Imaging the Néel vector switching in the monolayer antiferromagnet MnPSe3 with strain-controlled Ising order

Observation of Three-State Nematicity and Domain Evolution in Atomically Thin Antiferromagnetic NiPS3

Nickel phosphorus trisulfide (NiPS3), a van der Waals 2D antiferromagnet, has received significant interest for its intriguing properties in recent years. However, despite its fundamental importance in the physics of low-dimensional magnetism and promising potential for technological applications, the study of magnetic domains in NiPS3 down to an atomically thin state is still lacking. Here, we report the layer-dependent magnetic characteristics and magnetic domains in NiPS3 by employing linear dichroism spectroscopy, polarized microscopy, spin-correlated photoluminescence, and Raman spectroscopy. Our results reveal the existence of the paramagnetic-to-antiferromagnetic phase transition in bulk to bilayer NiPS3 and provide evidence of the role of stronger spin fluctuations in thin NiPS3. Furthermore, our study identifies three distinct antiferromagnetic domains within atomically thin NiPS3 and captures the thermally activated domain evolution. Our findings provide crucial insights for the development of antiferromagnetic spintronics and related technologies.

Acoustomagnonic Spin Hall Effect in Honeycomb Antiferromagnets

The recently discovered Van der Waals antiferromagnets have suffered from the lack of a comprehensive method to study their magnetic properties. Here, we propose an ac intrinsic magnon spin Hall current driven by surface acoustic waves as a novel probe for such antiferromagnets. Our results pave the way towards mechanical detection and manipulation of the magnetic order in two-dimensional antiferromagnets. Furthermore, they will overcome the difficulties with weak magnetic responses inherent in the use of antiferromagnets and hence provide a building block for future antiferromagnetic spintronics.

Giant Modulation of the Second Harmonic Generation by Magnetoelectricity in Two-Dimensional Multiferroic CuCrP2S6

Multiferroic materials have attracted considerable attention owing to their unique magnetoelectric or magnetooptical properties. The recent discovery of few-layer van der Waals multiferroic crystals provides a new research direction for controlling the multiferroic properties in the atomic layer limit. However, research on few-layer multiferroic crystals is limited and the effect of thickness-dependent symmetries on those properties is less explored. In this study, the symmetries and magnetoelectric responses of van der Waals multiferroic CuCrP2S6 are investigated by optical second harmonic generation (SHG). Structural and magnetic phase transitions are successfully probed by the temperature-dependent SHG signals, revealing significant changes by applying the magnetic field reflecting the magnetoelectric effect. Moreover, it is found that symmetries and resultant magnetoelectric responses can be modulated by the number of layers. These results offer a new principle of controlling the multiferroicity and indicate that 2D van der Waals multiferroic material is a promising building block for functional nanodevices.

A Route to Two-Dimensional Room-Temperature Organometallic Multiferroics: The Marriage of d-p Spin Coupling and Structural Inversion Symmetry Breaking

Two-dimensional (2D) room-temperature multiferroic materials are highly desirable but still very limited. Herein, we propose a potential strategy to obtain such materials in 2D metal-organic frameworks (MOFs) by utilizing the d-p direct spin coupling in conjunction with center-symmetry-breaking six-membered heterocyclic rings. Based on this strategy, a screening of 128 2D MOFs results in the identification of three multiferroics, that is, Cr(1,2-oxazine)2, Cr(1,2,4-triazine)2, and Cr(1,2,3,4-trazine)2, simultaneously exhibiting room-temperature ferrimagnetism and ferroelectricity/antiferroelectricity. The room-temperature ferrimagnetic order (306-495 K) in these MOFs originates from the strong d-p direct magnetic exchange interaction between Cr cations and ligand anions. Specifically, Cr(1,2-oxazine)2 exhibits ferroelectric behavior with an out-of-plane polarization of 4.24 pC/m, whereas the other two manifest antiferroelectric characteristics. Notably, all three materials present suitable polarization switching barriers (0.18-0.31 eV). Furthermore, these MOFs are all bipolar magnetic semiconductors with moderate band gaps, in which the spin direction of carriers can be manipulated by electrical gating.

