Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets

Multistep skyrmion phase transition driven by light-induced uniaxial strain

Strain engineering in skyrmion-hosting multilayers holds promising potential for spintronic devices. However, conventional strain is below 0.5%, limiting exploration of unique properties under substantial strain. In addition, while uniaxial strain modifies magnetic interactions anisotropically, its influence on skyrmions is underexplored. Here, we integrate skyrmion-hosting multilayers with a flexible liquid crystal film, enabling multistep skyrmion phase transitions through light-induced uniaxial strain up to 1%. Our results demonstrate that skyrmion transitions are sensitive to strain magnitude and orientation. Strain below 0.6% parallel to stripes transforms them into skyrmions. Above 0.6%, skyrmions elongate perpendicularly to the strain direction, exhibiting a negative Poisson effect, with deformation up to 40% at 0.8% strain. Further strain reverts skyrmions back into stripes. Micromagnetic simulations reveal that these phenomena stem from strain-induced anisotropic modulation of Dzyaloshinskii-Moriya interaction. This approach, which combines flexibility, light activation, and substantial uniaxial strain, offers a promising strategy for low-power, multistate spintronic devices.

Topological transitions, pinning and ratchets for driven magnetic hopfions in nanostructures

Using atomistic simulations, we examine the dynamics of three-dimensional magnetic hopfions interacting with an array of line defects or posts as a function of defect spacing, defect strength, and current. We find a pinned phase, a sliding phase where a hopfion can move through the posts or hurdles by distorting, and a regime where the hopfion becomes compressed and transforms into a toron that is half the size of the hopfion and moves at a lower velocity. The toron states occur when the defects are strong; however, in the toron regime, it is possible to stabilize sliding hopfions by increasing the applied current. Hopfions move without a Hall angle, while the toron moves with a finite Hall angle. We also show that when a hopfion interacts with an asymmetric array of planar defects, a ratchet effect consisting of a net dc motion can be realized under purely ac driving.

Skyrmion Sliding Switch in a 90 nm-Wide Nanostructured Chiral Magnet

Magnetic skyrmions, renowned for their fascinating electromagnetic properties, hold the potential for next-generation topological spintronic devices. Recent advancements have unveiled a rich tapestry of 3D topological magnetism. Nevertheless, the practical application of 3D topological magnetism in the development of topological spintronic devices remains a challenge. Here, we showcase the experimental utilization of 3D topological magnetism through the exploitation of skyrmion-edge attractive interactions in 90 nm-wide confined chiral FeGe and CoZnMn magnetic nanostructures. These attractive interactions result in two degenerate equilibrium positions that can be naturally interpreted as binary bits for a skyrmion sliding switch. Our theory and simulation reveal current-driven spiral motions of skyrmions, governed by the anisotropic gradient of the potential landscape. Our experiments validate the theory that predicts a tunable threshold current density via magnetic field and temperature modulation of the energy barrier. Our results offer an approach for implementing universal on-off switch functions in 3D topological spintronic devices.

Research on Quantum Materials and Quantum Technology at RIKEN

RIKEN covers fundamental research on physics, chemistry, biology, life and medical science, information and mathematical science, and engineering. Here, we outline research activities on quantum materials and quantum technology that include topological and correlated materials, spintronics, nanoscale materials and structures, atomic and quantum optics, and quantum computing.

Skyrmionium dynamics and stability on one dimensional anisotropy patterns

We examine a skyrmionium driven over a periodic anisotropy pattern, which consists of disorder free regions and disordered regions. For small defect densities, the skyrmionium flows for an extended range of currents, and there is a critical current above which it transforms into a skyrmion. For increased amounts of quenched disorder, the current needed for the skyrmionium to transform into a skyrmion decreases, and there is a critical disorder density above which a moving skyrmionium is not stable. In the moving state, the skyrmionium to skyrmion transformation leads to a drop in the velocity and the onset of a finite skyrmion Hall angle. We also find a reentrance effect in which the pinned skyrmionium transforms into a skyrmion just above depinning, restabilizes into skyrmionium at larger drives, and becomes unstable again at large currents. We also show that adding a transverse shaking drive can increase the lifetime of a moving skyrmionium by reducing the effect of the pinning in the direction of the drive.

