Topological Spin Textures Research Articles

Real-Space Topology-Engineering of Skyrmionic Spin Textures in a van der Waals Ferromagnet Fe3GaTe2.

Realizing magnetic skyrmions in two-dimensional (2D) van der Waals (vdW) ferromagnets offers unparalleled prospects for future spintronic applications. The room-temperature ferromagnet Fe3GaTe2 provides an ideal platform for tailoring these magnetic solitons. Here, skyrmions of distinct topological charges are artificially introduced and engineered by using magnetic force microscopy (MFM). The skyrmion lattice is realized by a specific field-cooling process and can be further erased and painted via delicate manipulation of the tip stray field. The skyrmion lattice with opposite topological charges (S = ±1) can be tailored at the target regions to form topological skyrmion junctions (TSJs) with specific configurations. The delicate interplay of TSJs and spin-polarized device current were finally investigated via the in situ transport measurements, alongside the topological stability of TSJs. Our results demonstrate that Fe3GaTe2 not only serves as a potential building block for skyrmion-based spintronic devices, but also presents prospects for Fe3GaTe2-based heterostructures with the engineered topological spin textures.

Stability and dynamics of magnetic skyrmions in FM/AFM heterostructures

Magnetic skyrmions have garnered attention for their potential roles in spintronic applications, such as information carriers in computation, data storage, and nano-oscillators due to their small size, topological stability, and the requirement of small electric currents to manipulate them. Two key challenges in harnessing skyrmions are the stabilization requirement through a strong out-of-plane field, and the skyrmion Hall effect (SkHE). Here, we present a systematic model study of skyrmions in ferromagnetic/antiferromagnetic (FM/AFM) multilayer structures by employing both atomistic Monte Carlo and atomistic spin dynamics simulations. We demonstrate that skyrmions stabilized by exchange bias have superior stability to field-stabilized skyrmions due to the formation of a magnetic imprint within the AFM layer. Additionally, stacking two skyrmion hosting FM layers between two AFM layers suppresses the SkHE and enables the transport of AFM-coupled skyrmions with high velocity in the order of a few km/s. This proposed multilayer configuration could serve as a pathway to overcome existing limitations in the development of skyrmion-based devices, and the insights obtained through this study contribute significantly to the broader understanding of topological spin textures in magnetic materials. Published by the American Physical Society 2024

Magnetostriction related to skyrmion-lattice formation in chiral magnet FeGe

The B20 type chiral magnet FeGe exhibits the formation of skyrmion-lattice (SkL) phases in the vicinity of the magnetic ordering temperature. The SkL is a magnetic superlattice composed of vortex-type topological spin objects, and it has experimentally been known that its formation requires the existence of an intermediate (IM) phase between SkL and the paramagnetic (PM) phases. We take interest in how the crystal lattice experiences the formation of these topological spin texture. In this study, we observed the so-called spin–orbit coupling induced magnetostriction related to these topological spin texture formation, in addition to the ac magnetization anomalies. The temperature and magnetic field dependences of the lattice parameter reflected the transformation of phases, such as helimagnetic (HM), SkL, IM, conical (CM), and PM phases. In the PM region, a phase characterized as gaseous skyrmions was detected similarly to the case of the same B20 type MnSi. Furthermore, the HM, CM, and IM phases were also divided into two regions. Thus, the precise phase diagram near Tc was reconstructed from the prospect of the magnetostriction such that we demonstrated that the stabilization of skyrmions needs a finite magnetic field.

Gesture recognition with Brownian reservoir computing using geometrically confined skyrmion dynamics

Physical reservoir computing leverages the dynamical properties of complex physical systems to process information efficiently, significantly reducing training efforts and energy consumption. Magnetic skyrmions, topological spin textures, are promising candidates for reservoir computing systems due to their enhanced stability, non-linear interactions and low-power manipulation. Traditional spin-based reservoir computing has been limited to quasi-static detection or real-world data must be rescaled to the intrinsic timescale of the reservoir. We address this challenge by time-multiplexed skyrmion reservoir computing, that allows for aligning the reservoir’s intrinsic timescales to real-world temporal patterns. Using millisecond-scale hand gestures recorded with Range-Doppler radar, we feed voltage excitations directly into our device and detect the skyrmion trajectory evolution. This method scales down to the nanometer level and demonstrates competitive or superior performance compared to energy-intensive software-based neural networks. Our hardware approach’s key advantage is its ability to integrate sensor data in real-time without temporal rescaling, enabling numerous applications.