Magnetic order in 2D antiferromagnets revealed by spontaneous anisotropic magnetostriction

The temperature dependent order parameter provides important information on the nature of magnetism. Using traditional methods to study this parameter in two-dimensional (2D) magnets remains difficult, however, particularly for insulating antiferromagnetic (AF) compounds. Here, we show that its temperature dependence in AF MPS3 (M(II) = Fe, Co, Ni) can be probed via the anisotropy in the resonance frequency of rectangular membranes, mediated by a combination of anisotropic magnetostriction and spontaneous staggered magnetization. Density functional calculations followed by a derived orbital-resolved magnetic exchange analysis confirm and unravel the microscopic origin of this magnetization-induced anisotropic strain. We further show that the temperature and thickness dependent order parameter allows to deduce the material's critical exponents characterising magnetic order. Nanomechanical sensing of magnetic order thus provides a future platform to investigate 2D magnetism down to the single-layer limit.

Spin-Driven Ferroelectricity in Two-Dimensional Magnetic Heterostructures

Achieving magnetic control of ferroelectricity or electric control of magnetism is usually challenging in material systems as their magnetism and ferroelectricity have distinct fundamental origins and are subject to different symmetry constraints. However, such control has significant promise for a wide range of device applications. In this work, we employ first-principles density functional theory calculations to demonstrate the emergence of spin-driven ferroelectricity in a vertically stacked two-dimensional (2D) van der Waals magnetic heterostructure, formed by two ferromagnetic (FM) CrBr3 layers separated by an antiferromagnetic (AFM) MnPSe3 layer, delicately designed to be structurally inversion symmetric but magnetically asymmetric. The spin-induced out-of-plane electric polarization of the entire heterostructure can be reversibly controlled by an external magnetic field. We further validate the effectiveness of this design strategy in several other lattice-matched FM/AFM/FM heterostructures, thereby providing a novel family of multiferroic systems based on 2D materials.

First-principles study of magnetic interactions and excitations in antiferromagnetic van der Waals material MPX3(M=Mn, Fe, Co, Ni; X=S, Se)

Transition metal phosphorus trichalcogenides MPX3(M = Mn, Fe, Co, Ni; X = S, Se), as layered van der Waals antiferromagnetic (AFM) materials, have emerged as a promising platform for exploring two-dimensional (2D) magnetism. Based on density functional theory, we present a comprehensive investigation of the electronic and magnetic properties of MPX3. We calculated the spin exchange interactions as well as magnetocrystalline anisotropy energy. The numerical results reveal thatJ3is AFM in all cases, andJ2is significantly smaller compared to bothJ3andJ1. This behavior can be understood with regard to exchange paths and electron filling. Compared to other materials within this family, FePS3and CoPS3demonstrate significant easy-axis anisotropy. Using the obtained parameters, we estimated the Néel temperatureTNand Curie-Weiss temperatureθCW, and the results are in good agreement with the experimental observations. We further calculated the magnon spectra and successfully reproduce several typical features observed experimentally. Finally, we give helpful suggestions for the strong constraints about the range of non-negligible magnetic interactions based on the relations between magnon eigenvalues at high-symmetrykpoints in honeycomb lattices.

Proximity-induced chiral quantum light generation in strain-engineered WSe2/NiPS3 heterostructures

Quantum light emitters capable of generating single photons with circular polarization and non-classical statistics could enable non-reciprocal single-photon devices and deterministic spin-photon interfaces for quantum networks. To date, the emission of such chiral quantum light relies on the application of intense external magnetic fields, electrical/optical injection of spin-polarized carriers/excitons or coupling with complex photonic metastructures. Here we report the creation of free-space chiral quantum light emitters via the nanoindentation of monolayer WSe2/NiPS3 heterostructures at zero external magnetic field. These quantum light emitters emit with a high degree of circular polarization (0.89) and single-photon purity (95%), independent of pump laser polarization. Scanning diamond nitrogen-vacancy microscopy and temperature-dependent magneto-photoluminescence studies reveal that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of defects in the antiferromagnetic order of NiPS3, both of which are co-localized by strain fields associated with the nanoscale indentations.