The role of magnetic dipolar interactions in skyrmion lattices

Magnetic skyrmions are topological two-dimensional (2D) spin textures that can be stabilized at room temperature and low magnetic fields in magnetic multilayer stacks. Besides their envisioned applications in data storage and processing, these 2D quasiparticles constitute an ideal model system to study 2D particle properties. More precisely, the role of inter-particle dipolar interactions in 2D ensembles can be fully captured in skyrmion lattices. We engineer a multilayer stack hosting skyrmion lattices and increase the relevance of the dipolar coupling by increasing the number of repetitions n from n = 1 to n = 30 . To ascertain the impact on the spin structure, we carry out a series of imaging experiments and find a drastic change of the skyrmion size. We develop an analytical description for the skyrmion radius in the whole multilayer regime, from thin to thick film limits, identifying the key impact of the nucleation process leading to the skyrmion lattice. Our work provides a detailed understanding of the skyrmion-skyrmion interaction, clarifying the role of dipolar interactions as the multilayer stack is expanded in the z direction.

Observation of Topological Spin Textures in Ferrimagnetic Mn2 - xZnxSb

Ferrimagnets, which have both ferromagnetic and antiferromagnetic coupling, are attracting increased attention in the realm of spintronic devices due to advantages such as ultrafast dynamics and a suppressed skyrmion Hall effect. Thus, understanding the behavior of nontrivial spin textures in ferrimagnets is crucial; however, comprehensive reports on this topic remain limited. Here, the magnetic spin textures of ferrimagnetic Mn2 - xZnxSb (x = 0.85) is explored as a function of temperature and applied magnetic field. The spin textures can be tuned to a variety of states, including stripes, skyrmion bags, and a skyrmion lattice. Chiral Néel-type magnetic structures are visualized using Lorentz transmission electron microscopy. Mn(I) ions are slightly shifted toward the Sb sites, which may be due to a strong electrostatic interaction between Mn and Sb ions. This local structural distortion breaks the inversion symmetry and introduces an effective Dzyaloshinkii-Moriya interaction. This work thus provides a pathway to use doping and heterogeneity in a ferrimagnet to control and generate chiral nontrivial spin textures.

Computational study of skyrmion stability and transport on W/CoFeB

Skyrmions are topologically protected magnetic structures originating from Dzyaloshinbskii-Moriya Interaction (DMI) which can be driven by a spin-polarized current making it a candidate for many different novel spintronic devices. However, the transport velocity is proportional to the size of the skyrmion rendering the effort of miniaturizing spintronics devices useless indicating that it is not possible to realise high-speed transport, small size and low operating current at the same time. One approach to solving the trilemma is to increase the spin Hall angle [Formula: see text], the conversion ratio between charge current and spin current, in the heavy metal layer. For example, beta-tungsten (β-W) has attracted attention due to its high spin Hall angle, abundance in nature and the potential to combine with other materials to form complex structures. To characterise the use of β-W as a heavy metal layer along with a CoFeB magnetic layer, the interfacial DMI and the external field required to generate skyrmions were estimated to be 1.5 [Formula: see text] and 0.1 T respectively, which were confirmed to be realistic. In that case, the about 10 nm diameter skyrmion was transported under SOT at a velocity of about 40 m/s, which has the potential for skyrmion-based unconventional computing devices like skyrmion race track memory and logic gate.

Anomalous Hall spin current drives self-generated spin-orbit torque in a ferromagnet

Spin-orbit torques enable energy-efficient manipulation of magnetization by electric current and hold promise for applications ranging from non-volatile memory to neuromorphic computing. Here we report the discovery of a giant spin-orbit torque induced by anomalous Hall current in ferromagnetic conductors. This anomalous Hall torque is self-generated as it acts on the magnetization of the ferromagnet that engenders the torque. The magnitude of the anomalous Hall torque is sufficiently large to fully negate magnetic damping of the ferromagnet, which allows us to implement a microwave spin torque nano-oscillator driven by this torque. The peculiar angular symmetry of the anomalous Hall torque favours its use over the conventional spin Hall torque in coupled nano-oscillator arrays. The universal character of the anomalous Hall torque makes it an integral part of the description of coupled spin transport and magnetization dynamics in magnetic nanostructures.

