry. Here, by using first-principles calculations, we demonstrate that a significant DMI can be obtained in a series of Janus monolayers of dichalcogenides XSeTe (X = Nb, Re) in which the difference between Se and Te on the opposite sides of X breaks the inversion symmetry. Remarkably, the DMI amplitudes of NbSeTe (1.78 meV) and ReSeTe (4.82 meV) are larger than the experimental value of Co/graphene (0.16 meV), and NbSeTe and ReSeTe monolayers have a high Curie temperature of 1023 K and 689 K, respectively. Through the micromagnetic simulation of XSeTe (X= Nb, Re) simulations, we also find that the ReSeTe monolayer can performance for skyrmion states by applying an external magnetic field, and importantly, the skyrmion states can be regulated and controlled under external strain. The findings pave the way for device concepts using chiral magnetic structures in specially designed 2D ferromagnetic materials.

Even-Odd-Layer-Dependent Ferromagnetism in 2D Non-van-der-Waals CrCuSe2

Van der Waals (vdW) layered materials with strong magnetocrystalline anisotropy have attracted significant interest as the long-range magnetic order in these systems can survive even when their thicknesses is reduced to the 2D limit. Even though the interlayer coupling between the neighboring magnetic layers is very weak, it has a determining effect on the magnetism of these atomic-thickness materials. Herein, a new 2D ferromagnetic material, namely, non-vdW CuCrSe₂ nanosheets with even-odd-layer-dependent ferromagnetism when laminated from an antiferromagnetic bulk is reported. Monolayer and even-layer CuCrSe₂ exhibit the anomalous Hall effect and a significantly enhanced magnetic ordering temperature of more than 125 K. In contrast, the linear Hall effect exists in the odd-layer samples. Theoretical calculations indicate that the layer-dependent magnetic coupling is attributable to the orbital shift of the Cr atoms in the CrSe₂ layers owing to the Cu-induced breaking of the centrosymmetry. Thus, this work sheds light on the exotic magnetic properties of layered materials that exhibit phenomena beyond weak interlayer interactions.

Enhanced Curie temperature in partially decorated CrSnSe3 monolayer with alkali metals (Li, Na, and K)

A two-dimensional ferromagnetic layer with a large Curie temperature is highly desired for spintronics applications. Herein, we investigated the effect of the partial decoration of the CrSnSe₃ monolayer with alkali metals Li, Na, and K on the structure, electronic and magnetic properties. The calculated formation energy, phonon dispersion curves, and ab initio molecular dynamics indicated that the decorated CrSnSe₃ layers are stable and can be fabricated. The Li, Na, and K decorated systems display semiconducting band features, with bandgaps of 0.53, 0.55, and 0.55 eV, respectively, with the HSE06 hybrid functional. We found a ferromagnetic ground state and an in-plane magnetic anisotropy of -2.12, -2.42, and -2.39 meV per cell in the Li, Na, and K-decorated systems, respectively. Based on Monte Carlo Simulations, we obtained largely enhanced Curie temperatures of 241, 256, and 265 K in the Li, Na, and K decorated systems, respectively. Our findings suggest that the decorated layers could be used as potential candidates for spintronics applications.

Vibrationally Induced Magnetism in Supramolecular Aggregates

Magnetic phenomena in chemistry and condensed matter physics are considered to be associated with low temperatures. That a magnetic state or order is stable below a critical temperature as well as becoming stronger the lower the temperature is a nearly unquestioned paradigm. It is, therefore, surprising that recent experimental observations made on supramolecular aggregates suggest that, for instance, the magnetic coercivity may increase with an increasing temperature and the chiral-induced spin selectivity effect may be enhanced. Here, a mechanism for vibrationally stabilized magnetism is proposed, and a theoretical model is introduced with which the qualitative aspects of the recent experimental findings can be explained. It is argued that anharmonic vibrations, which become increasingly occupied with an increasing temperature, enable nuclear vibrations to both stabilize and sustain magnetic states. The theoretical proposal, hence, pertains to structures without inversion and/or reflection symmetries, for instance, chiral molecules and crystals.