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.

Intrinsic Nonlinear Hall Detection of the Néel Vector for Two-Dimensional Antiferromagnetic Spintronics

The respective unique merit of antiferromagnets and two-dimensional (2D) materials in spintronic applications inspires us to exploit 2D antiferromagnetic spintronics. However, the detection of the Néel vector in 2D antiferromagnets remains a great challenge because the measured signals usually decrease significantly in the 2D limit. Here we propose that the Néel vector of 2D antiferromagnets can be efficiently detected by the intrinsic nonlinear Hall (INH) effect which exhibits unexpected significant signals. As a specific example, we show that the INH conductivity of the monolayer manganese chalcogenides MnX (X=S, Se, Te) can reach the order of nm·mA/V^{2}, which is orders of magnitude larger than experimental values of paradigmatic antiferromagnetic spintronic materials. The INH effect can be accurately controlled by shifting the chemical potential around the band edge, which is experimentally feasible via electric gating or charge doping. Moreover, we explicitly demonstrate its 2π-periodic dependence on the Néel vector orientation based on an effective k·p model. Our findings enable flexible design schemes and promising material platforms for spintronic memory device applications based on 2D antiferromagnets.

2D Ferroic Materials for Nonvolatile Memory Applications

The emerging nonvolatile memory technologies based on ferroic materials are promising for producing high-speed, low-power, and high-density memory in the field of integrated circuits. Long-range ferroic orders observed in 2D materials have triggered extensive research interest in 2D magnets, 2D ferroelectrics, 2D multiferroics, and their device applications. Devices based on 2D ferroic materials and heterostructures with an atomically smooth interface and ultrathin thickness have exhibited impressive properties and significant potential for developing advanced nonvolatile memory. In this context, a systematic review of emergent 2D ferroic materials is conducted here, emphasizing their recent research on nonvolatile memory applications, with a view to proposing brighter prospects for 2D magnetic materials, 2D ferroelectric materials, 2D multiferroic materials, and their relevant devices.

Probing the Néel-Type Antiferromagnetic Order and Coherent Magnon-Exciton Coupling in Van Der Waals VPS3

2D van der Waals (vdW) antiferromagnets have received intensive attention due to their terahertz resonance, multilevel magnetic-order states, and ultrafast spin dynamics. However, accurately identifying their magnetic configuration still remains a challenge owing to the lack of net magnetization and insensitivity to external fields. In this work, the Néel-type antiferromagnetic (AFM) order in 2D antiferromagnet VPS₃ with the out-of-plane anisotropy, which is demonstrated by the temperature-dependent spin-phonon coupling and second-harmonic generation (SHG), is experimentally probed. This long-range AFM order even persists at the ultrathin limit. Furthermore, strong interlayer exciton-magnon coupling (EMC) upon the Néel-type AFM order is detected based on the monolayer WSe₂ /VPS₃ heterostructure, which induces an enhanced excitonic state and further certifies the Néel-type AFM order of VPS₃ . The discovery provides optical routes as the novel platform to study 2D antiferromagnets and promotes their potential applications in magneto-optics and opto-spintronic devices.