Inverse Size-Scaling Ferroelectricity in Centrosymmetric Insulating Perovskite Oxide DyScO3

The breaking of inversion symmetry dictates the emergence of electric polarization, whose topological states in superlattices and bulks have received tremendous attention for their intriguing physics brought for novel device design. However, as for substrate oxides such as LaAlO3, KTaO3, RScO3 (R = rare earth element), their centrosymmetric trivial attributes make their functionality poorly explored. Here, the discovery of nanoscale thickness gradient-induced nonpolar-to-polar phase transition in band insulator DyScO3 is reported by using atomic resolution transmission electron microscopy. As the free-standing specimen reduces to a critical thickness ≈5 nm, its inversion symmetry is spontaneously broken by surface charge transfer, which gives rise to asymmetric Dy atomic displacements and ferrodistortive octahedral order, as substantiated by the first-principles calculations. Apart from the observation of migratable polar vortex structures, the switchable electric polarization by applied electric field is demonstrated by the piezoresponse force microscopy experiments. Given the decisive role of critical size in generating ferroelectricity, a concept of "inverse size-scaling ferroelectric" is proposed to define a class of such materials. Distinct from the proper and improper ferroelectrics, the findings offer a new platform to explore novel low-dimensional ferroelectrics and device applications in the future.

Realizing Quantitative Quasiparticle Modeling of Skyrmion Dynamics in Arbitrary Potentials

We demonstrate fully quantitative Thiele model simulations of magnetic skyrmion dynamics on previously unattainable experimentally relevant large length and time scales by ascertaining the key missing parameters needed to calibrate the experimental and simulation timescales and current-induced forces. Our work allows us to determine complete spatial pinning energy landscapes that enable quantification of experimental studies of diffusion in arbitrary potentials within the Lifson-Jackson framework. Our method enables us to ascertain the timescales, and by isolating the effect of ultralow current density (order 10^{6}  A/m^{2}) generated torques we directly infer the total force acting on the skyrmion for a quantitative modeling.

Effects of Chiral Polypeptides on Skyrmion Stability and Dynamics

Magnetic skyrmions, topologically stabilized chiral spin textures in magnetic thin films, have garnered considerable interest due to their efficient manipulation and resulting potential as efficient nanoscale information carriers. One intriguing approach to address the challenge of tuning skyrmion properties involves using chiral molecules. Chiral molecules can locally manipulate magnetic properties by inducing magnetization through spin exchange interactions and by creating spin currents. Here, Magneto-Optical Kerr Effect (MOKE) microscopy is used to image the impact of chiral polypeptides on chiral magnetic structures. The chiral polypeptides shift the spin reorientation transition temperature, reduce thermal skyrmion motion, and alter the coercive field locally, enhancing skyrmion stability and thus enabling local control. These findings demonstrate the potential of chiral molecules to address challenges for skyrmion based devices, thus paving the way to applications such as the racetrack memory, reservoir computing and others.

Room-temperature creation and conversion of individual skyrmion bags in magnetic multilayered disks

Skyrmion bags, with arbitrary topological charge Q, have recently attracted much interest, since such high-Q topological systems could open a way for topological magnetism research and are promising for spintronic applications with high flexibility for information encoding. Investigation on room-temperature skyrmion bags in magnetic multilayered structures is essential for applications and remains unexplored so far. Here, we demonstrate room-temperature creation and manipulation of individual skyrmion bags in magnetic multilayered disks. Individual skyrmion bags with varying topological charges are identified to remain stable at zero field. Furthermore, we realize intriguing field-driven topological conversion of skyrmion bags, as well as local manipulation of skyrmion bags via magnetic tips. Micromagnetic simulations indicate that the special boundary condition of the disks is responsible for skyrmion-bag formation and stability. These findings provide a platform to investigate individual skyrmion bags in confined multilayered structures, which could be useful for developing high-Q-based topological 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.