Confined antiskyrmion motion driven by electric current excitations

Current-driven dynamics of topological spin textures, such as skyrmions and antiskyrmions, have garnered considerable attention in condensed matter physics and spintronics. As compared with skyrmions, the current-driven dynamics of their antiparticles – antiskyrmions − remain less explored due to the increased complexity of antiskyrmions. Here, we design and employ fabricated microdevices of a prototypical antiskyrmion host, (Fe0.63Ni0.3Pd0.07)3P, to allow in situ current application with Lorentz transmission electron microscopy observations. The experimental results and related micromagnetic simulations demonstrate current-driven antiskyrmion dynamics confined within stripe domains. Under nanosecond-long current pulses, antiskyrmions exhibit directional motion along the stripe regardless of the current direction, while the antiskyrmion velocity is linearly proportional to the current density. Significantly, the antiskyrmion mobility could be enhanced when the current flow is perpendicular to the stripe direction. Our findings provide novel and reliable insights on dynamical antiskyrmions and their potential implications on spintronics.

Spin order and dynamics in the topological rare-earth germanide semimetals

The REAl(Si,Ge) (RE = rare earth) family, known to break both the inversion- and time-reversal symmetries, represents one of the most suitable platforms for investigating the interplay between correlated-electron phenomena and topologically nontrivial bands. Here, we report on systematic magnetic, transport, and muon-spin rotation and relaxation (uSR) measurements on (Nd,Sm)AlGe single crystals, which exhibit antiferromagnetic (AFM) transitions at TN = 6.1 and 5.9 K, respectively. In addition, NdAlGe undergoes also an incommensurate-to-commensurate ferrimagnetic transition at 4.5 K. Weak transverse-field µSR measurements confirm the AFM transitions, featuring a ∼90% magnetic volume fraction. Zero-field (ZF) µSR measurements reveal a more disordered internal field distribution in NdAlGe than in SmAlGe, reflected in a larger transverse muon-spin relaxation rate λT at T ≪ TN. This may be due to the complex magnetic structure of NdAlGe, which undergoes a series of metamagnetic transitions in an external magnetic field, while SmAlGe shows only a robust AFM order. In NdAlGe, the topological Hall effect (THE) appears between the first and the second metamagnetic transitions for H ∥ c, while it is absent in SmAlGe. Such THE in NdAlGe is most likely attributed to the field-induced topological spin textures. The longitudinal muon-spin relaxation rate λL, diverges near the AFM order, followed by a clear drop at T < TN. In the magnetically ordered state, spin fluctuations are significantly stronger in NdAlGe than in SmAlGe. In general, our longitudinal-field μSR data indicate vigorous spin fluctuations in NdAlGe, thus providing valuable insights into the origin of THE and of the possible topological spin textures in REAl(Si,Ge) Weyl semimetals.

Eliminating Skyrmion Hall Effect in Ferromagnetic Skyrmions.

Skyrmion Hall effect (SkHE) remains an obstacle for the application of magnetic skyrmions. While methods have been established to cancel or compensate SkHE in artificial antiferromagnets and ferrimagnets, eliminating intrinsic SkHE in ferromagnets is still a big challenge. Here, we propose a strategy to eliminate SkHE by intercalating nonmagnetic elements into van der Waals bilayer ferromagnets featuring in-plane ferromagnetism. The in-plane magnetism, along with a delicate balance among exchange interactions, Dzyaloshinskii-Moriya interactions (DMI), and magnetocrystalline anisotropy, creates interlayer bimerons/quadmerons, whose polarity can be controlled by DMI. Opposite DMI in the upper and lower layers results in opposite polarity and topological charge number Q-locking of topological spin texture, therefore, eliminating the SkHE. By intercalating Sr (Ba) in bilayer VSe2, we identify ten topological magnetic structures with zero topological charge number. Furthermore, we present a phase diagram illustrating diverse magnetic configurations achievable within the bimagnetic atomic layer, offering valuable guidance for future investigations.

Magnetic Structure-Dependent Spin Texture Lattice in Hexagonal MnFeCoGe Magnets.

Topological spin textures are of great significance in magnetic information storage and spintronics due to their high storage density and low drive current. In this work, the transformation of magnetic configuration from chaotic labyrinth domains to uniform stripe domains was observed in MnFe1-xCoxGe magnets. This change occurs due to the noncollinear magnetic structure switching to a uniaxial ferromagnetic structure with increasing Co content, as identified by neutron diffraction results and Lorentz transmission electron microscopy (L-TEM). Of utmost importance, a hexagonal lattice of high-density robust type-II magnetic bubble lattice was established for x = 0.8 through out-of-plane magnetic field stimulation and field-cooling. The dimensions of the type-II magnetic bubbles were found to be tuned by the sample thickness. Therefore, the stabilization of complex magnetic spin textures, associated with enhanced uniaxial ferromagnetic interaction and magnetic dipole-dipole interaction in MnFe1-xCoxGe through magnetic structure manipulation, as further confirmed by the micromagnetic simulations, will provide a convenient and efficient strategy for designing topological spin textures with potential applications in spintronic devices.

