Spin Textures Research Articles

Theory of Moiré Magnetism and Multidomain Spin Textures in Twisted Mott Insulator-Semimetal Heterobilayers.

Motivated by the recent experimental developments in van der Waals heterostructures, we investigate the emergent magnetism in Mott insulator-semimetal moiré superlattices by deriving effective spin models and exploring their phase diagram by Monte Carlo simulations. Our analysis indicates that the stacking-dependent interlayer Kondo interaction can give rise to different types of magnetic order, forming domains within the moiré unit cell. In particular, we find that the AB (AA) stacking regions tend to order (anti)ferromagnetically for an extended range of parameters. The remaining parts of the moiré unit cell form ferromagnetic chains that are coupled antiferromagnetically. We show that the decay length of the Kondo interaction can control the extent of these phases. Our results highlight the importance of stacking-dependent interlayer exchange and the rich magnetic spin textures that can be obtained in van der Waals heterostructures.

Influence of Magnetic Sublattice Ordering on Skyrmion Bubble Stability in 2D Magnet Fe5GeTe2.

The realization of above room-temperature ferromagnetism in the two-dimensional (2D) magnet Fe5GeTe2 represents a major advance for the use of van der Waals (vdW) materials in practical spintronic applications. In particular, observations of magnetic skyrmions and related states within exfoliated flakes of this material provide a pathway to the fine-tuning of topological spin textures via 2D material heterostructure engineering. However, there are conflicting reports as to the nature of the magnetic structures in Fe5GeTe2. The matter is further complicated by the study of two types of Fe5GeTe2 crystals with markedly different structural and magnetic properties, distinguished by their specific fabrication procedure: whether they are slowly cooled or rapidly quenched from the growth temperature. In this work, we combine X-ray and electron microscopy to observe the formation of magnetic stripe domains, skyrmion-like type-I, and topologically trivial type-II bubbles, within exfoliated flakes of Fe5GeTe2. The results reveal the influence of the magnetic ordering of the Fe1 sublattice below 150 K, which dramatically alters the magnetocrystalline anisotropy and leads to a complex magnetic phase diagram and a sudden change of the stability of the magnetic textures. In addition, we highlight the significant differences in the magnetic structures intrinsic to slow-cooled and quenched Fe5GeTe2 flakes.

Interplay between Topological States and Rashba States as Manifested on Surface Steps at Room Temperature.

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, we report room-temperature scanning tunneling microscopy/spectroscopy observation of interaction between Rashba states and topological surface states, which manifests local electronic structure along step edges controllable by the layer thickness of thin films. The first-principles theoretical calculation elucidates the robust Rashba states coexisting with topological surface states along the surface steps with characteristic spin textures in momentum space. Furthermore, the Rashba edge states can be switched off by reducing the thickness of a topological insulator Bi2Se3 to bolster their interaction with the hybridized topological surface states. The study unveils a manipulating mechanism of the spin textures at room temperature, reinforcing the necessity of thin film technology in controlling the quantum states.

Emergent chiral spin textures in centrosymmetric iron garnet with spin alignment constraints

Chiral spin textures have emerged as a captivating class of topological matter. These intriguing structures harbor localized and topologically protected magnetic textures, rendering them promising candidates for novel spintronic applications. Here, the emergent chiral spin textures in an iron garnet with nanodisk configuration have been micromagnatically simulated based on the Landau-Lifshitz-Gilbert equation (LLG) of COMSOL Multiphysics. The modification of magnetic spin texture in the considered iron garnet has been controlled by introducing the interfacial Dzyaloshinskii-Moriya interaction (I-DMI) and spin alignment constraints as locally pinned spins and curvilinear defects at edges. These boundary conditions induced interesting chiral textures not commonly observed in such iron garnet due to its centrosymmetric crystal structure. The simulation results showed the presence of two distinct regions of spin texture: inside and outside the pinning boundary. By systematically varying the DMI strength (D) with the size of the pinning boundary (Rpin) and the number of defects (Ndef), the emerging chiral spin textures have been explored and reported, including the existence of ramified helical stripes, skyrmions, and skyrmions-like textures such as elongated skyrmions, chiral horseshoe domains, biskyrmions and target skyrmions (TSk) (2π-TSk and 3π-TSk). Also, it has been reported that various collapses occur in the final spin texture states, leading to the transition from one state to another when varying the values of Rpin and Ndef with D.

