Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

Lattice thermal conductivity in CrSBr: the effects of interlayer interaction, magnetic ordering and external strain

With the continuous development of digital information and big data technologies, the ambient temperature and heat generation during the operation of magnetic storage devices play an increasingly crucial role in ensuring data security and device stability. In this study, we conducted a thorough investigation on in-plane lattice thermal conductivity of the van der Waals (vdWs) magnetic semiconductor CrSBr from bulk to monolayer using first-principles calculations and phonon Boltzmann transport equation. Our findings indicated that CrSBr show strong anisotropic thermal transport behaviors and layer number and magnetic ordering dependent lattice thermal conductivity. The lowest thermal conductivity is observed in y direction of antiferromagnetic CrSBr bilayer at all temperatures. Through the analysis of phonon spectra, phonon lifetime, heat capacity, scattering probability, phonon-phonon interaction strength, we demonstrated that out of plane acoustic phonon modes soften, the shift of Cr-Br antisymmetrical stretching vibrations, and large phonon band gap are the main factors. These results offer a comprehensive insight into phonon transport phenomena in vdWs magnetic materials.

Proximity-Induced Superconductivity in Ferromagnetic Fe3GeTe2 and Josephson Tunneling through a van der Waals Heterojunction

Synergy between superconductivity and ferromagnetism may offer great opportunities in nondissipative spintronics and topological quantum computing. Yet at the microscopic level, the exchange splitting of the electronic states responsible for ferromagnetism is inherently incompatible with the spin-singlet nature of conventional superconducting Cooper pairs. Here, we exploit the recently discovered van der Waals ferromagnets as enabling platforms with marvelous controllability to unravel the myth between ferromagnetism and superconductivity. We report unambiguous experimental evidence of superconductivity in few-layer ferromagnetic Fe3GeTe2 (FGT) proximity coupled to a superconducting NbSe2 overlayer through an insulating spacer, demonstrating coexistence of these two seemingly antagonistic orderings. Our transport measurements reveal a sudden resistance drop to zero in FGT below the superconducting critical temperature of NbSe2 and detect a Josephson supercurrent through the NbSe2/insulator/FGT van der Waals junction. Furthermore, using anomalous Hall effect and magnetic force microscopy characterizations, we confirm that FGT preserves its ferromagnetism in the superconducting regime. Our central findings reveal the microscopic harmony between ferromagnetism and superconductivity and render these systems immense technological potentials.

Emerging Physics in Magnetic Organic-Inorganic Hybrid Systems

The hybrid magnetic heterostructures and superlattices, composed of organic and inorganic materials, have shown great potential for quantum computing and next-generation information technology. Organic materials generally possess designable structural motifs and versatile optical, electronic, and magnetic properties, but are too delicate for robust integration into solid-state devices. In contrast, inorganic systems provide robust solid-state interface and excellent electronic properties but with limited customization space. Combining these two systems and taking respective advantages to exploit exotic physical properties has been a promising research direction but with tremendous challenges. Herein, we review the material preparation methods and discuss the emerging physical properties discovered in such magnetic organic-inorganic hybrid systems (MOIHSs), including recent progress on designable magnetic property modification, exchange bias effect, and the interplay of ferromagnetism and superconductivity, which provide a promising material platform for emerging magnetic memory and spintronic device applications.

Exploring van der Waals Cuprate Superconductors Using a Hybrid Microwave Circuit

The advent of two-dimensional van der Waals materials is a frontier of condensed matter physics and quantum devices. However, characterizing such materials remains challenging due to the limitations of bulk material techniques, necessitating the development of specialized methods. Here, we investigate the superconducting properties of Bi2Sr2CaCu2O8+x flakes by integrating them with a hybrid superconducting microwave resonator. The hybrid resonator is significantly modified by the interaction with the flake while maintaining a high quality factor (3 × 104). We also observe a significant upshift of the resonator frequency with increasing temperature, as well as a positive nonlinearity. These effects originate from a presently unknown microscopic mechanism within the flake, and can be modeled as a two-level system bath interacting with the resonant mode. Our findings open a path for high quality hybrid circuits with van der Waals flakes for exploring novel materials and developing new devices for quantum technology.

Octupolar Vortex Crystal and Toroidal Moment in Twisted Bilayer MnPSe3

Experimental detection of antiferromagnetic order in two-dimensional materials is a challenging task. Identifying multidomain antiferromagnetic textures via the current techniques is even more difficult. Therefore, we investigate the higher-order multipole moments in twisted bilayer MnPSe3. While the monolayers of MnPSe3 exhibit antiferromagnetic order, the moiré superlattices display a two-domain phase. We show that the octupolar moments M33+ and M33- are significant in this multidomain phase. In addition, when [M33+,M33-] are represented by the x and y components of a vector, the resultant pattern of these octupole moments forms vortex crystals which leads to octupolar toroidal moments, Txyz and Tzβ. Txyz and Tzβ can give rise to a magnetoelectric effect and gyrotropic birefringence that may provide indirect ways of detecting multidomain antiferromagnetic order. Our results highlight the importance of higher-order multipole moments for identification of complex spin textures in moiré magnets.

Ultrahigh Exchange Bias Field/Coercive Field Ratio in In Situ Formed Two-Dimensional Magnetic Te-Cr2O3/Cr5Te6 Heterostructures

The exchange bias (EB) effect is a fundamental magnetic phenomenon, in which the exchange bias field/coercive field ratio (|HEB/HC|) can improve the stability of spintronic devices. Two-dimensional (2D) magnetic heterostructures have the potential to construct low-power and high-density spintronic devices, while their typically air unstable and |HEB/HC| lesser, limiting the possibility of applications. Here, 2D Cr5Te6 nanosheets have been systematically synthesized with an in situ formed ≈2 nm-thick Te doped Cr2O3 layer (Te-Cr2O3) on the upper surface by chemical vapor deposition (CVD) method. The strong and air stable EB effect, achieving a |HEB/HC| of up to 80% under an ultralow cooling field of 0.01 T, which is greater than that of the reported 2D magnetic heterostructures. Meanwhile, the uniformity of thickness and chemical composition of the Te-Cr2O3 layer can be controlled by the growth conditions which are highly correlated with the EB effect of 2D Te-Cr2O3/Cr5Te6 heterostructures. First-principles calculations show that the Te-Cr2O3 can provide uncompensated spins in the Cr2O3, thus forming strong spin pinning effect. The systematical investigation of the EB effect in 2D Te-Cr2O3/Cr5Te6 heterostructures with high |HEB/HC| will open up exciting opportunities in low-power and high-stability 2D spintronic devices.

Two-Dimensional Materials for Brain-Inspired Computing Hardware

Recent breakthroughs in brain-inspired computing promise to address a wide range of problems from security to healthcare. However, the current strategy of implementing artificial intelligence algorithms using conventional silicon hardware is leading to unsustainable energy consumption. Neuromorphic hardware based on electronic devices mimicking biological systems is emerging as a low-energy alternative, although further progress requires materials that can mimic biological function while maintaining scalability and speed. As a result of their diverse unique properties, atomically thin two-dimensional (2D) materials are promising building blocks for next-generation electronics including nonvolatile memory, in-memory and neuromorphic computing, and flexible edge-computing systems. Furthermore, 2D materials achieve biorealistic synaptic and neuronal responses that extend beyond conventional logic and memory systems. Here, we provide a comprehensive review of the growth, fabrication, and integration of 2D materials and van der Waals heterojunctions for neuromorphic electronic and optoelectronic devices, circuits, and systems. For each case, the relationship between physical properties and device responses is emphasized followed by a critical comparison of technologies for different applications. We conclude with a forward-looking perspective on the key remaining challenges and opportunities for neuromorphic applications that leverage the fundamental properties of 2D materials and heterojunctions.