Evidence for Topological Magnon-Phonon Hybridization in a 2D Antiferromagnet down to the Monolayer Limit

Topological phonons and magnons potentially enable low-loss, quantum coherent, and chiral transport of information and energy at the atomic scale. Van der Waals magnetic materials are promising to realize such states due to their recently discovered strong interactions among the electronic, spin, and lattice degrees of freedom. Here, we report the first observation of coherent hybridization of magnons and phonons in monolayer antiferromagnet FePSe₃ by cavity-enhanced magneto-Raman spectroscopy. The robust magnon-phonon cooperativity in the 2D limit occurs even in zero magnetic field, which enables nontrivial band inversion between longitudinal and transverse optical phonons caused by the strong coupling with magnons. The spin and lattice symmetry theoretically guarantee magnetic-field-controlled topological phase transition, verified by nonzero Chern numbers calculated from the coupled spin-lattice model. The 2D topological magnon-phonon hybridization potentially offers a new route toward quantum phononics and magnonics with an ultrasmall footprint.

Switchable Giant Bulk Photocurrents and Photo-spin-currents in Monolayer PT-Symmetric Antiferromagnet MnPSe3

Converting light into steady currents and spin-currents in two-dimensional (2D) platform is essential for future energy harvesting and spintronics. We show that the giant and modulable bulk photovoltaic effects (BPVEs) can be achieved in air-stable 2D antiferromagnet (AFM) monolayer MnPSe₃, with nonlinear photoconductance >4000 nm·μA/V² and photo-spin-conductance >2000 (nm·μA/V²ℏ/2e) in the visible spectrum. The propagation and the spin-polarizations of photocurrents can be switched via simply rotating the Néel vector. We unveil that the PT-symmetry, mirror symmetries, and spin-orbital-couplings are the keys for the observed sizable and controllable 2D BPVEs. All the results provide insights into the BPVEs of 2D AFM and suggest that the layered MnPSe₃ is an outstanding 2D platform for energy device and photo-spintronics.

Unexpected spontaneous symmetry breaking and diverse ferroicity in two-dimensional mono-metal phosphorus chalcogenides

Mono-metal phosphorus trichalcogenides (MPX₃) have attracted intensive interest due to their intriguing magnetic properties and potential applications. Generally, single-layer two-dimensional (2D) MPX₃ are believed to be centrosymmetric. However, we discovered that unexpected spontaneous symmetry breaking may occur in some 2D MPX₃, i.e., vertical P-P dimers move out of the plane and become tilted, leading to the structural stability being enhanced, the inversion symmetry being simultaneously broken, and ferroelectricity or ferroelasticity emerging. By systematically investigating the family (176) of 2D MPX₃, we found that 34 members undergo such symmetry breaking during geometric optimization, in which ten are identified to be dynamically stable. We show that the mismatch between the triangular sublattice of P-P dimers and the hexagonal sublattice of M atoms and the variable accommodation of P lone-pair electrons in different valence states of M atoms play dominant roles in the inversion symmetry breaking and the emergence of ferroicity. We obtained a ferroic atlas of the whole 2D MPX₃ family, which also includes many stable antiferromagnetic and non-ferroic members that have never been reported. Our work not only presents ferroelectricity in the 2D MPX₃ family but also reveals how diverse ferroicity emerges with various spontaneous symmetry breakings, which will be helpful for further exploration of 2D ferroic materials.

Anomalous Second Harmonic Generation from Atomically Thin MnBi2Te4

MnBi₂Te₄ is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature-dependent SHG measurements on MnBi₂Te₄ flakes ranging from bulk to monolayer thickness. We find that the dominant SHG signal from MnBi₂Te₄ is unexpectedly unrelated to both magnetic state and layer number. We suggest that surface SHG is the likely source of the observed strong SHG, whose symmetry matches that of the MnBi₂Te₄-vacuum interface. Our results highlight the importance of considering the surface contribution to inversion symmetry-breaking in van der Waals centrosymmetric magnets.