New approach to measuring topological phase transitions utilizing Floquet technology

Abstract The Floquet technique provides a novel anomalous topological phase for non-equilibrium phase transitions. Based on the high symmetry of the quantum anomalous Hall model, the findings suggest a one-to-one correspondence between the average spin texture and the Floquet quasi-energy spectrum. A new approach is proposed to directly measure the quasi-energy spectrum, replacing previous measurements of the average spin texture. Finally, we proposed a reliable experimental scheme based on ion trap platforms. This scheme markedly reduces the measurement workload, improves the measurement fidelity, and is applicable to multiple platforms such as cold atoms and nuclear magnetic resonance.

Controlling Noncollinear Ferromagnetism in van der Waals Metal-Organic Magnets.

Van der Waals (vdW) magnets both allow exploration of fundamental 2D physics and offer a route toward exploiting magnetism in next generation information technology, but vdW magnets with complex, noncollinear spin textures are currently rare. We report here the syntheses, crystal structures, magnetic properties and magnetic ground states of four bulk vdW metal-organic magnets (MOMs): FeCl2(pym), FeCl2(btd), NiCl2(pym), and NiCl2(btd), pym = pyrimidine and btd = 2,1,3-benzothiadiazole. Using a combination of neutron diffraction and bulk magnetometry we show that these materials are noncollinear magnets. Although only NiCl2(btd) has a ferromagnetic ground state, we demonstrate that low-field hysteretic metamagnetic transitions produce states with net magnetization in zero-field and high coercivities for FeCl2(pym) and NiCl2(pym). By combining our bulk magnetic data with diffuse scattering analysis and broken-symmetry density-functional calculations, we probe the magnetic superexchange interactions, which when combined with symmetry analysis allow us to suggest design principles for future noncollinear vdW MOMs. These materials, if delaminated, would prove an interesting new family of 2D magnets.

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, we summarize current research on promising topological spin structures in addition to skyrmions. Beyond two-dimensional structures, exploration also extends to one-dimensional magnetic solitons and three-dimensional spin textures. In addition, we introduce a diverse array of emerging magnetic materials, including antiferromagnets and two-dimensional 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, we offer a comprehensive overview of experimental and theoretical advances in the research of topological magnetism. Finally, we summarize both conventional and unconventional applications based on spin textures proposed thus far. This review provides an outlook on future development in applied spintronics. This article is protected by copyright. All rights reserved.

Topological Hall effect in a non-magnetic metal interfaced to a canted antiferromagnetic insulator in perovskite oxide heterostructures

We report interfacial transport properties in in situ grown orthorhombic perovskite oxide heterostructures consisting of an antiferromagnetic insulator DyFeO3 and a paramagnetic conductor CaRuO3. We observe Hall effect with a step-like increase amounting to an effective magnetic field of 30 T at 20 K. We provide a plausible explanation in the context of topological Hall effect originating from a non-coplanar spin texture and resultant emergent field in DyFeO3 associated with the scalar spin chirality. Our results demonstrate that the proximity effect of the emergent field at heterointerfaces is a universal physical phenomenon, while it has been reported originally in a heterointerface composed of pyrochlore oxides. This will greatly expand the choice of materials to the heterointerfaces for the research in emergent transport phenomena, which has been limited to single compounds with both metallic properties and special spin textures. Additionally, this will pave the way for possible device application of the emergent field by designing and combining perovskite oxides with versatile functionalities such as multiferroicity.

Asymmetric energy barrier induced magnetic skyrmion diodes in synthetic antiferromagnetic nanotracks

Magnetic skyrmions are nanoscale spin textures with topological protection properties, which are regarded as promising information carriers. When skyrmions move along nanotracks, their trajectories will shift if affected by skyrmion Hall effect and pinning effect. In this work, the influence of trapezoidal notches in synthetic antiferromagnetic nanotracks on the dynamics of skyrmions is investigated through micromagnetic simulation. The results exhibit that the asymmetric energy barrier induced by the asymmetric geometric structure can hinder reverse motion of skyrmions to realize the unidirectional conduction function of the diodes. Adjusting the height and inclination angle of the trapezoidal notch can effectively expand the current operating windows of the diodes, and the larger the height or inclination angle, the higher the threshold current density to overcome the pinning effect of the geometric energy barrier. Furthermore, the failure mode has a crucial impact on the implementation of skyrmion diodes. By utilizing the asymmetric structure, the forward conduction and reverse cutoff of skyrmion-based diodes can be achieved within 4 ns, with lower energy consumption than traditional diodes, resulting in high efficiency and low energy consumption of diodes. This work proposes a method for realizing the diode functionality by simply altering the geometric structure of the nanotracks, which may play an important role in the design and application of skyrmion-based diodes.

Unconventional Magnetic and Magneto-Transport Properties in Quasi-2D Ni0.28TaSeS Single Crystal.