Spin Canting Promoted Manipulation of Exchange Bias in a Perpendicular Coupled Fe3GaTe2/CrSBr Magnetic van der Waals Heterostructure

Recently, two-dimensional (2D) van der Waals (vdW) magnetic materials have emerged as a promising platform for studying exchange bias (EB) phenomena due to their atomically flat surfaces and highly versatile stacking configurations. Although complex spin configurations between 2D vdW interfaces introduce challenges in understanding their underlying mechanisms, they can offer more possibilities in realizing effective manipulations. In this study, we present a spin-orthogonal arranged 2D Fe3GaTe2 (FGaT)/CrSBr vdW heterostructure, realizing the EB effect with the bias field as large as 1730 Oe at 2 K. Interestingly, this structure induces a positive EB under low cooling field, in contrast to conventional phenomena. Moreover, by employing asymmetric field sweeping methods, we effectively manipulate the zero-field cooling EB of the device with a switchable sign and a tunable magnitude. Thus, these findings not only elucidate a distinct mechanism analysis for EB phenomena with perpendicular coupled spin configurations but also hold promise for promoting contemporary 2D spintronic device applications.

Ab initio investigation of layered TMGeTe3 alloys for phase-change applications

Chalcogenide phase-change materials (PCMs) are among the most mature candidates for next-generation memory technology. Recently, CrGeTe3 (CrGT) emerged as a promising PCM due to its enhanced amorphous stability and fast crystallization for embedded memory applications. The amorphous stability of CrGT was attributed to the complex layered structure of the crystalline motifs needed to initiate crystallization. A subsequent computational screening work identified several similar compounds with good thermal stability, such as InGeTe3, CrSiTe3 and BiSiTe3. Here, we explored the substitution of Cr in CrGT with other 3d metals and predicted four additional layered alloys to be dynamically stable, namely ScGeTe3, TiGeTe3, ZnGeTe3 and MnGeTe3. Thorough ab initio simulations performed on both crystalline and amorphous models of these materials indicate the former three alloys to be potential PCMs with sizable resistance contrast. Furthermore, we found that crystalline MnGeTe3 exhibits ferromagnetic behavior, whereas the amorphous state probably forms a spin glass phase. This makes MnGeTe3 a promising candidate for magnetic phase-change applications.

Novel Magnetic-Field-Free Switching Behavior in vdW-Magnet/Oxide Heterostructure

Magnetization switching by charge current without a magnetic field is essential for device applications and information technology. It generally requires a current-induced out-of-plane spin polarization beyond the capability of conventional ferromagnet/heavy-metal systems, where the current-induced spin polarization aligns in-plane orthogonal to the in-plane charge current and out-of-plane spin current. Here, a new approach is demonstrated for magnetic-field-free switching by fabricating a van-der-Waals magnet and oxide Fe3GeTe2/SrTiO3 heterostructure. This new magnetic-field-free switching is possible because the current-driven accumulated spins at the Rashba interface precess around an emergent interface magnetism, eventually producing an ultimate out-of-plane spin polarization. This interpretation is further confirmed by the switching polarity change controlled by the in-plane initialization magnetic fields with clear hysteresis. Van-der-Waals magnet and oxide are successfully combined for the first time, especially taking advantage of spin-orbit torque on the SrTiO3 oxide. This allows this study to establish a new way of magnetic field-free switching. This work demonstrates an unusual perpendicular switching application of large spin Hall angle materials and precession of accumulated spins, and in doing so, opens up a new field and opportunities for van-der-Waals magnets and oxide spintronics.

High Néel temperature and magnetism modulation in 2D pentagon-based XN2 (X = B, Al, and Ga) structures with spin-polarized non-metallic atoms

Magnetic semiconductors with spin-polarized non-metallic atoms are usually overlooked in applications because of their poor performances in magnetic moments and under critical temperatures. Herein, magnetic characteristics of 2D pentagon-based XN2 (X = B, Al, and Ga) are revealed based on first-principles calculations. It was proven that XN2 structures are antiferromagnetic semiconductors with bandgaps of 2.15 eV, 2.42 eV and 2.16 eV for X = B, Al, and Ga, respectively. Through analysis of spin density distributions and molecular orbitals, the magnetic origin was found to be located at the antibonding orbitals (π*2px and π*2pz) of covalently bonded N atoms. Furthermore, it was demonstrated that XN2 semiconductors exhibit Néel temperatures (TN) of as high as 136 K, 266 K and 477 K, as found through Monte Carlo (MC) simulations of the Ising model. More significantly, the phase transition of the magnetic ground state from antiferromagnetic order to ferromagnetic order, continuous distribution of bandgaps from 2.0 eV to 2.5 eV, and enhancement of magnetic moment from 0.3μB to 1.2μB could be realized by exerting external fields. Our work proposes a novel spin-polarized phenomenon based on covalent bonds, ameliorating the performances of magnetic semiconductors with spin-polarized p-orbit electrons and providing immense application potentials for XN2 in spintronic devices.

First-principles predictions of two-dimensional Ce-based ferromagnetic semiconductors: CeF2 and CeFCl monolayers

Two-dimensional (2D) ferromagnetic (FM) semiconductors hold great promise for the next generation spintronics devices. By performing density functional theory first-principles calculations, both CeF2 and CeFCl monolayers are studied, our calculation results show that CeF2 is a FM semiconductor with sizable magneto-crystalline anisotropy energy (MAE) and high Curie temperature (290 K), but a smaller band gap and thermal instability indicate that it is not applicable at higher temperature. Its isoelectronic analogue, the CeFCl monolayer, is a bipolar FM semiconductor, its dynamics, elastic, and thermal stability are confirmed, our results demonstrate promising applications of the CeFCl monolayer for next-generation spintronic devices owing to its high Curie temperature (200 K), stable semiconducting features, and stability. Under biaxial strain from -5% to 5%, the CeFCl monolayer is a semiconductor with sizable MAE, its Curie temperature can increase to 240 K, the easy magnetization axes for CeFCl monolayer are still along the out-of-plane directions because the couplings between Cef y(3x 2-y 2) and f x(x 2-3y 2) orbitals in the different spin channels contribute most to the MAE according to second-order perturbation theory.

Ferro-Valleytricity with In-Plane Spin Magnetization

Ferro-valleytricity that manifests spin-orbit coupling (SOC)-induced spontaneous valley polarization is generally considered to occur in two-dimensional (2D) materials with out-of-plane spin magnetization. Here, we propose a mechanism to realize SOC-induced valley polarization and ferro-valleytricity in 2D materials with in-plane spin magnetization, wherein the physics correlates to non-collinear magnetism in triangular lattice. Our model analysis provides comprehensive ingredients that allow for ferro-valleytricity with in-plane spin magnetization, revealing that mirror symmetry favors remarkable valley polarization and time-reversal-mirror joint symmetry should be excluded. Through modulating the in-plane spin magnetization offset, the SOC-induced valley polarization could be reversed. Followed by first-principles, such a mechanism is demonstrated in a multiferroic triangular lattice of single-layer W3Cl8. We further show that the reversal of valley polarization could also be driven by applying an electric field that modulates ferroelectricity. Our findings greatly enrich valley physics research and significantly extend the scope for material classes of ferro-valleytricity.