Enabling Room-Temperature Triferroic Coupling in Dual Transition-Metal Dichalcogenide Monolayers Via Electronic Asymmetry

Triferroic compounds are the ideal platform for multistate information devices but are rare in the two-dimensional (2D) form, and none of them can maintain macroscopic order at room temperature. Herein, we propose a general strategy for achieving 2D triferroicity by imposing electric polarization into a ferroelastic magnet. Accordingly, dual transition-metal dichalcogenides, for example, 1T'-CrCoS₄, are demonstrated to display room-temperature triferroicity. The magnetic order of 1T'-CrCoS₄ undergoes a magnetic transition during the ferroic switching, indicating robust triferroic magnetoelectric coupling. In addition, the negative out-of-plane piezoelectricity and strain-tunable magnetic anisotropy make the 1T'-CrCoS₄ monolayer a strong candidate for practical applications. Following the proposed scheme, a new class of 2D room-temperature triferroic materials is introduced, providing a promising platform for advanced spintronics.

Two-dimensional chalcogenide-based ferromagnetic semiconductors

Two-dimensional (2D) magnetic materials draw an enormous amount of attention due to their novel physical properties and potential spintronics device applications. Room-temperature ferromagnetic (FM) semiconductors have long been pursued in 2D magnetic materials, which show a long range magnetic order down to atomic-layer thickness. The intrinsic ferromagnetism has been predicted in a series of 2D materials and verified in experiments and the magnetism can be modulated by multiple physical fields, exhibiting promising application prospects. In this review, we overview several types of 2D chalcogenide-based FM semiconductors discovered in recent years. We summary and compare their basic physical properties, including the crystal structures, electronic structures, and mechanical stability. The 2D magnetism can be described by several physical models. We also focus on the recent progresses about theoretical prediction of FM semiconductors and experimental observation of external-field regulation. Most of investigations have shown that 2D chalcogenide-based FM semiconductors have relatively high Curie temperature (Tc) and structural stability. These materials are promising to realize the room-temperature ferromagnetism in atomic-layer thickness, which is significant to design spintronics devices.

Designing a ferrimagnetic-ferroelastic multiferroic semiconductor in FeMoClO4 nanosheets via element substitution

Exploring two-dimensional multiferroic semiconductors, combined with ferro-/ferrimagnetism and ferroelasticity as well as large spin polarization around the valence band maximum (VBM) and conduction band minimum (CBM), is highly desirable but remains a challenging task. Here, via first-principles calculations, we predict such a material based on the square phase FeMoClO₄ nanosheet, which is experimentally accessible by exfoliating its layered bulk. Pristine FeMoClO₄ nanosheets are a weak antiferromagnet with zero spin polarization. After substituting nonmagnetic Mo with magnetic Mn, the resulting FeMnClO₄ nanosheet becomes ferrimagnetic with magnetic ordering temperature significantly enhanced from 14 to 127 K. Besides, the FeMnClO₄ nanosheet is a half semiconductor with its VBM and CBM 100% spin-polarized in the same spin direction. Interestingly, the initial square lattice is distorted into a rectangular one, inducing an in-plane ferroelasticity in the FeMnClO₄ nanosheet with a switching barrier of 27 meV per atom. Moreover, under ferroelastic transition, the orientation of the magnetic easy axis can be reversibly rotated by 90°, indicating a strong magnetoelastic coupling.