Van der Waals (vdW) magnetic materials have broad application prospects in next-generation spintronics. Inserting magnetic elements into nonmagnetic vdWmaterials can introduce magnetism and enhance various transport properties. Herein, the unconventional magnetic and magneto-transport phenomena is reported in Ni0.28TaSeS crystal by intercalating Ni atoms into nonmagnetic 2H-TaSeS matrix. Magnetic characterization reveals a canted magnetic structure in Ni0.28TaSeS, which results in an antiferromagnetic (AFM) order along the c-axis and a ferromagnetic (FM) moment in the ab-plane. The presence of spin-flop (SF) behavior can also be attributed to the canted magnetic structure. Temperature-dependent resistivity exhibits a metallic behavior with an abrupt decrease corresponding to the magnetic transition. Magneto-transport measurements demonstrate a positive magnetoresistance (MR) with a plateau that is different from conventional magnetic materials. The field-dependent Hall signal exhibits nonlinear field dependence when the material is in magnetically ordered state. These unconventional magneto-transport behaviors are attributed to the field-induced formation of a complex spin texture in Ni0.28TaSeS. In addition, it further investigated the angle dependence of MR and observed an unusual fourfold anisotropic magnetoresistance (AMR) effect. This work inspires future research on spintronic devices utilizing magnetic atom-intercalated quasi-2D materials.

L -gap surface resonance at Pt(111): Influence of atomic structure, d bands, and spin-orbit interaction

Pt(111) hosts a surface resonance with peculiar properties concerning energy vs momentum dispersion and spin texture. At variance with the free-electron-like behavior of the L-gap Shockley-type surface states on the fcc(111) surfaces of Au, Ag, and Cu, it splits into several branches with distinct spin polarization around the center of the surface Brillouin zone Γ¯. Theoretical predictions based on density-functional theory vary depending on the particular functionals used. To clarify this issue, we investigate the atomic structure of Pt(111) by low-energy electron diffraction and the unoccupied electronic structure by spin- and angle-resolved inverse photoemission. The experimental results are backed by theoretical studies using different functionals, which show that the characteristics of the surface band depend critically on the lattice constant. From the analysis of the energy-dependent low-energy electron diffraction intensities, we derive structural parameters of the Pt(111) surface relaxation with high accuracy. In addition, we give an unambiguous definition of the nonequivalent mirror-plane directions Γ¯M¯ and Γ¯M¯′ at fcc(111) surfaces, which is consistent with band-structure calculations and inverse-photoemission data. Concerning the surface resonance at the bottom of the L gap, we identified a delicate interplay of several contributions. Lattice constant, hybridization with d bands, and the influence of spin-orbit interaction are critical ingredients for understanding the peculiar energy dispersion and spin character of the unoccupied surface resonance. Published by the American Physical Society 2024

Tunable spike-timing-dependent plasticity in magnetic skyrmion manipulation chambers

Magnetic skyrmions, as scalable and nonvolatile spin textures, can dynamically interact with fields and currents, making them promising for unconventional computing. This paper presents a neuromorphic device based on skyrmion manipulation chambers to implement spike-timing-dependent plasticity (STDP), a mechanism for unsupervised learning in brain-inspired computing. STDP adjusts synaptic weights based on the timing of pre-synaptic and post-synaptic spikes. The proposed three-chamber design encodes synaptic weight in the number of skyrmions in the center chamber, with left and right chambers for pre- and post-synaptic spikes, respectively. Micromagnetic simulations demonstrate that the timing between applied currents across the chambers controls the final skyrmion count (weight). The device exhibits adaptability and learning capabilities by manipulating chamber parameters, mimicking Hebbian and dendritic location-based plasticity. The device's ability to maintain state post-write highlights its potential for advancing adaptable neuromorphic devices.

Large-Gap and Topological Crystalline Insulating Phase in RbZnBi and CsZnBi.

Topological insulators (TIs) are a new class of materials with gapless boundary states inside the bulk insulating gap. This metallic boundary state hosts intriguing phenomena such as helical spin textures and Dirac crossing points. Here, we theoretically propose RbZnBi and CsZnBi as a new family of TIs exhibiting large bulk band gaps and unique gapless surface states. Our first-principles density functional calculations show that two materials can be stabilized in two different structures depending on the stacking order of hexagonal ZnBi layers. While both materials in the AA-stacked structure become TI, the AB-stacked RbZnBi and CsZnBi are topological crystalline insulators with hourglass-shaped Fermion surface states protected by nonsymmorphic glide symmetry. The calculated bulk gap is about 1.5-1.8 times larger than that of Bi2Se3, which makes RbZnBi and CsZnBi promising candidates for future applications.