CO2-broken Ti-O bonds in the TiO6 octahedron of CaTiO3 for greatly enhanced room-temperature ferromagnetism

Preparation of two-dimensional (2D) ferromagnetic nanomaterials and the study of their magnetic sources are crucial for the exploration of new materials with multiple applications. Herein, two-dimensional room-temperature ferromagnetic (FM) CaTiO3 nanosheets are successfully constructed with the assistance of supercritical carbon dioxide (SC CO2). In this process, the SC CO2-induced strain effect can lead to lattice expansion and introduction of O vacancies. More importantly, experimentally it can be found out that the breakage of the Ti-O2 bond by CO2 directly results in the equatorial plane of the TiO6 octahedron being exposed. This leads to more opportunities for oxygen vacancies and low-valent titanium to appear, where Ti3+ can optimize the spin structure, releasing the macroscopic magnetization. Greatly improved room-temperature ferromagnetic behavior, with an optimal magnetization of 0.1661 emu g-1 and a high Curie temperature (Tc) of 300 K can be achieved.

Recent Advances in Two-Dimensional Ferromagnetic Materials-Based van der Waals Heterostructures

Two-dimensional (2D) ferromagnetic materials are subjects of intense research owing to their intriguing physicochemical properties, which hold great potential for fundamental research and spintronic applications. Specifically, 2D van der Waals (vdW) ferromagnetic materials retain both structural integrity and chemical stability even at the monolayer level. Moreover, due to their atomic thickness, these materials can be easily manipulated by stacking them with other 2D vdW ferroic and nonferroic materials, enabling precise control over their physical properties and expanding their functional applications. Consequently, 2D vdW ferromagnetic materials-based heterostructures offer a platform to tailor magnetic properties and explore advanced spintronic devices. This review aims to provide an overview of recent developments in emerging 2D vdW ferromagnetic materials-based heterostructures and devices. The fabrication approaches for 2D ferromagnetic vdW heterostructures are primarily summarized, followed by a review of two categories of heterostructures: ferromagnetic/ferroic and ferromagnetic/nonferroic vdW heterostructures. Subsequently, the progress made in modulating magnetic properties and emergence of various phenomena in these heterostructures is highlighted. Furthermore, the applications of such heterostructures in spintronic devices are discussed along with their future perspectives and potential directions in this exciting field.

Electronic and magnetic properties of GeP monolayer modulated by Ge vacancies and doping with Mn and Fe transition metals

In this work, Ge vacancies and doping with transition metals (Mn and Fe) are proposed to modulate the electronic and magnetic properties of GeP monolayers. A pristine GeP monolayer is a non-magnetic two-dimensional (2D) material, exhibiting indirect gap semiconductor behavior with an energy gap of 1.34(2.04) eV obtained from PBE(HSE06)-based calculations. Single Ge vacancy (VaGe) and pair Ge vacancies (pVaGe) magnetize the monolayer significantly with total magnetic moments of 2.00 and 2.02 μ B, respectively. Herein, P atoms around the defect sites are the main contributors to the system magnetism. Similarly, the monolayer magnetization is induced by doping with Mn (MnGe) and Fe (FeGe) atoms. In these cases, total magnetic moments of 3.00 and 4.00 μ B are obtained, respectively, and the system magnetism originates mainly from transition metal impurities. The calculated band structures assert the diluted magnetic semiconductor nature of VaGe and FeGe systems, while pVaGe and MnGe systems can be classified as 2D half-metallic materials. Further, the spin orientation in Mn- and Fe-doped GeP monolayers is studied. Results indicate the antiferromagnetic state in the case of doping with pair transition metal atoms. Regardless of the interatomic distance between dopant atoms, Mn-doped systems exhibit ferromagnetic half-metallicity, where the parallel spin orientation is energetically more favorable than the antiparallel configuration. In contrast, the antiparallel spin orientation is stable in Fe-doped systems, which show the antiferromagnetic semiconductor nature. Results presented herein may introduce new prospective 2D spintronic materials made from non-magnetic GeP monolayers.

Two-dimensional anion-rich NaCl2 crystal under ambient conditions

The two-dimensional (2D) "sandwich" structure composed of a cation plane located between two anion planes, such as anion-rich CrI3, VS2, VSe2, and MnSe2, possesses exotic magnetic and electronic structural properties and is expected to be a typical base for next-generation microelectronic, magnetic, and spintronic devices. However, only a few 2D anion-rich "sandwich" materials have been experimentally discovered and fabricated, as they are vastly limited by their conventional stoichiometric ratios and structural stability under ambient conditions. Here, we report a 2D anion-rich NaCl2 crystal with sandwiched structure confined within graphene oxide membranes with positive surface potential. This 2D crystal has an unconventional stoichiometry, with Na:Cl ratio of approximately 1:2, resulting in a molybdenite-2H-like structure with cations positioned in the middle and anions in the outer layer. The 2D NaCl2 crystals exhibit room-temperature ferromagnetism with clear hysteresis loops and transition temperature above 320 K. Theoretical calculations and X-ray magnetic circular dichroism (XMCD) spectra reveal the ferromagnetism originating from the spin polarization of electrons in the Cl elements of these crystals. Our research presents a simple and general approach to fabricating advanced 2D unconventional stoichiometric materials that exhibit half-metal and ferromagnetism for applications in electronics, magnetism, and spintronics.

Magnetic exchange coupling and photodetection multifunction characteristics of an MnSe/LaMnO3 heterostructure

Artificial heterostructures are often realized by stacking different materials to present new emerging properties that are not exhibited by their individual constituents. In this work, non-layered two-dimensional α-MnSe nanosheets were transferred onto LaMnO3 (LMO) films to obtain a multifunctional heterostructure. The high crystal quality of the MnSe/LMO heterostructure was revealed by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy measurements. The enhancement of the saturated magnetization and coercive field and synchrotron X-ray measurements indicated the magnetic exchange coupling effect present in this MnSe/LMO heterostructure. The exchange bias field and coercive field reached 400 Oe and 1013 Oe under a positive 5k Oe field-cooling process. Thus, an outstanding photodetector with photoresponsivity of 4.1 × 10-4 A W-1 and photo detectivity of 2.6 × 108 jones was obtained with a luminescence of 532 nm for this MnSe/LMO heterostructure. The multifunction characteristics of magnetic exchange coupling and photodetection in this heterostructure are very useful for next-generation devices.

Nonlinear Optics in Two-Dimensional Magnetic Materials: Advancements and Opportunities

Nonlinear optics, a critical branch of modern optics, presents unique potential in the study of two-dimensional (2D) magnetic materials. These materials, characterized by their ultra-thin geometry, long-range magnetic order, and diverse electronic properties, serve as an exceptional platform for exploring nonlinear optical effects. Under strong light fields, 2D magnetic materials exhibit significant nonlinear optical responses, enabling advancements in novel optoelectronic devices. This paper outlines the principles of nonlinear optics and the magnetic structures of 2D materials, reviews recent progress in nonlinear optical studies, including magnetic structure detection and nonlinear optical imaging, and highlights their role in probing magnetic properties by combining second harmonic generation (SHG) and multispectral integration. Finally, we discuss the prospects and challenges for applying nonlinear optics to 2D magnetic materials, emphasizing their potential in next-generation photonic and spintronic devices.