The impacts of molecular adsorption on antiferromagnetic MnPS3 monolayers: enhanced magnetic anisotropy and intralayer Dzyaloshinskii-Moriya interaction

In two-dimensional (2D) magnetic systems, significant magnetic anisotropy is required to protect magnetic ordering against thermal fluctuation. In this paper, we explored the effect of molecular adsorption on the magnetic anisotropy and intralayer Dzyaloshinskii-Moriya interaction (DMI) of monolayer MnPS₃, combining the first-principles calculation and theoretical analysis. We find that molecular adsorption can break the spatial inversion symmetry in a 2D magnet, and results in a significant DMI, which is rare in pristine 2D magnets. For example, in an MPS-NO system, the magnitude of the asymmetric DMI vector increases 9 times, and the magnetocrystalline anisotropy increases 600 times compared with the pristine MPS monolayer. It is found the DMI mainly comes from the structural deformation after adsorption, whereas the increase of magnetocrystalline anisotropy mainly originates from a new 'bridge' super-exchange interaction between Mn ions and NO gas molecules. The calculated Mn-NO-Mn 'bridge' super-exchange coupling strength is much higher than the Mn-S-Mn coupling strength. Our findings offer a new strategy to increase the magnetic anisotropy and induce chiral magnetic structures in 2D magnets.

Tunable Strong Magnetic Anisotropy in Two-Dimensional van der Waals Antiferromagnets

We show that the anisotropic energy of a 2D antiferromagnet is greatly enhanced via stacking on a magnetic substrate layer, arising from the sublattice-dependent interlayer magnetic interaction that defines an effective anisotropic energy. Interestingly, this effective energy couples strongly with the interlayer stacking order and the magnetic order of the substrate layer, providing unique mechanical and magnetic means to control the antiferromagnetic order. These two types of control methods distinctly affect the sublattice magnetization dynamics, with a change in the ratio of sublattice precession amplitudes in the former and its chirality in the latter. In moiré superlattices formed by a relative twist or strain between the layers, the coupling with stacking order introduces a landscape of effective anisotropic energy across the moiré, which can be utilized to create nonuniform antiferromagnetic textures featuring periodically localized low-energy magnons.

Magnon-Coupled Intralayer Moiré Trion in Monolayer Semiconductor-Antiferromagnet Heterostructures

Moiré fringe patterns created by stacking different 2D layered materials as artificial van der Waals (vdW) heterostructures have become a novel platform to study and engineer optically generated excitonic properties. The moiré patterns contribute to the formation of spatially ordered excitonic states (excitons and trions), which can be used in the quantum simulation of many-body systems and ensembles of coherent quantum light emitters. The intriguing moiré excitonic properties are affected by and controlled via the interaction with magnetic elements. Here, a moiré excitonic system interacting with the magnetic elementary excitation of antiferromagnetic orders in MoSe₂ /MnPS₃ vdW heterostructures is reported. The low-temperature photoluminescence spectra with additional fine spectral structures on the low-energy side, which are coupled magnon-trion peaks below the Néel temperature of MnPS₃ , are carefully investigated. The fine spectral structures with long lifetime and coherence time are assigned to intralayer trion-magnon complexes trapped in the moiré potentials (moiré trion-magnon complexes). These findings highlight the emergence of moiré trion-magnon complexes and provide a new way to explore novel quantum phenomena in moiré excitonic systems with magnetic functionalities.

Observation of Giant Surface Second-Harmonic Generation Coupled to Nematic Orders in the van der Waals Antiferromagnet FePS3

Second-harmonic generation has been applied to study lattice, electronic, and magnetic proprieties in atomically thin materials. However, inversion symmetry breaking is usually required for the materials to generate a large signal. In this work, we report a giant second-harmonic generation that arises below the Néel temperature in few-layer centrosymmetric FePS₃. A layer-dependent study indicates the detected signal is from the second-order nonlinearity of the surface. The magnetism-induced surface second-harmonic response is 2 orders of magnitude larger than those reported in other magnetic systems, with the surface nonlinear susceptibility reaching 0.08-0.13 nm²/V in 2-5 L samples. By combing linear dichroism and second-harmonic generation experiments, we further confirm the giant second-harmonic generation is coupled to nematic orders formed by the three possible Zigzag antiferromagnetic domains. Our study shows that the surface second-harmonic generation is also a sensitive tool to study antiferromagnetic states in centrosymmetric atomically thin materials.