Transition metal dichalcogenide magnetic atomic chains

Reducing the dimensions of a material to the atomic scale endows them with novel properties that are significantly different from their bulk counterparts. A family of stoichiometric transition metal dichalcogenide (TMD) MX₂ (M = Ti to Mn, and X = S to Te) atomic chains is proposed. The results reveal that the MX₂ atomic chains, the smallest possible nanostructure of a TMD, are lattice-dynamically stable, as confirmed from their phonon spectra and ab initio molecular dynamics simulations. In contrast to their bulk and two-dimensional (2D) counterparts, the TiX₂ atomic chains are nonmagnetic semiconductors, while the VX₂, CrX₂, and MnX₂ chains are unipolar magnetic, bipolar magnetic, and antiferromagnetic semiconductors, respectively. In addition, the VX₂, CrX₂, and MnX₂ chains can be converted via carrier doping from magnetic semiconductors to half metals with reversible spin-polarization orientation at the Fermi level. Of these chains, the MnX₂ chains exhibit either ferromagnetic or antiferromagnetic half metallicity depending on the injected carrier type and concentration. The diverse and tunable electronic and magnetic properties in the MX₂ chains originate, based on crystal field theory, from the occupation of the metal d orbitals and the exchange interaction between the tetrahedrally coordinated metal atoms in the atomic chain. The calculated interaction between the carbon nanotubes and the MX₂ chains implies that armchair (7,7) or armchair (8,8) carbon nanotubes are appropriate sheaths for growing MX₂ atomic single-chains in a confined channel. This study reveals the diverse magnetic properties of MX₂ atomic single-chains and provides a promising building block for nanoscale electronic and spintronic devices.

Electronic structure, magnetoresistance and spin filtering in graphene|2 monolayer-CrI33|graphene van der Waals magnetic tunnel junctions

In the pursuit of designing van der Waals magnetic tunneling junctions (vdW-MTJs) with two-dimensional (2D) intrinsic magnets, as well as to quantitatively reveal the microscopic nature governing the vertical tunneling pathways beyond the phenomenological descriptions on CrI₃-based vdW-MTJs, we investigate the structural configuration, electronic structure and spin-polarized quantum transport of graphene|2 monolayer(2ML)-CrI₃|graphene heterostructure with Ag(111) layers as the electrode, using density functional theory (DFT) and its combination of non-equilibrium Green's function (DFT-NEGF) methods. The in-plane lattice of CrI₃ layers is found to be stretched when placed on the graphene (Gr) layer, and the layer-stacking does not show any site selectivity. The charge transfer between CrI₃ and Gr layers make the CrI₃ layer lightly electron-doped, and the Gr layer hole-doped. Excitingly, the inter-layer hybridization between graphene and CrI₃ layers render the CrI₃ layer metallic in the majority spin channel, giving rise to an insulator-to-half-metal transition. Due to the metallic/insulator characteristics of the spin-majority/minority channel of the 2ML-CrI₃ barrier in vdW-MTJs, Gr|2ML-CrI₃|Gr heterostructures exhibit an almost perfect spin filtering effect (SFE) near the zero bias in parallel magnetization, a giant tunneling magnetoresistance (TMR) ratio up to 2 × 10⁴%, and remarkable negative differential resistance (NDR). Our results not only give an explanation for the observed giant TMR in CrI₃-based MTJs but also show the direct implications of 2D magnets in vdW-heterostructures.

A generic dual d-band model for interlayer ferromagnetic coupling in a transition-metal doped MnBi2Te4 family of materials

Realization of ferromagnetic (FM) interlayer coupling in magnetic topological insulators (TIs) of the MnBi₂Te₄ family of materials (MBTs) may pave the way for realizing the high-temperature quantum anomalous Hall effect (high-T QAHE). Here we propose a generic dual d-band (DDB) model to elucidate the energy difference (ΔE = E_AFM - E_FM) between the AFM and FM coupling in transition-metal (TM)-doped MBTs, where the valence of TMs splits into d-t_2g and d-e_g sub-bands. Remarkably, the DDB shows that ΔE is universally determined by the relative position of the dopant (X) and Mn d-e_g/t_2g bands, . If ΔE_d > 0, then ΔE > 0 and the desired FM coupling is favored. This surprisingly simple rule is confirmed by first-principles calculations of hole-type 3d and 4d TM dopants. Significantly, by applying the DDB model, we predict the high-T QAHE in the V-doped Mn₂Bi₂Te₅, where the Curie temperature is enhanced by doubling of the MnTe layer, while the topological order mitigated by doping can be restored by strain.