Electrically and magnetically readable memory with a graphene/1T-CrTe2 heterostructure: anomalous Hall transistor

Using first-principles theoretical analysis, we demonstrate the spin-polarized anomalous Hall conductivity (AHC) response of a 2D vdW heterostructure of graphene and ferromagnetic CrTe2 that can be controlled with a perpendicular electric field E. The origins of AHC and linear magnetoelectric responses are traced to (a) the transfer of electronic charge from graphene to ferromagnetic CrTe2 causing an out-of-plane electric polarization P = 1.69 μC cm-2 and (b) the crystal field and spin-split Dirac points of graphene. Through H' = -VP·E coupling, E controls the charge transfer, magnetization and carrier density, switching the spin-polarized Berry curvature as the Fermi energy crosses the split Dirac points of graphene. Based on these, we propose an Anomalous Hall Transistor (AHT) that exploits electronic spin and charge to store binary information, opening up a route to quantum devices based on quantum geometry and magnetoelectric transport.

Monolayer Magnetic Metal with Scalable Conductivity

2D magnets have emerged as a class of materials highly promising for studies of quantum phenomena and applications in ultra-compact spintronics. Current research aims at design of 2D magnets with particular functional properties. A formidable challenge is to produce metallic monolayers: the material landscape of layered magnetic systems is strongly dominated by insulators; rare metallic magnets, such as Fe3GeTe2, become insulating as they approach the monolayer limit. Here, electron transport measurements demonstrate that the recently discovered 2D magnet GdAlSi - graphene-like AlSi layers coupled to layers of Gd atoms - remains metallic down to a single monolayer. Band structure analysis indicates the material to be an electride, which may stabilize the metallic state. Remarkably, the sheet conductance of 2D GdAlSi is proportional to the number of monolayers - a manifestation of scalable conductivity. The GdAlSi layers are epitaxially integrated with silicon, facilitating applications in electronics.

Substantially Enhanced Spin Polarization in Epitaxial CrTe2 Quantum Films

2D van der Waals (vdW) magnets, which extend to the monolayer (ML) limit, are rapidly gaining prominence in logic applications for low-power electronics. To improve the performance of spintronic devices, such as vdW magnetic tunnel junctions, a large effective spin polarization of valence electrons is highly desired. Despite its considerable significance, direct probe of spin polarization in these 2D magnets has not been extensively explored. Here, using 2D vdW ferromagnet of CrTe2 as a prototype, the spin degrees of freedom in the thin films are directly probed using Mott polarimetry. The electronic band of 50 ML CrTe2 thin film, spanning the Brillouin zone, exhibits pronounced spin-splitting with polarization peaking at 7.9% along the out-of-plane direction. Surprisingly, atomic-layer-dependent spin-resolved measurements show a significantly enhanced spin polarization in a 3 ML CrTe2 film, achieving 23.4% polarization even in the absence of an external magnetic field. The demonstrated correlation between spin polarization and film thickness highlights the pivotal influence of perpendicular magnetic anisotropy, interlayer interactions, and itinerant behavior on these properties, as corroborated by theoretical analysis. This groundbreaking experimental verification of intrinsic effective spin polarization in CrTe2 ultrathin films marks a significant advance in establishing 2D ferromagnetic atomic layers as a promising platform for innovative vdW-based spintronic devices.

High Electrical Conductance in Magnetic Emission Junction of Fe3GeTe2/ZnO/Ni Heterostructure via Selective Spin Emission through ZnO Ohmic Barrier

The insulator is essential for magnetic tunneling junction (MTJ) that increases magnetoresistance (MR) by decoupling magnetization directions between two ferromagnets. However, wide bandgap tunnel barrier blocks the thermionic emission of electrons, significantly reducing electrical conductance through MTJ. Here, a magnetic emission junction (MEJ) is demonstrated for the first time using an Fe3GeTe2 (FGT)/ZnO/Ni heterostructure with very high electrical conductance. The conduction band of ZnO (electron affinity 4.6 eV) aligns with Fermi levels (EF) of FGT (4.47 eV) and Ni (4.58 eV) ferromagnets and forms an Ohmic barrier, enabling free spin-electron emission through ZnO barrier and high electrical conductance. In contrast to the typical positive MR in MTJ by majority spin tunneling, negative MR is observed in FGT/ZnO/Ni MEJ. The minority spin electrons of Ni, with maximum states near the EF, are dominantly emitted to FGT over the ZnO barrier, while majority spin electrons of Ni, with maximum states below the EF, are blocked by it. In the FGT/FGT/ZnO/Ni heterostructure, the MR ratio is further increased by combining positive and negative MR at the MTJ (FGT/FGT) and MEJ (FGT/ZnO/Ni), respectively. As a result, FGT-MEJ exhibits 10-1000 orders higher conductance than other 2D-MTJs, while MR ratio remains similar to other 2D-MTJs.

Vacancies Engineering in Molybdenum Boride MBene Nanosheets to Activate Room-Temperature Ferromagnetism

The rapid development of low energy dissipation spintronic devices has stimulated the search for air-stable 2D nanomaterials possessing room-temperature ferromagnetism. Here the experimental realization of 2D Mo4/3B2 nanosheets is reported with intrinsic room-temperature ferromagnetic characteristics by vacancy engineering. These nanosheets are synthesized by etching the bulk MAB phase (Mo2/3Y1/3)2AlB2 into Mo4/3B2 nanosheets in ZnCl2 molten salt. The Mo4/3B2 nanosheets show robust intrinsic ferromagnetic properties, with a saturation magnetic moment of 0.044 emu g-1 at 300 K, while vacancy-free MoB MBene exhibits paramagnetism. It is elucidated that the Mo-vacancy defect generates large density of states near the Fermi surface and spontaneously spin-split bands through first-principles calculations, which contributes to the non-zero magnetic moment in Mo4/3B2 nanosheets. This work lays the groundwork for activating the magnetic properties of MBene nanosheets by vacancy engineering, offering the possibilities for development of practical spintronic devices.

Topological Spin Textures: Basic Physics and Devices

In the face of escalating modern data storage demands and the constraints of Moore's Law, exploring spintronic solutions, particularly the devices based on magnetic skyrmions, has emerged as a promising frontier in scientific research. Since the first experimental observation of skyrmions, topological spin textures have been extensively studied for their great potential as efficient information carriers in spintronic devices. However, significant challenges have emerged alongside this progress. This review aims to synthesize recent advances in skyrmion research while addressing the major issues encountered in the field. Additionally, current research on promising topological spin structures in addition to skyrmions is summarized. Beyond 2D structures, exploration also extends to 1D magnetic solitons and 3D spin textures. In addition, a diverse array of emerging magnetic materials is introduced, including antiferromagnets and 2D van der Waals magnets, broadening the scope of potential materials hosting topological spin textures. Through a systematic examination of magnetic principles, topological categorization, and the dynamics of spin textures, a comprehensive overview of experimental and theoretical advances in the research of topological magnetism is provided. Finally, both conventional and unconventional applications are summarized based on spin textures proposed thus far. This review provides an outlook on future development in applied spintronics.