Reversible strain-induced magnetic phase transition in a van der Waals magnet

Mechanical deformation of a crystal can have a profound effect on its physical properties. Notably, even small modifications of bond geometry can completely change the size and sign of magnetic exchange interactions and thus the magnetic ground state. Here we report the strain tuning of the magnetic properties of the A-type layered antiferromagnetic semiconductor CrSBr achieved by designing a strain device that can apply continuous, in situ uniaxial tensile strain to two-dimensional materials, reaching several percent at cryogenic temperatures. Using this apparatus, we realize a reversible strain-induced antiferromagnetic-to-ferromagnetic phase transition at zero magnetic field and strain control of the out-of-plane spin-canting process. First-principles calculations reveal that the tuning of the in-plane lattice constant strongly modifies the interlayer magnetic exchange interaction, which changes sign at the critical strain. Our work creates new opportunities for harnessing the strain control of magnetism and other electronic states in low-dimensional materials and heterostructures.

Visualizing rotation and reversal of the Néel vector through antiferromagnetic trichroism

Conventional magnetic memories rely on bistable magnetic states, such as the up and down magnetization states in ferromagnets. Increasing the number of stable magnetic states in each cell, preferably composed of antiferromagnets without stray fields, promises to achieve higher-capacity memories. Thus far, such multi-stable antiferromagnetic states have been extensively studied in conducting systems. Here, we report on a striking optical response in the magnetoelectric collinear antiferromagnet Bi₂CuO₄, which is an insulating version of the representative spintronic material, CuMnAs, with four stable Néel vector orientations. We find that, due to a magnetoelectric effect in a visible range, which is enhanced by a peculiar local environment of Cu ions, absorption coefficient takes three discrete values depending on an angle between the propagation vector of light and the Néel vector—a phenomenon that we term antiferromagnetic trichroism. Furthermore, using this antiferromagnetic trichroism, we successfully visualize field-driven reversal and rotation of the Néel vector.

Antiferromagnets have great promise for use in spin-based electronics; however, detecting the Neel vector is challenging due to the lack of a net magnetization. Here, Kimura et al demonstrate an intriguing optical response, where the optical absorption depends on the angle of the Neel vector.

Anisotropic Second-Harmonic Generation Induced by Reduction of In-Plane Symmetry in 2D Materials with Strain Engineering

Strain engineering is an attractive method to induce and control anisotropy for polarized optoelectronic applications with two-dimensional (2D) materials. Herein, we have investigated the nonlinear optical coefficient dispersion relationship and the second-harmonic generation (SHG) pattern evolution under the uniaxial strains for graphene, WS₂, GaSe, and In₂Se₃ monolayers. The uniaxial strain can break the in-plane symmetry of 2D materials, leading to both trade-off breaking of the nonlinear coefficient and new emergent nonlinear coefficients. In such a case, a classical sixfold ϕ-dependent SHG pattern is transformed into a distorted sixfold SHG pattern under the strain. Due to the lattice symmetry breaking and the uneven charge density distribution in strained 2D materials, the SHG patterns also depend on the excitation photon energy. The results could give a guide for the SHG pattern analysis in experiments, suggesting strain engineering on 2D materials for the tunable anisotropy in polarized and flexible nonlinear optical devices.