Control of structure and spin texture in the van der Waals layered magnet CrSBr

Controlling magnetism at nanometer length scales is essential for realizing high-performance spintronic, magneto-electric and topological devices and creating on-demand spin Hamiltonians probing fundamental concepts in physics. Van der Waals (vdW)-bonded layered magnets offer exceptional opportunities for such spin texture engineering. Here, we demonstrate nanoscale structural control in the layered magnet CrSBr with the potential to create spin patterns without the environmental sensitivity that has hindered such manipulations in other vdW magnets. We drive a local phase transformation using an electron beam that moves atoms and exchanges bond directions, effectively creating regions that have vertical vdW layers embedded within the initial horizontally vdW bonded exfoliated flakes. We calculate that the newly formed two-dimensional structure is ferromagnetically ordered in-plane with an energy gap in the visible spectrum, and weak antiferromagnetism between the planes, suggesting possibilities for creating spin textures and quantum magnetic phases.

Tunable magnetocrystalline anisotropy of two-dimensional Fe3GeTe2 with adsorbed 5d-transition metal

The demand for ultra-compact spintronic devices with lower energy consumption and higher storage density requires two-dimensional (2D) magnetic materials with tunable magnetocrystalline anisotropy (MCA) energy. Employing first-principles calculations, we have investigated the influence of W atom adsorption and biaxial strain on the magnetic properties of layered Fe₃GeTe₂. We demonstrate that the adsorption mode and applied strain play a critical role in determining their MCA. The Fe₃GeTe₂ adsorbed with W atoms undergoes a change in spin reorientation from out-of-plane to in-plane magnetization, yielding a colossal MCA up to -13.112 erg cm^-2. The dominant contribution to these unexpected changes mainly arises from the W atoms with emerged magnetism and large SOC. Moreover, our results reveal distinct strain-driven modulation behaviors of the MCA in different adsorption configurations. The underlying atomistic mechanism mainly involves the alteration of various W-derived 5d-orbital states under the strain effect, leading to competitive changes of the corresponding spin-orbit coupling energies between the spin-parallel and spin-flip channels. Our findings not only provide useful guidance in optimizing the MCA performance of 2D magnetic crystals but also highlight the potential of W-adsorbed Fe₃GeTe₂ in the applications of new-generation magnetic memory storage devices.

Intrinsic ferromagnetism and the quantum anomalous Hall effect in two-dimensional MnOCl2 monolayers

Due to their potential application in spintronic devices, two-dimensional (2D) ferromagnetic materials are highly desired. We used first-principles calculations and Monte Carlo simulations to investigate the electronic structure and magnetic characteristics of the MnOCl₂ monolayers. We discovered two stable monolayer structures, Pmna-MnOCl₂ and Pmmn-MnOCl₂. Our findings show that the Pmna-MnOCl₂ monolayer is an intrinsic ferromagnetic semiconductor with an indirect band gap of 0.152 eV and a Curie temperature (T_C) of 202 K, while the Pmmn-MnOCl₂ monolayer is an intrinsic ferromagnetic Dirac semimetal with a high T_C (910 K) and triaxial magnetic anisotropy. We also show that a Pmmn-MnOCl₂ monolayer with a nontrivial band gap of 6.2 meV can achieve the quantum anomalous Hall effect (QAHE) with Chern number C = 1. Additionally, the existence of a gapless edge state can be flexibly regulated by choosing the terminal edges. Our studies reveal that the Pmmn-MnOCl₂ monolayer can serve as a candidate material to achieve high-temperature QAHE.

Strain-tunable magnetic and electronic properties of a CuCl3 monolayer

Recently, theoretical search has found that a two-dimensional CuCl₃ monolayer is a ferromagnetic semiconductor. Here, we apply density functional theory to study its geometrical structure, magnetic and electronic properties under the influence of a biaxial strain ε. It is found that the CuCl₃ monolayer exhibits ferromagnetic ordering at the ground state with ε = 0 and its Curie temperature increases monotonously with respect to the biaxial strain, which can be increased to about 100 K at 10% tensile strain. When a compressive strain of about 6.8% is applied, a transition from the ferromagnetic to the antiferromagnetic state occurs. In addition to the transition of the magnetic ground state, the electronic band gaps of spin-up and spin-down electrons undergo direct-indirect and indirect-direct-indirect transitions at the tensile strains, respectively. The tunable magnetic and electronic properties investigated in this work are helpful in understanding the magnetism in the CuCl₃ monolayer, which is useful for the design of spintronic devices based on ferromagnetic semiconductors.