Unconventional Anomalous Hall Effect Driven by Self-Intercalation in Covalent 2D Magnet Cr2Te3

Covalent 2D magnets such as Cr2Te3, which feature self-intercalated magnetic cations located between monolayers of transition-metal dichalcogenide material, offer a unique platform for controlling magnetic order and spin texture, enabling new potential applications for spintronic devices. Here, it is demonstrated that the unconventional anomalous Hall effect (AHE) in Cr2Te3, characterized by additional humps and dips near the coercive field in AHE hysteresis, originates from an intrinsic mechanism dictated by the self-intercalation. This mechanism is distinctly different from previously proposed mechanisms such as topological Hall effect, or two-channel AHE arising from spatial inhomogeneities. Crucially, multiple Weyl-like nodes emerge in the electronic band structure due to strong spin-orbit coupling, whose positions relative to the Fermi level is sensitively modulated by the canting angles of the self-intercalated Cr cations. These nodes contribute strongly to the Berry curvature and AHE conductivity. This component competes with the contribution from bands that are less affected by the self-intercalation, resulting in a sign change in AHE with temperature and the emergence of additional humps and dips. The findings provide compelling evidence for the intrinsic origin of the unconventional AHE in Cr2Te3 and further establish self-intercalation as a control knob for engineering AHE in complex magnets.

Robust ferromagnetism in wafer-scale Fe3GaTe2 above room-temperature

The discovery of ferromagnetism in van der Waals (vdW) materials has enriched the understanding of two-dimensional (2D) magnetic orders and opened new avenues for fundamental physics research and next generation spintronics. However, achieving ferromagnetic order at room temperature, along with strong perpendicular magnetic anisotropy, remains a significant challenge. In this work, we report wafer-scale growth of vdW ferromagnet Fe3GaTe2 using molecular beam epitaxy. The epitaxial Fe3GaTe2 films exhibit robust ferromagnetism, exemplified by high Curie temperature (TC = 420 K) and large perpendicular magnetic anisotropy (PMA) constant KU = 6.7 × 105 J/m3 at 300 K for nine-unit-cell film. Notably, the ferromagnetic order is preserved even in the one-unit-cell film with TC reaching 345 K, benefiting from the strong PMA (KU = 1.8×105 J/m3 at 300 K). In comparison to exfoliated Fe3GaTe2 flakes, our epitaxial films with the same thickness show the significant enhancement of TC, which could be ascribed to the tensile strain effect from the substrate. The successful realization of wafer-scale ferromagnetic Fe3GaTe2 films with TC far above room temperature represents a substantial advancement (in some aspects or some fields, e.g. material science), paving the way for the development of 2D magnet-based spintronic devices.

Spin Glass State and Griffiths Phase in van der Waals Ferromagnetic Material Fe5GeTe2

The discovery of two-dimensional (2D) van der Waals ferromagnetic materials opens up new avenues for making devices with high information storage density, ultra-fast response, high integration, and low power consumption. Fe5GeTe2 has attracted much attention because of its ferromagnetic transition temperature near room temperature. However, the investigation of its phase transition is rare until now. Here, we have successfully synthesized a single crystal of the layered ferromagnet Fe5GeTe2 by chemical vapor phase transport, soon after characterized by X-ray diffraction (XRD), DC magnetization M(T), and isotherm magnetization M(H) measurements. A paramagnetic to ferromagnetic transition is observed at ≈302 K (TC) in the temperature dependence of the DC magnetic susceptibility of Fe5GeTe2. We found an unconventional potential spin glass state in the low-temperature regime that differs from the conventional spin glass states and Griffiths phase (GP) in the high-temperature regime. The physical mechanisms behind the potential spin glass state of Fe5GeTe2 at low temperatures and the Griffith phase at high temperatures need to be further investigated.

Magnetointeractive Cr2Te3-Coated Liquid Metal Droplets for Flexible Memory Arrays and Wearable Sensors

Magnetic liquid metal droplets, featured by unique fluidity, metallic conductivity, and magnetic reactivity, are of growing significance for next-generation flexible electronics. Conventional fabrication routes, which typically incorporate magnetic nanoparticles into liquid metals, otherwise encounter the pitfall pertaining to surface adhesivity and corrosivity over device modules. Here, an innovative approach of synergizing liquid metals with 2D magnetic materials is presented, accordingly creating chromium(III)-telluride-coated liquid metal (CT-LM) droplets via a simple self-assembly process. The CT-LM droplets exhibit controllable deformation and locomotion under magnetic fields, demonstrate nonadherence to various surfaces, and enable cost-effective recycling of components. The functionality of CT-LM droplets is validated through their use in magnetointeractive memory devices to enable sensing/storing 64 magnetic paths and in wearable sensors as the flexible vibrator for dynamic gesture recognition with machine learning assistance. This work opens new avenues for the functional droplet design and broadens the horizons of flexible electronics.

Intrinsic Localized Excitons in MoSe2/CrSBr Heterostructure

Despite extensive studies on magnetic proximity effects, the fundamental excitonic properties of the 2D semiconductor-magnet heterostructures remain elusive. Here, the presence of localized excitons in MoSe2/CrSBr heterostructures is unveiled, represented by a new photoluminescence emission feature, X*. Our findings reveal that X* originates from excitons confined by intrinsic defects in the CrSBr layer. Additionally, the degrees of valley polarization of the X* and trion peaks exhibit opposite polarities under a magnetic field and closely correlate with the magnetic order of CrSBr. This is attributed to spin-dependent charge transfer across the heterointerface, supported by density functional theory calculations which reveal a type-II band alignment. Furthermore, the strong in-plane anisotropy of CrSBr induces unique polarization-dependent responses in MoSe2 emissions. This study highlights the crucial role of defects in shaping excitonic properties and offers valuable insights into spectrally resolved proximity effects in semiconductor-magnet van der Waals heterostructures.

Pressure-driven magnetic phase change in the CrI3/Br3Cr2I3 heterostructure

Vertically stacked van der Waals (vdW) heterostructures not only provide a promising platform in terms of band alignment, but also constitute fertile ground for fundamental science and attract tremendous practical interest towards their use in various device applications. Beyond most two-dimensional (2D) materials, which are intrinsically non-magnetic, CrI3 is a novel material with magnetism dependent on its vdW-bonded layers, promising potential spintronics applications. However, for particular device applications, a heterostructure is commonly fabricated and it is necessary to examine the effect of the interface or contact atoms on the magnetic properties of the heterostructure. Most importantly, the effect of assembly stress on the electronic and magnetic properties remains unclear. In this study, we design a vdW heterostructure from two-chromium tri-halides, namely the CrI3/Br3Cr2I3 heterostructure, where the Janus equivalent of the CrI3 monolayer, Br3Cr2I3, is also an intrinsically magnetic 2D material. Using state-of-the-art first-principles calculations, we uncover the effects of the contact atoms, as well as external pressure, on the electronic and magnetic properties of the CrI3/Br3Cr2I3 heterostructure. It is found that the heterostructure transitions from an antiferromagnetic (AFM) to ferromagnetic (FM) ground state with pressure larger than certain threshold. We also investigate the magneto-crystalline anisotropy energy (MAE) of the CrI3/Br3Cr2I3 heterostructure. Remarkably, it is found that the MAE is significantly influenced by both the stacking and the contact atoms, varying abruptly and inconsistently with the contact atoms and external pressure. Further, we also reveal a correlation between the MAE and the polar angle. The pressure-regulated magnetic properties of the CrI3/Br3Cr2I3 heterostructure as revealed in this study highlight its potential applications in spintronic devices.