Quantifying the polar skyrmion motion barrier in an oxide heterostructure

Exotic polar topologies such as polar skyrmions have been widely observed in ferroelectric superlattice systems. The dynamic motion of polar skyrmions under external forces holds promise for applications in advanced electronic devices such as race-track memory. Meanwhile, the polar skyrmion motion has proven to be challenging due to the strong skyrmion-skyrmion interaction and a lack of a mechanism similar to the spin-transfer torque. In this study, we have developed a nudged elastic band (NEB) method to quantify the polar skyrmion motion barrier along a specific trajectory. It is indicated that the skyrmion motion barrier can be significantly reduced with the reduction of the periodicity to 8 uc, due to the large reduction of the skyrmion size. Moreover, this barrier can also be greatly reduced with a small external electric potential. Following the analysis, we further performed phase-field simulation to verify the collective motion of the polar skyrmion. We have demonstrated the collective skyrmion motion by applying a 5 μN mechanical force using a blade-shaped indenter with a periodicity of 8 unit cells, under an external applied voltage of 1.5 V. This study further paves the way for the design of polar skyrmion-based electronic devices.

Magnetic domain wall and skyrmion manipulation by static and dynamic strain profiles

Magnetic domain walls and skyrmions in thin film micro- and nanostructures have been of interest to a growing number of researchers since the turn of the millennium, motivated by the rich interplay of materials, interface and spin physics as well as by the potential for applications in data storage, sensing and computing. This review focuses on the manipulation of magnetic domain walls and skyrmions by piezoelectric strain, which has received increasing attention recently. Static strain profiles generated, for example, by voltage applied to a piezoelectric-ferromagnetic heterostructure, and dynamic strain profiles produced by surface acoustic waves, are reviewed here. As demonstrated by the success of magnetic random access memory, thin magnetic films have been successfully incorporated into complementary metal-oxide-semiconductor back-end of line device fabrication. The purpose of this review is therefore not only to highlight promising piezoelectric and magnetic materials and their properties when combined, but also to galvanise interest in the spin textures in these heterostructures for a variety of spin- and straintronic devices.

Room Temperature Magnetic Skyrmions in Gradient-Composition Engineered CoPt Single Layers

Topologically protected magnetic skyrmions in magnetic materials are stabilized by an interfacial or bulk Dzyaloshinskii-Moriya interaction (DMI). Interfacial DMI decays with an increase of the magnetic layer thickness in just a few nanometers, and bulk DMI typically stabilizes magnetic skyrmions at low temperatures. Consequently, more flexibility in the manipulation of DMI is required for utilizing nanoscale skyrmions in energy-efficient memory and logic devices at room temperature (RT). Here, we demonstrate the observation of RT skyrmions stabilized by gradient DMI (g-DMI) in composition gradient-engineered CoPt single-layer films by employing the topological Hall effect, magnetic force microscopy, and nitrogen-vacancy scanning magnetometry. Skyrmions remain stable over a wide range of applied magnetic fields and are confirmed to be nearly Bloch-type from micromagnetic simulation and analytical magnetization reconstruction. Furthermore, we observe skyrmion pairs, which may be explained by skyrmion-antiskyrmion interactions. Our findings expand the family of magnetic materials hosting RT magnetic skyrmions by tuning g-DMI via gradient polarity and a choice of magnetic elements.