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.

The Magnetic Genome of Two-Dimensional van der Waals Materials

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

Atomic Intercalation Induced Spin-Flip Transition in Bilayer CrI3

The recent discovery of 2D magnets has induced various intriguing phenomena due to the modulated spin polarization by other degrees of freedoms such as phonons, interlayer stacking, and doping. The mechanism of the modulated spin-polarization, however, is not clear. In this work, we demonstrate theoretically and computationally that interlayer magnetic coupling of the CrI3 bilayer can be well controlled by intercalation and carrier doping. Interlayer atomic intercalation and carrier doping have been proven to induce an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in the spin-polarization of the CrI3 bilayer. Our results revealed that the AFM to FM transition induced by atom intercalation was a result of enhanced superexchange interaction between Cr atoms of neighboring layers. FM coupling induced by O intercalation mainly originates from the improved superexchange interaction mediated by Cr 3d-O 2p coupling. FM coupling induced by Li intercalation was found to be much stronger than that by O intercalation, which was attributed to the much stronger superexchange by electron doping than by hole doping. This comprehensive spin exchange mechanism was further confirmed by our results of the carrier doping effect on the interlayer magnetic coupling. Our work provides a deep understanding of the underlying spin exchange mechanism in 2D magnetic materials.

The exchange between anions and cations induced by coupled plasma and thermal annealing treatment for room-temperature ferromagnetism

Two-dimensional (2D) materials, with outstanding magnetic properties at room temperature, are highly desirable for the future spintronic and nanoscale electronic industry. However, most of the 2D systems are not of magnetic nature due to thermal fluctuations. Herein, we propose a novel strategy to induce robust room-temperature ferromagnetism in the originally nonmagnetic 2D ReS₂ by the exchange between anions and cations. The vacancies are created by argon plasma treatment, which lowers the formation energy of point defects. The subsequent annealing facilitates the movement of the cations into the anion sites, giving rise to antisite defects, which leads to a significant increase in the magnetization. First-principles calculations demonstrate that the point defect with respect to the antisite substitution from Re to S is responsible for the extraordinary room-temperature ferromagnetism. This work opens a new door to the design of spin electronic structures by controllable antisite defects.

Recent advances in two-dimensional ferromagnetism: strain-, doping-, structural- and electric field-engineering toward spintronic applications

Since the first report on truly two-dimensional (2D) magnetic materials in 2017, a wide variety of merging 2D magnetic materials with unusual physical characteristics have been discovered and thus provide an effective platform for exploring the associated novel 2D spintronic devices, which have been made significant progress in both theoretical and experimental studies. Herein, we make a comprehensive review on the recent scientific endeavors and advances on the various engineering strategies on 2D ferromagnets, such as strain-, doping-, structural- and electric field-engineering, toward practical spintronic applications, including spin tunneling junctions, spin field-effect transistors and spin logic gate, etc. In the last, we discuss on current challenges and future opportunities in this field, which may provide useful guidelines for scientists who are exploring the fundamental physical properties and practical spintronic devices of low-dimensional magnets.

Atomically Thin 2D van der Waals Magnetic Materials: Fabrications, Structure, Magnetic Properties and Applications

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

Coexistence of intrinsic room-temperature ferromagnetism and piezoelectricity in monolayer BiCrX3 (X = S, Se, and Te)