Stacking-Engineered Ferroelectricity and Multiferroic Order in van der Waals Magnets

Two-dimensional (2D) materials that exhibit spontaneous magnetization, polarization, or strain (referred to as ferroics) have the potential to revolutionize nanotechnology by enhancing the multifunctionality of nanoscale devices. However, multiferroic order is difficult to achieve, requiring complicated coupling between electron and spin degrees of freedom. We propose a universal method to engineer multiferroics from van der Waals magnets by taking advantage of the fact that changing the stacking between 2D layers can break inversion symmetry, resulting in ferroelectricity as well as magnetoelectric coupling. We illustrate this concept using first-principles calculations in bilayer NiI_{2}, which can be made ferroelectric upon rotating two adjacent layers by 180° with respect to the bulk stacking. Furthermore, we discover a novel strong magnetoelectric coupling between the interlayer spin order and interfacial electronic polarization. Our approach is not only general but also systematic and can enable the discovery of a wide variety of 2D multiferroics with strong magnetoelectric coupling.

Emergent Skyrmions in Cr0.85Te nanoflakes at Room Temperature

Chiral noncollinear magnetic nanostructures, such as skyrmions, are intriguing spin configurations with significant potential for magnetic memory technologies. However, the limited availability of 2D magnetic materials that host skyrmions with Curie temperatures above room temperature presents a major challenge for practical implementation. Chromium tellurides exhibit diverse spin configurations and remarkable stability under ambient conditions, making them a promising platform for fundamental spin physics research and the development of innovative 2D spintronic devices. Here, domain structures of Cr0.85Te nanoflakes synthesized via chemical vapor deposition are investigated, using magnetic force microscopy at room temperature. The results reveal that the domain width of the as-grown nanoflakes scales with the square root of their thicknesses. Notably, the emergence and annihilation of skyrmions are observed, which can be reversibly controlled by external magnetic fields and thermal excitation in ambient air. Micromagnetic simulations suggest that the emergence of skyrmions in Cr0.85Te nanoflakes arises from inversion symmetry breaking due to compositional gradients across the sample thickness, rather than the interfacial Dzyaloshinskii-Moriya interaction. These findings provide new insights into the mechanisms underlying skyrmion formation in 2D ferromagnets and open exciting possibilities for manipulating domain structures at room temperature, offering practical pathways for developing next-generation spintronic devices.

Multiferroic Tuning of Magnetic Anisotropy in MnTe2 Monolayer with Li/Na Adsorption

Two-dimensional (2D) magnetic materials with tunable magnetic anisotropy energy (MAE) are of great scientific interest and hold immense promise for ultracompact spintronic devices with lower energy consumption and higher storage density. Here, we demonstrate a practical approach for manipulating MAE in layered MnTe2 through the alkali metal adsorption and ferroelectric (FE) polarization effect. Our results reveal colossal MAE values of up to -12.428 erg/cm2 under Li/Na adsorption, accompanied by a spin reorientation and enhanced ferromagnetic (FM) coupling stability. Their negative MAE show a linear enhancement in response to the external strain. Moreover, we find that the FE In2Se3 substrate enhances the perpendicular magnetic anisotropy (PMA) of MnTe2 up to 2.318 erg/cm2 depending on the polarization direction. Ferroelectric switching at In2Se3-based interfaces could also induce significant MAE changes with the value of 3.838 erg/cm2. We elucidate that the underlying mechanisms for these modulations are primarily attributed to alterations in the electron occupancy of interfacial Te1-derived py and pz states, which affect their competitive spin-orbit coupling (SOC) strengths. These findings highlight the potential of interfacial engineering in tailoring magnetism in 2D materials, opening exciting possibilities for the development of advanced spintronic devices with enhanced functionality.

Electric Field Control Of Moiré Skyrmion Phases in Twisted Multiferroic NiI2 Bilayers

Twisted magnetic van der Waals materials provide a flexible platform to engineer unconventional magnetism. Here we demonstrate the emergence of electrically tunable topological moiré magnetism in twisted bilayers of the spin-spiral multiferroic NiI2. We establish a rich phase diagram featuring uniform spiral phases, a variety of kπ-skyrmion lattices, and nematic spin textures ordered at the moiré scale. The emergence of these phases is driven by the local stacking and the resulting moiré modulated frustration. Notably, when the spin-spiral wavelength is commensurate with the moiré length scale by an integer k, multiwalled skyrmions become pinned to the moiré pattern. We show that the strong magnetoelectric coupling displayed by the moiré multiferroic allows electric control of the kπ-skyrmion lattices by an out-of-plane electric field. Our results establish a highly tunable platform for skyrmionics based on twisted van der Waals multiferroics, potentially enabling a new generation of ultrathin topologically protected spintronic devices.

Above-Room-Temperature Ferromagnetism Regulation in Two-Dimensional Heterostructures by van der Waals Interfacial Magnetochemistry

Most methods for regulating physical and chemical properties of materials involve the breaking and formation of chemical bonds, which inevitably change local structures. Two-dimensional (2D) ferromagnets are critical for spintronic memory and quantum devices, but most of them maintain ferromagnetism at low temperature, and multiaspect control of 2D ferromagnetism at room temperature or above is still missing. Here, we report a nondestructive, van der Waals (vdW) interfacial magnetochemistry strategy for above-room-temperature, multiaspect 2D ferromagnetism regulation. By vdW coupling nonmagnetic MoS2, WSe2, or Bi1.5Sb0.5Te1.7Se1.3 with 2D vdW ferromagnet Fe3GaTe2, the Curie temperature is enhanced up to 400 K, best for 2D ferromagnets, with 26.8% tuning of room-temperature perpendicular magnetic anisotropy and an unconventional anomalous Hall effect up to 340 K. These phenomena originate from changes in magnetic exchange interactions and magnetic anisotropy energy by interfacial charge transfer and spin-orbit coupling. This work opens a pathway for engineering multifunctional 2D heterostructures by vdW interfacial magnetochemistry.

Coexisting Ferromagnetic-Antiferromagnetic State and Giant Anomalous Hall Effect in Chemical Vapor Deposition-Grown 2D Cr5Te8

Two-dimensional (2D) magnets, as an important member of the 2D material family, have emerged as a promising platform for spintronic devices. Herein, we report the chemical vapor deposition (CVD) growth of highly crystalline submillimeter-scale self-intercalated metallic 2D ferromagnetic (FM) trigonal chromium telluride (Cr5Te8) flakes on inert mica substrates. Through magneto-optical and magnetotransport measurements, we unveil the exceptional magnetic properties of these 2D flakes. The trigonal Cr5Te8 flakes exhibit a strong anisotropic FM order with a Curie temperature above 220 K. Notably, an emergent antiferromagnetic (AFM) state is observed in the MOKE signal from ultrathin Cr5Te8 flakes around the Curie temperature. The AFM state has a relatively weak interlayer exchange coupling, allowing a switching between the interlayer AFM and FM states by tuning the temperature. Meanwhile, the trigonal Cr5Te8 flakes exhibit a giant anomalous Hall effect (AHE), with an anomalous Hall conductivity of 710 Ω-1 cm-1 and an anomalous Hall angle of 3.5% at zero magnetic field, surpassing typical itinerant ferromagnets. Further analysis suggests that the AHE in trigonal Cr5Te8 is primarily driven by the skew-scattering mechanism rather than the intrinsic or extrinsic side-jump mechanism. These findings demonstrate the potential of CVD-grown ultrathin Cr5Te8 flakes as a promising 2D magnetic material with exceptional AHE properties for future spintronic applications.