Ultrafast optical pumping induced polarity and chirality reversal of Néel skyrmions in amorphous GdFeCo ferrimagnet

The development of next-generation data storage devices relies on the efficient control of topological spin textures at ultrafast timescales with minimal energy consumption. Here, we theoretically investigate the switching of the magnetic skyrmion in ferrimagnetic GdFeCo utilizing the helicity-independent all-optical switching (HI-AOS) driven by femtosecond laser pulses. Our study demonstrates the switching of Néel skyrmion between the two degenerate skyrmion ground states having opposite polarity and chirality. A systematic study was done by varying the laser fluence, and we found that single-shot skyrmion switching is observed for a range of fluence values, where optically induced magnetization switching is observed. The present study proves that HI-AOS is a potential mechanism for switching magnetic skyrmion at ultrafast timescales. Our results offer significant insight into implementing optical writing skyrmion-based memory devices.

Anisotropic magnon frequency comb based on antiferromagnetic bimerons

The interaction between propagating magnons and topological spin textures is attracting a lot of recent attention from the magnonic community. It has been shown that the three-wave mixing between magnons and breathing skyrmion can induce the magnon frequency comb (MFC) with equidistant coherent peaks. However, a magnetic bimeron is a nontrivial spin texture and is regarded as the counterpart of the skyrmion in easy-plane magnets with Dzyaloshinskii–Moriya interaction, which allows anisotropic magnon propagations. This raises the question of whether the nonlinear interaction between magnons and bimerons can generate an MFC. If so, how does the direction of magnon propagation affect the characteristics of the MFC? In this Letter, we demonstrate that the three-wave mixing between propagating magnons and locally breathing bimerons can induce a terahertz MFC in easy-plane antiferromagnets. Micromagnetic simulations reveal that the three-wave coupling strength weakly depends on the driving frequency, but it strongly relies on the propagation direction of incident magnons. Our findings uncover the anisotropic nature of MFC in bimeron structures, which may have potential applications for ultrafast magnonic devices with spectroscopy, metrology, and sensing functionalities.

Combining Lorentz TEM and SEM with Polarization Analysis to Uncover Fractional Topological Spin Textures in Fe/Gd Multilayer Thin Films

Combining Lorentz TEM and SEM with Polarization Analysis to Uncover Fractional Topological Spin Textures in Fe/Gd Multilayer Thin Films

Effect of collective spin excitations on electronic transport in topological spin textures

Effect of collective spin excitations on electronic transport in topological spin textures

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.

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.

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.

Boundary flat bands with topological spin textures protected by subchiral symmetry

Boundary flat bands with topological spin textures protected by subchiral symmetry

Bimerons create bimerons: Proliferation and aggregation induced by currents and magnetic fields

AbstractThe aggregation of topological spin textures at nano and micro scales has practical applications in spintronic technologies. Here, the authors report the in‐plane current‐induced proliferation and aggregation of bimerons in a bulk chiral magnet. It is found that the spin‐transfer torques can induce the proliferation and aggregation of bimerons only in the presence of an appropriate out‐of‐plane magnetic field. It is also found that a relatively small damping and a relatively large non‐adiabatic spin‐transfer torque could lead to more pronounced bimeron proliferation and aggregation. Particularly, the current density should be larger than a certain threshold in order to trigger the proliferation; namely, the bimerons may only be driven into translational motion under weak current injection. Besides, the authors find that the aggregate bimerons could relax into a deformed honeycomb bimeron lattice with a few lattice structure defects after the current injection. The results are promising for the development of bio‐inspired spintronic devices that use a large number of aggregate bimerons. The findings also provide a platform for studying aggregation‐induced effects in spintronic systems, such as the aggregation‐induced lattice phase transitions.

3D skyrmion strings and their melting dynamics revealed via scalar-field electron tomography

Two-dimensional skyrmion, a topological spin texture, has been extensively investigated in non-centrosymmetric magnets. It is thermodynamically stable in bulk materials and can extend perpendicularly to the spin-rotating plane, forming the three-dimensional (3D) skyrmion string that occasionally terminates inside the bulk or at its surfaces, thereby producing spin (anti)hedgehogs. While magnetically manipulated skyrmion strings have been demonstrated thus far, an understanding of metastable skyrmion strings and their melting dynamics remain elusive. The challenge arises from the absence of a suitable 3D magnetic imaging technique that is capable of tracking their dynamics while varying the temperature. Here, we show a high-resolution 3D phase imaging to show the metastable skyrmions and their melting dynamics accompanied by the emergence of (anti)hedgehogs over a temperature range from 95 K to room temperature in helimagnets.