Two-dimensional (2D) materials with intrinsic ferromagnetism and piezoelectricity have received growing attention due to their potential applications in nanoscale spintronic devices. However, their applications are highly limited by the low Curie temperatures (T_C) and small piezoelectric coefficients. Here, using first-principles calculations, we have successfully predicted that BiCrX₃ (X = S, Se, and Te) monolayers simultaneously possess ferromagnetism and piezoelectricity by replacing one layer of Bi atoms with Cr atoms in Bi₂X₃ monolayers. Our results demonstrate that BiCrX₃ monolayers are not only intrinsic ferromagnetic semiconductors with indirect band gaps, adequate T_C values of higher than 300 K, and significant out-of-plane magnetic anisotropic energies, but also exhibit appreciable in-plane and out-of-plane piezoelectricity. In particular, the in-plane piezoelectric coefficients of BiCrX₃ monolayers with ABCAB configuration are up to 15.16 pm V^-1, which is higher than those of traditional three-dimensional piezoelectric materials such as α-quartz. The coexistence of ferromagnetism and piezoelectricity in BiCrX₃ monolayers gives them promising applications in spintronics and nano-sized sensors.

Nodal-loop half metallicity in a two-dimensional Fe4N2 pentagon crystal with room-temperature ferromagnetism

Two-dimensional (2D) materials with fully spin-polarized nodal-loop band crossing are a class of topological magnetic materials, holding promise for high-speed low-dissipation spintronic devices. Recently, several 2D nodal-loop materials have been reported in theory and experiment, such as Cu₂Si, Be₂C, CuSe, and Cr₂S₃ monolayers, adopting triangular, tetragonal, hexagonal, or complex lattices. However, a 2D nodal-loop half metal with room-temperature magnetism is still less reported. Here, we report that the 2D Fe₄N₂ pentagon crystal is a nodal-loop half metal with room-temperature magnetism over 428 K and a global minimum structure via first-principles calculations and global structure search. The Dirac nodal lines in Fe₄N₂ form a flat nodal loop at the Fermi level and a spin-polarized type-II nodal-loop above the Fermi level, which are protected by mirror symmetry. Our results establish Fe₄N₂ as a platform to obtain nodal-loop half metallicity in the 2D pentagon lattice and provide opportunities to build high-speed low-dissipation spintronics in the nanoscale.

Designing Two-Dimensional Versatile Room-Temperature Ferromagnets via Assembling Large-Scale Magnetic Quantum Dots

Two-dimensional (2D) ferromagnets possess astonishing potential in new-concept spintronics. However, most of the reported intrinsic 2D ferromagnets show a low Curie temperature far below room temperature. Here, we propose a series of 2D magnetic covalent and metal organic frameworks (COFs/MOFs) by assembling triangular zigzag graphene quantum dots (TZGDs) with various linkages, involving small-sized TZGDs, nonmetal atoms, magnetic metal atoms, and molecules. Upon first-principles calculations, we demonstrate 2D magnetic semiconductors with an enhanced Curie temperature of up to 472 K can be realized through the strong p(d)-p direct exchange interaction between TZGDs and linkages. Particularly, the TZGD size hardly affects the Curie temperature, whereas linkages can modulate the Curie temperature significantly. The TZGD size and linkages can regulate the electronic and magnetic properties of TZGD-based 2D ferromagnets. Our results confirm the possibility of designing 2D ferromagnets based on TZGDs and motivate the research of 2D ferromagnets on magnetic quantum dots and molecular magnets.

Recent Developments in van der Waals Antiferromagnetic 2D Materials: Synthesis, Characterization, and Device Implementation

Magnetism in two dimensions is one of the most intriguing and alluring phenomena in condensed matter physics. Atomically thin 2D materials have emerged as a promising platform for exploring magnetic properties, leading to the development of essential technologies such as supercomputing and data storage. Arising from spin and charge dynamics in elementary particles, magnetism has also unraveled promising advances in spintronic devices and spin-dependent optoelectronics and photonics. Recently, antiferromagnetism in 2D materials has received extensive attention, leading to significant advances in their understanding and emerging applications; such materials have zero net magnetic moment yet are internally magnetic. Several theoretical and experimental approaches have been proposed to probe, characterize, and modulate the magnetic states efficiently in such systems. This Review presents the latest developments and current status for tuning the magnetic properties in distinct 2D van der Waals antiferromagnets. Various state-of-the-art optical techniques deployed to investigate magnetic textures and dynamics are discussed. Furthermore, device concepts based on antiferromagnetic spintronics are scrutinized. We conclude with remarks on related challenges and technological outlook in this rapidly expanding field.