Field-Induced Antiferromagnetic Correlations in a Nanopatterned Van der Waals Ferromagnet: A Potential Artificial Spin Ice

Nano-patterned magnetic materials have opened new venues for the investigation of strongly correlated phenomena including artificial spin-ice systems, geometric frustration, and magnetic monopoles, for technologically important applications such as reconfigurable ferromagnetism. With the advent of atomically thin 2D van der Waals (vdW) magnets, a pertinent question is whether such compounds could make their way into this realm where interactions can be tailored so that unconventional states of matter can be assessed. Here, it is shown that square islands of CrGeTe3 vdW ferromagnets distributed in a grid manifest antiferromagnetic correlations, essential to enable frustration resulting in an artificial spin-ice. By using a combination of SQUID-on-tip microscopy, focused ion beam lithography, and atomistic spin dynamic simulations, it is shown that a square array of CGT island as small as 150 × 150 × 60 nm3 have tunable dipole-dipole interactions, which can be precisely controlled by their lateral spacing. There is a crossover between non-interacting islands and significant inter-island anticorrelation depending on how they are spatially distributed allowing the creation of complex magnetic patterns not observable at the isolated flakes. These findings suggest that the cross-talk between the nano-patterned magnets can be explored in the generation of even more complex spin configurations where exotic interactions may be manipulated in an unprecedented way.

Crystal Chemistry and Design Principles of Altermagnets

Altermagnetism was very recently identified as a new type of magnetic phase beyond the conventional dichotomy of ferromagnetism (FM) and antiferromagnetism (AFM). Its globally compensated magnetization and directional spin polarization promise new properties such as spin-polarized conductivity, spin-transfer torque, anomalous Hall effect, tunneling, and giant magnetoresistance that are highly useful for the next-generation memory devices, magnetic detectors, and energy conversion. Though this area has been historically led by the thin-film community, the identification of altermagnetism ultimately relies on precise magnetic structure determination, which can be most efficiently done in bulk materials. Our review, written from a materials chemistry perspective, intends to encourage materials and solid-state chemists to make contributions to this emerging topic through new materials discovery by leveraging neutron diffraction to determine the magnetic structures as well as bulk crystal growth for exploring exotic properties. We first review the symmetric classification for the identification of altermagnets with a summary of chemical principles and design rules, followed by a discussion of the unique physical properties in relation to crystal and magnetic structural symmetry. Several major families of compounds in which altermagnets have been identified are then reviewed. We conclude by giving an outlook for future directions.

Polarity-Reversal of Exchange Bias in van der Waals FePS3/Fe3GaTe2 Heterostructures

Exchange bias (EB) in antiferromagnetic (AFM)/ferromagnetic heterostructures is crucial for the advancement of spintronic devices and has attracted significant attention. The common EB effect in van der Waals heterostructures features a low blocking temperature (Tb) and a single polarity. In this work, a significant EB effect with a Tb up to 150 K is observed in FePS3/Fe3GaTe2 heterostructures, and in particular, the EB exhibits an unusual temperature-dependent polarity-reversal behavior. Under a high positive field-cooling condition (e.g., μ0H ≥ 0.5 T), a negative EB field (HEB) is observed at low temperatures, and with increasing temperature, the HEB crosses zero at ≈20 K, subsequently becomes positive and later approaches zero again at Tb. A model composed of a top FePS3/interfacial FePS3/Fe3GaTe2 sandwich structure is proposed. The charge transfer from Fe3GaTe2 to FePS3 at the interface induces net magnetic moments (∆M) in FePS3. The interface favors AFM coupling, and thus the reversal of ∆M of the interfacial FePS3 leads to the polarity-reversal of EB. Moreover, the EB can be extended to the bare Fe3GaTe2 region of the Fe3GaTe2 flake partially covered by FePS3. This work provides opportunities for a deeper understanding of the EB effect and opens a new route toward constructing novel spintronic devices.

Magnetic stability, Fermi surface topology, and spin-correlated dielectric response in monolayer 1T-CrTe2

We have carried out density-functional theory (DFT) calculations to study the magnetic stability of both ferromagnetic (FM) and anti-ferromagnetic (AFM) states in monolayer 1T-CrTe2. Our results show that the AFM order is lower in energy and thus is the ground state. By tuning the lattice parameters, the AFM order can transition to the FM order, in good agreement with experimental observation. We observe a commensurate spin density wave (SDW) alongside the previously predicted charge density wave (CDW), and attribute the AFM order to the SDW. The SDW order leads to distinct hole and electron Fermi pockets and a pronounced optical anisotropy, suggesting quasi-one-dimensional behavior in this material.

Engineering Two-Dimensional Magnetic Heterostructures: A Theoretical Perspective

Two-dimensional (2D) magnetic materials have attracted great attention due to their promise for applications in future high-speed, low-energy quantum computing and memory devices. By integrating 2D magnetic materials with other magnetic or nonmagnetic materials to form heterostructures, the synergistic effects of interlayer orbital hybridization, spin-orbit coupling, and symmetry breaking can surpass the performance of single-layer materials and lead to novel physical phenomena. This review provides a comprehensive theoretical analysis of engineering 2D magnetic heterostructures, emphasizing the fundamental physics of interlayer interactions and the resulting enhancements and novel properties. It reviews the mechanisms and progress in tuning the magnetic ordering, enhancing the Curie temperature (Tc) and modulating properties such as topological magnetic structures, spin polarization, electronic band topology, valley polarization, and magnetoelectric coupling through the construction of 2D magnetic heterostructures. Additionally, this review discusses the current challenges faced by 2D magnetic heterostructures, aiming to guide the future design of higher-performance magnetic heterostructures.

Defect states in magnetically "seasoned" WSSe - adsorption and doping effects of magnetic transition metals X = V, Cr, Mn, Fe, Co - a comprehensive first-principles study

Besides the symmetry breaking of Janus transition metal dichalcogenides (TMDs), Janus-based Diluted Magnetic Semiconductors (DMS) are attractive to study considering the local symmetry of transition metal (TM) dopant/adatom. This study conducts a first-principles calculation of magnetic properties in TM (V, Cr, Mn, Fe, and Co) -- doped and adsorbed Janus WSSe. Our results reveal that TM's atomic/ionic size impacts d-p-d orbital overlap, affecting bond length/angle and defect state positions. The result shows that V-doped WSSe exhibits long-range ferromagnetic order sustained by itinerant carriers and W atom spin-orbital coupling (SOC). The mechanism of d-p-d orbital hybridization is highlighted with an overlapping density of states and spin-density plots. Moreover, an enhanced magnetic anisotropy energy (MAE) is observed in Fe/Co-doped and Mn/Fe-adsorbed systems, with the easy axis aligning to the c-axis. The orbital contribution of MAE provided explains the relationship between states near the Fermi and MAE. On the other hand, the adsorption of V and Cr aligns the easy axis to the a-b plane, for which we have systematically explained the contribution of the spin-flip term in MAE. This research provides insights and a guideline for further exploration of 2D DMS for spintronics and spin-related phenomena.