Fundamental theory of current-induced motion of magnetic skyrmions

Magnetic skyrmions are topological spin textures that appear in magnets with broken spatial inversion symmetry as a consequence of competition between the (anti)ferromagnetic exchange interactions and the Dzyaloshinskii-Moriya interactions in a magnetic field. In the research of spintronics, the current-driven dynamics of skyrmions has been extensively studied aiming at their applications to next-generation spintronic devices. However, current-induced skyrmion motion exhibits diverse behaviors depending on various factors and conditions such as the type of skyrmion, driving mechanism, system geometry, direction of applied current, and type of the magnet. While this variety attracts enormous research interest of fundamental science and enriches their possibilities of technical applications, it is, at the same time, a source of difficulty and complexity that hinders their comprehensive understandings. In this article, we discuss fundamental and systematic theoretical descriptions of current-induced motion of skyrmions driven by the spin-transfer torque and the spin-orbit torque. Specifically, we theoretically describe the behaviors of current-driven skyrmions depending on the factors and conditions mentioned above by means of analyses using the Thiele equation. Furthermore, the results of the analytical theory are visually demonstrated and quantitatively confirmed by micromagnetic simulations using the Landau-Lifshitz-Gilbert-Slonczewski equation. In particular, we discuss dependence of the direction and velocity of motion on the type of skyrmion (Bloch type and Néel type) and its helicity, the system geometry (thin plate and nanotrack), the direction of applied current (length and width direction of the nanotrack) and its spin-polarization orientation, and the type of magnet (ferromagnet and antiferromagnet). The comprehensive theory provided by this article is expected to contribute significantly to research on the manipulation and control of magnetic skyrmions by electric currents for future spintronics applications.

Experimental progress in Eu(Al,Ga)4topological antiferromagnets

The non-trivial magnetic and electronic phases occurring in topological magnets are often entangled, thus leading to a variety of exotic physical properties. Recently, the BaAl4-type compounds have been extensively investigated to elucidate the topological features appearing in their real- and momentum spaces. In particular, the topological Hall effect and the spin textures, typical of the centrosymmetric Eu(Al,Ga)4family, have stimulated extensive experimental and theoretical research. In this topical review, we discuss the latest findings on the Eu(Al,Ga)4topological antiferromagnets and related materials, arising from a wide range of experimental techniques. We show that Eu(Al,Ga)4represents a suitable platform to explore the interplay between lattice-, charge-, and spin degrees of freedom, and associated emergent phenomena. Finally, we address some key questions open to future investigation.

Skyrmion flow in periodically modulated channels

Magnetic skyrmions, topologically stabilized chiral magnetic textures with particlelike properties, have so far primarily been studied statically. Here, we experimentally investigate the dynamics of skyrmion ensembles in metallic thin film conduits where they behave as quasiparticle fluids. By exploiting our access to the full trajectories of all fluid particles by means of time-resolved magneto-optical Kerr microscopy, we demonstrate that boundary conditions of skyrmion fluids can be tuned by modulation of the channel geometry. We observe as a function of channel width deviations from classical flow profiles even into the no- or partial-slip regime. Unlike conventional colloids, the skyrmion Hall effect can also introduce transversal flow asymmetries and even local motion of single skyrmions against the driving force which we explore with particle-based simulations, demonstrating the unique properties of skyrmion liquid flow that uniquely deviates from previously known behavior of other quasiparticles.

Dynamic transition and Galilean relativity of current-driven skyrmions

The coupling of conduction electrons and magnetic textures leads to quantum transport phenomena described by the language of emergent electromagnetic fields1-3. For magnetic skyrmions, spin-swirling particle-like objects, an emergent magnetic field is produced by their topological winding4-6, resulting in the conduction electrons exhibiting the topological Hall effect (THE)7. When the skyrmion lattice (SkL) acquires a drift velocity under conduction electron flow, an emergent electric field is also generated8,9. The resulting emergent electrodynamics dictate the magnitude of the THE by the relative motion of SkL and conduction electrons. Here we report the emergent electrodynamics induced by SkL motion in Gd2PdSi3, facilitated by its giant THE10,11. With increasing current excitation, we observe the dynamic transition of the SkL motion from the pinned to creep regime and finally to the flow regime, in which the THE is totally suppressed. We argue that the Galilean relativity required for the total cancellation of the THE may be generically recovered in the flow regime, even in complex multiband systems such as the present compound. Moreover, the observed THE voltages are large enough to enable real-time measurement of the SkL velocity-current profile, which shows the inertial-like motion of the SkL in the creep regime, appearing as the current hysteresis of the skyrmion velocity.