Multi-terminal logic transmission via orthogonal currents controlling magnetic skyrmion motion in a perpendicular ferromagnetic multilayer nanotrack

Magnetic skyrmions are topologically nontrivial whirling spin textures with great potential for next-generation magnetic logic and memory applications as spintronic information carriers. The abilities to precisely manipulate skyrmion generation and motion using electric current are considered to be important issues for developing the high-efficiency skyrmion-based devices. In this paper, a skyrmionic logic device structure is proposed to realize the generation of Néel-type skyrmions from the topologically trivial domains. Micromagnetic simulations are employed to investigate the controllable skyrmion motions manipulated by the orthogonal current excitations in a perpendicular ferromagnetic multilayer nanotrack. The transversal skyrmion motion induced by the skyrmion Hall effect is balanced through introducing a y-axial current. A three-terminals logic output is achieved by adjusting the strength of currents along x and y directions. The interlayer coupling effect for stabilizing skyrmion nucleation is finely tuned via a ferromagnetic pinning layer in absent of the external magnetic field. It is found that both the perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction are critical for the formation of stabilized skyrmions. The controllable skyrmion motion and multi-terminal information transmission offer a novel path for designing skyrmion-based logic and memory devices.

Unusual Dzyaloshinskii-Moriya Interaction in Graphene/Fe3GeTe2 Van der Waals Heterostructure.

Dzyaloshinskii-Moriya interaction (DMI) is shown to induce a topologically protected chiral spin texture in magnetic/nonmagnetic heterostructures. In the context of van der Waals spintronic devices, graphene emerges as an excellent candidate material. However, due to its negligible spin-orbit interaction, inducing DMI to stabilize topological spins when coupled to 3d-ferromagnets remains challenging. Here, it is demonstrated that, despite these challenges, a sizeable Rashba-type spin splitting followed by significant DMI is induced in graphene/Fe3GeTe2. This is made possible due to an interfacial electric field driven by charge asymmetry together with the broken inversion symmetry of the heterostructure. These findings reveal that the enhanced DMI energy parameter, resulting from a large effective electron mass in Fe3GeTe2, remarkably contributes to stabilizing non-collinear spins below the Curie temperature, overcoming the magnetic anisotropy energy. These results are supported by the topological Hall effect, which coexists with the non-trivial breakdown of Fermi liquid behavior, confirming the interplay between spins and non-trivial topology. This work paves the way toward the design and control of interface-driven skyrmion-based devices.

Ferroelectric Control of Spin‐Orbitronics

AbstractSpin‐orbit coupling refers to the relativistic interaction between the spin and orbital motions of electrons. This interaction leads to numerous intriguing phenomena, including spin‐orbit torques, spin–momentum locking, topological spin textures, etc., that have recently gained prominence in the field of spin‐orbitronics. In particular, the emerging ferroelectric control is recognized and validated as an effective means to enhance energy efficiency across a broad spectrum of spin‐orbitronic devices. Here, cutting‐edge research on ferroelectric control of spin‐orbitronics (FECSO) by means of spontaneous polarization, reversible ferroelectric switching, and multiferroic coupling, are comprehensively reviewed. Two fascinating topics are mainly discussed: topological spin texture and spin‐charge interconversion. The classification of control mechanisms for different interactions in FECSO is summarized first. Then, from the perspective of material classification, the ferroelectric‐controlled spin‐orbit coupling with tunable topological spin texture in oxide systems, magnetic metal multilayers, and 2D van der Waals materials is reviewed. Subsequently, the ferroelectric‐tunable spin‐charge interconversion on heavy metal layers, oxide interfaces, and ferroelectric Rashba semiconductors is highlighted. In the end, the challenges and forthcoming prospects of FECSO are discussed. This work may provide pertinent and forward‐thinking guidance to accelerate the ongoing advancement of this field.

Field-free transformations of topological spin textures in ferrimagnetic TbFeCo films

The generation of topological spin textures under ultrafast laser pulse excitations and field-free manipulation of topological transitions between different spin textures has attracted enormous interest from the perspective of spintronic applications. Here, we utilize ultrafast electron microscopy to showcase the femtosecond laser pulses excitation on magnetic materials and have generated multiple topological spin textures in an amorphous ferrimagnetic TbFeCo film. Furthermore, the following field-free topological transitions between skyrmions and bubbles with diversified topology are identified via in situ heating the sample in Lorentz transmission electron microscopy. The critical role of uniaxial anisotropy variation on changing the magnetic textures during the heating process is confirmed by micromagnetic simulations. Our results provide a perspective on the generation and transformation of topological spin textures.