Identifying Band Structure Changes of FePS3 across the Antiferromagnetic Phase Transition

Magnetic 2D materials enable interesting tuning options of magnetism. As an example, the van der Waals material FePS3, a zig-zag-type intralayer antiferromagnet, exhibits very strong magnetoelastic coupling due to the different bond lengths along different ferromagnetic and antiferromagnetic coupling directions enabling elastic tuning of magnetic properties. The likely cause of the length change is the intricate competition between direct exchange of the Fe atoms and superexchange via the S and P atoms. To elucidate this interplay, we study the band structure of exfoliated FePS3 by μm scale ARPES (angular resolved photoelectron spectroscopy), both, above and below the Néel temperature TN. We found three characteristic changes across TN. They involve S 3p-type bands, Fe 3d-type bands and P 3p-type bands, respectively, as attributed by comparison with density functional theory calculations (DFT + U). This highlights the involvement of all the atoms in the magnetic phase transition providing independent evidence for the intricate exchange paths.

A Quantitative Arrott Analysis Methodology for Magnetic Susceptibility of Microscale Ferromagnetic Nanoflakes

Probing magnetic susceptibility of a microsized ferromagnet is a long-standing problem in condensed matter physics. Among various measuring methods for magnetic susceptibility including vibrating sample magnetometry and superconducting quantum interference device magnetometry, almost all require large-scale bulk samples or thick films. However, the quantitative measurement for magnetic susceptibility on a microscale nanoflake is a great challenge. Here, we demonstrate a new analysis method to quantitatively evaluate the magnetic susceptibility of a microscale ferromagnetic nanoflake. Based on the Arrott plot of magnetization isotherms obtained from anomalous Hall resistance, we achieve an in situ evaluation of the value of magnetic susceptibility of a microscale ferromagnetic Fe5GeTe2 nanoflake, identification of the out-of-plane and in-plane magnetization, and investigation of the magnetic anisotropy transition with quantifying critical exponents. Our method reveals critical information on magnetic phase transition in microscale ferromagnetic materials, providing deep insight into spin dynamics of correlated electron systems.

High Chern number quantum anomalous Hall effect in monolayer Co3X3SSe (X = Sn, Pb) kagomes

The high Chern number quantum anomalous Hall effect can offer an ideal platform to develop exotic quantum materials with a dissipationless chiral edge. The investigation of kagome monolayer Co3X3SSe (X = Sn, Pb) materials enables a comprehensive exploration of their structural, magnetoelectric, and topological characteristics through first-principles calculations. The monolayers Co3Sn3SSe and Co3Pb3SSe are classified as kagome ferromagnets, and they exhibit stable perpendicular magnetic anisotropy energy. These materials can achieve the intrinsic high Chern number quantum anomalous Hall effect with C = -3. The band gaps of Co3Sn3SSe and Co3Pb3SSe are 46 and 59 meV, respectively, which are larger than the thermal energy at room-temperature scale. Additionally, our findings demonstrate that both the band gap and magnetic anisotropy energy of the monolayers Co3Sn3SSe and Co3Pb3SSe are sensitive to applied strain. This research presents intriguing and alternative possibilities for advancing intrinsic high Chern number quantum anomalous Hall devices.

Monolayer M2X2O as potential 2D altermagnets and half-metals: a first principles study

Realizing novel two-dimensional (2D) magnetic states would accelerate the development of advanced spintronic devices and the understandings of 2D magnetic physics. In this paper, we have examined the magnetic and electronic properties of 20 dynamically stable and exfoliable M2X2O (M = Ti-Ni; X = S-Te; excluding Co2Te2O). It has been unveiled that [X4O2]-D2hand [M4]-D4hcrystal fields govern the M-3dorbital splittings in M2X2O. The splittings further lead to the antiferromagnetic (AFM) orderings in Ti2S2O/Fe2S2O/Fe2Se2O/M2X2O (M = V, Cr, Mn and Ni; X = S-Se) as well as the ferromagnetic orderings in Ti2Se2O/Ti2Te2O/Fe2Te2O/Co2S2O/Co2Se2O through kinetic and superexchange mechanisms. Notably, all the AFM M2X2O are 2D altermagnets, and Ti2Se2O/Ti2Te2O/Co2S2O/Co2Se2O are 2D half-metals. In particular, the anisotropicd-d/phoppings lead to the tunable altermagnetic splitting in Ti2S2O/Cr2Te2O, while the parity of V-3dyzorbital contributes to the symmetry-protected altermagnetic splitting within V2X2O. These altermagnetic and half-metallic monolayer M2X2O provide promising candidates applied in low-dimensional spintronic devices. In addition, the potential 2D altermagnetic Weyl semimetal of Fe2S2O/Fe2Se2O, nodal-loop half-metal of Ti2Se2O and half-semi metal of Ti2Te2O facilitate to uncover novel low-dimensional topological physics. These theoretical results would expand the platform in particular for 2D altermagnets and nontrivial systems.

Multi-level chiral edge states in Janus M2XS2Se2 (M = V, Ti; X = W, Mo) monolayers with high Curie temperature and sizable nontrivial topological gaps

Quantum anomalous Hall (QAH) insulators with dissipation-less chiral edge channels provide ideal platforms for the exploration of topological materials and low-power spintronic devices. However, the ultralow operation temperature and small nontrivial gaps are the bottlenecks for QAH insulators towards future applications. Here, a new family of QAH insulators, that is, Janus M2XS2Se2 (M = V, Ti; X = W, Mo) monolayers, are proposed to be ferromagnets with large perpendicular magnetic anisotropy (PMA) and high Curie temperature above room temperature. Moreover, the present M2XS2Se2 monolayers hold sizable nontrivial topological gaps, resulting in the 1st chiral edge state with Chern number C = -1. Unexpectedly, there also exists an occupied 2nd chiral edge state below the Fermi level. Although all M2XS2Se2 monolayers retain their PMA characteristics on application of biaxial strain, various topological phase transitions are present. The V2WS2Se2 monolayer preserves the QAH state regardless of strain, while the V2MoS2Se2 and Ti2WS2Se2 monolayers transform from QAH states to metallic states under tensile strains. The present M2XS2Se2 monolayers show competitive advantages among the reported materials for the development of topological electronic devices.

All-in-One Magneto-optical Memory Arrays Based on a Two-Dimensional Ferromagnetic Metal

Two-dimensional (2D) van der Waals (vdW) magnetic materials with atomic-scale thickness and smooth interfaces promise the possibility of developing high-density, energy-efficient spintronic devices. However, it remains a challenge to effectively control the perpendicular magnetic anisotropy (PMA) of 2D vdW ferromagnetic materials, as well as the integration of multiple memory cells. Here, we report highly efficient magneto-optical memory arrays by utilizing the huge spin-orbit torques (SOT) induced by the in-plane current in Fe3GeTe2 (FGT) flake. The device is constructed from individual FGT flakes without heavy metal assistance and allows for a low current density. The magneto-optical memory arrays implement nonvolatile memories for three bits and can be repeatedly scrubbed for "writing" and "reading". Besides, we show that FGT nanoflakes possess current-controlled volatile switching behavior at zero magnetic field. These results provide a solution for the next generation of all-vdW-scalable, high-performance spintronic logic devices and SOT-Magnetic Random Access Memory (MRAM).