Hydrogen-Intercalated 2D Magnetic Bilayer: Controlled Magnetic Phase Transition and Half-Metallicity via Ferroelectric Switching

Electrically controlled magnetism in two-dimensional (2D) multiferroics is highly desirable for both fundamental research and the future development of low-power nanodevices. Herein, inspired by the recently experimentally realized 2D antiferromagnetic MnPSe₃ [ Nat. Nanotechnol. 2021, 16 (7), 782] and guided by a heteromagnetic structural design, we engineer strong magnetoelectric coupling in a hydrogen-intercalated 2D MnPSe₃ bilayer. Hydrogen functionalization breaks the centrosymmetry of bilayer MnPSe₃, leading to out-of-plane ferroelectricity. Moreover, there is a phase transition from antiferromagnetic semiconductor to ferromagnetic half-metal in the H-bonded MnPSe₃ layer, while the other remains antiferromagnetic and semiconducting. When reversing the electrical polarization, the intercalated H atom can flip between the top and bottom layers with an ultralow switching barrier, which allows one to tune the magnetic order and conductivity of the individual layers via an external electric field. Our results pave a new avenue to realize strong magnetoelectric coupling in single-phase multiferroic material. The ferroelectricity-controlled magnetic phase transition and half-metallicity offer promising applications in nanoscale spintronics such as electrically written and magnetically read memories.

Intrinsic staggered spin-orbit torque for the electrical control of antiferromagnets - application to CrI3

Spin-orbit torque enables the electrical control of the orientation of ferromagnets' or antiferromagnets' order parameter. In this work we consider antiferromagnets in which the magnetic sublattices are connected by inversion+time reversal symmetry, and in which the exchange and anisotropy energies are similar in magnitude. We identify the staggered dampinglike spin-orbit torque as the key mechanism for electrical excitation of the Néel vector for this case. To illustrate this scenario, we examine the 2-d Van der Waals antiferromagnetic bilayer CrI₃, in the n-doped regime. Using a combination of first-principles calculations of the spin-orbit torque and an analysis of the ensuing spin dynamics, we show that the deterministic electrical switching of the Néel vector is the result of dampinglike spin-orbit torque which is staggered on the magnetic sublattices.

Magnon-phonon hybridization in 2D antiferromagnet MnPSe3

Magnetic excitations in van der Waals (vdW) materials, especially in the two-dimensional (2D) limit, are an exciting research topic from both the fundamental and applied perspectives. Using temperature-dependent, magneto-Raman spectroscopy, we identify the hybridization of two-magnon excitations with two phonons in manganese phosphorus triselenide (MnPSe₃), a magnetic vdW material that hosts in-plane antiferromagnetism. Results from first-principles calculations of the phonon and magnon spectra further support our identification. The Raman spectra’s rich temperature dependence through the magnetic transition displays an avoided crossing behavior in the phonons’ frequency and a concurrent decrease in their lifetimes. We construct a model based on the interaction between a discrete level and a continuum that reproduces these observations. Our results imply a strong hybridization between each phonon and a two-magnon continuum. This work demonstrates that the magnon-phonon interactions can be observed directly in Raman scattering and provides deep insight into these interactions in 2D magnetic materials.

Direct Imaging of Antiferromagnetic Domains and Anomalous Layer-Dependent Mirror Symmetry Breaking in Atomically Thin MnPS_{3}

We have developed a sensitive cryogenic second-harmonic generation microscopy to study a van der Waals antiferromagnet MnPS_{3}. We find that long-range Néel antiferromagnetic order develops from the bulk crystal down to the bilayer, while it is absent in the monolayer. Before entering the long-range antiferromagnetic ordered phase in all samples, an upturn of the second harmonic generation below 200 K indicates the formation of the short-range order and magnetoelastic coupling. We also directly image the two antiphase (180°) antiferromagnetic domains and thermally induced domain switching down to bilayer. An anomalous mirror symmetry breaking shows up in samples thinner than ten layers for the temperature both above and below the Néel temperature, which indicates a structural change in few-layer samples. Minimal change of the second harmonic generation polar patterns in strain tuning experiments indicate that the symmetry crossover at ten layers is most likely an intrinsic property of MnPS_{3} instead of an extrinsic origin of substrate-induced strain. Our results show that second harmonic generation microscopy is a direct tool for studying antiferromagnetic domains in atomically thin materials, and opens a new way to study two-dimensional antiferromagnets.