Electrical detection of mobile skyrmions with 100% tunneling magnetoresistance in a racetrack-like device

Magnetic skyrmions are topological spin textures that are regarded as promising information carriers for next-generation spintronic memory and computing devices. For practical applications, their deterministic generation, manipulation, and efficient detection are the most critical aspects. Although the generation and manipulation of skyrmions have been extensively studied, efficient electrical detection of mobile skyrmions by using techniques that are compatible with modern magnetic memory technology, remains to be adequately addressed. Here, through integrating magnetic multilayers that host nanoscale skyrmions, together with the magnetic tunnel junctions (MTJ), we demonstrate the electrical detection of skyrmions by using the tunneling magnetoresistance (TMR) effect with a TMR ratio that reaches over 100% at room temperature. By building prototype three-terminal racetrack-like devices, we further show the electrical detection of mobile skyrmions by recording the time-dependent TMR ratios. Along with many recent developments, our results could advance the development of skyrmionic memory and logic devices.

Sym4state.jl: An efficient computation package for magnetic materials

Exploring magnetic configurations of magnets often involves utilizing the four-state method to obtain the magnetic interaction matrix, and Monte Carlo method to simulate spin textures and phase transition processes. However, computing the interaction matrix between magnetic atoms using the four-state method requires plenty of individual calculations. Despite manual simplifying the number of individual calculations based on material's symmetry is possible, there remains a necessity for an automated approach to streamline the process for high-throughput screening of magnetic materials. Meanwhile, the traditional sequential Monte Carlo simulation encounters challenges of low efficiency and long time consuming in dealing with large systems. Furthermore, the prior parallelism in the Heisenberg model was limited to parallel computation of the system's energy or run several replicas in parallel. Hence, in our pursuit of comprehensive parallelization for the Heisenberg model, we have introduced a novel adaptation of the checkerboard algorithm, enabling a fully parallelizable simulation of the Heisenberg model. To address these problems, we have developed Sym4state.jl, a program specifically designed to simplify the computation of magnetic interaction matrix and simulate spin textures under various environmental conditions. This program, available as a Julia package, can be freely accessed at https://github.com/A-LOST-WAPITI/Sym4state.jl. Program summaryProgram title: Sym4state.jlCPC Library link to program files:https://doi.org/10.17632/s6dkmgrjfw.1Developer's repository link:https://github.com/A-LOST-WAPITI/Sym4state.jlLicensing provisions: MITProgramming language: JuliaNature of problem: Employing the four-state method to calculate magnetic interaction matrix for magnetic materials can be simplified based on material symmetry, however, there is a lack of automated approach to streamline the simplification. Additionally, the commonly used Metropolis method for simulating magnetic texture can only make parallel computation of the system's energy or run several replicas in parallel, which could hardly boost the performance when simulating the large-scale magnetic textures.Solution method: We simplify the four-state method calculations by utilizing the principles of energy invariance under symmetry operations and time reversal operations. To enhance the efficiency of the Metropolis algorithm, we have designed a strategy to divide the entire 2D lattice into multiple domains. We then execute the Metropolis algorithm in parallel for each individual domain, thereby improving the overall computational efficiency.Additional comments including restrictions and unusual features: While the methods aimed at simplifying the four-state method and parallelizing the Metropolis algorithm are applicable to both 2D and 3D systems, the current program is specifically designed for the calculation and simulation of magnetism in 2D materials. As a result, compatibility with 3D systems has not yet been implemented.

Macroscopic tunneling probe of Moiré spin textures in twisted CrI3

Various noncollinear spin textures and magnetic phases have been predicted in twisted two-dimensional CrI3 due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange from moiré stacking—with potential spintronic applications even when the underlying material possesses a negligible Dzyaloshinskii–Moriya or dipole–dipole interaction. Recent measurements have shown evidence of coexisting FM and AFM layer order in small-twist-angle CrI3 bilayers and double bilayers. Yet, the nature of the magnetic textures remains unresolved and possibilities for their manipulation and electrical readout are unexplored. Here, we use tunneling magnetoresistance to investigate the collective spin states of twisted double-bilayer CrI3 under both out-of-plane and in-plane magnetic fields together with detailed micromagnetic simulations of domain dynamics based on magnetic circular dichroism. Our results capture hysteretic and anisotropic field evolutions of the magnetic states and we further uncover two distinct non-volatile spin textures (out-of-plane and in-plane domains) at ≈1° twist angle, with a different global tunneling resistance that can be switched by magnetic field.