Tailoring Dzyaloshinskii-Moriya Interaction and Spin-Hall Topological Hall Effect in Insulating Magnetic Oxides by Interface Engineering

Chiral spin textures, as exotic phases in magnetic materials, hold immense promise for revolutionizing logic, and memory applications. Recently, chiral spin textures have been observed in centrosymmetric magnetic insulators (FMI), due to an interfacial Dzyaloshinskii-Moriya interaction (iDMI). However, the source and origin of this iDMI remain enigmatic in magnetic insulator systems. Here, the source and origin of the iDMI in Pt/Y3Fe5O12 (YIG)/substrate structures are deeply delved by examining the spin-Hall topological Hall effect (SH-THE), an indication of chiral spin textures formed due to an iDMI. Through carefully modifying the interfacial chemical composition of Pt/YIG/substrate with a nonmagnetic Al3+ doping, the obvious dependence of SH-THE on the interfacial chemical composition for both the heavy metal (HM)/FMI and FMI/substrate interfaces is observed. The results reveal that both interfaces contribute to the strength of the iDMI, and the iDMI arises due to strong spin-orbit coupling and inversion symmetry breaking at both interfaces in HM/FMI/substrate. Importantly, it is shown that nonmagnetic substitution and interface engineering can significantly tune the SH-THE and iDMI in ferrimagnetic iron garnets. The approach offers a viable route to tailor the iDMI and associated chiral spin textures in low-damping insulating magnetic oxides, thus advancing the field of spintronics.

Realization of skyrmion shift register

The big data explosion demands novel data storage technology. Among many different approaches, solitonic racetrack memory devices hold great promise for accommodating nonvolatile and low-power functionalities. As representative topological solitons, magnetic skyrmions are envisioned as potential information carriers for efficient information processing. While their advantages as memory and logic elements have been vastly exploited from theoretical perspectives, the corresponding experimental efforts are rather limited. These challenges, which are key to versatile skyrmionic devices, will be studied in this work. Through patterning concaved surface topography with designed arrays of indentations on standard Si/SiO2 substrates, we demonstrate that the resultant non-flat energy landscape could lead to the formation of hexagonal and square skyrmion lattices in Ta/CoFeB/MgO multilayers. Based on these films, one-dimensional racetrack devices are subsequently fabricated, in which a long-distance deterministic shifting of skyrmions between neighboring indentations is achieved at room temperature. Through separating the word line and the bit line, a prototype shift register device, which can sequentially generate and precisely shift complex skyrmionic data strings, is presented. The deterministic writing and long-distance shifting of skyrmionic bits can find potential applications in transformative skyrmionic memory, logic as well as the in-memory computing devices.

Influence of the Hall-bar geometry on texture-induced topological spin transport in two-dimensional Rashba spin-orbit ferromagnets

While the recent prediction and observation of magnetic skyrmions bears inspiring promise for next-generation spintronic devices, how to detect and track their position becomes an important issue. In this work, we investigate the spin transport in a two-dimensional magnetic nanoribbon with the Hall-bar geometry in the presence of Rashba spin-orbit coupling and magnetic skyrmions. We employ the Kwant tight-binding code to compute the Hall conductance and local spin-polarized current density. We consider two versions of the model: One with single skyrmion and one with two separate skyrmions. It is found that the size and position of the skyrmions strongly modulate the Hall conductance near the Hall-bar position. The geometry of the Hall bar also has a strong influence on the Hall conductance of the system. With the decreasing of the width of Hall leads, the peak of Hall conductance becomes sharper. We also show the spatial distribution of the spin-polarized current density around a skyrmion located at different positions. We extend this study toward two separate skyrmions, where the Hall conductance also reveals a sizable dependence on the position of the skyrmions and their distance. Our numerical analysis offers the possibility of electrically detecting the skyrmion position, which could have potential applications in ultrahigh-density storage design.

Steady motion of 80-nm-size skyrmions in a 100-nm-wide track

The current-driven movement of magnetic skyrmions along a nanostripe is essential for the advancement and functionality of a new category of spintronic devices resembling racetracks. Despite extensive research into skyrmion dynamics, experimental verification of current-induced motion of ultra-small skyrmions within an ultrathin nanostripe is still pending. Here, we unveil the motion of individual 80 nm-size skyrmions in an FeGe track with an ultrathin width of 100 nm. The skyrmions can move steadily along the track over a broad range of current densities by using controlled pulse durations of as low as 2 ns. The potential landscape, arising from the magnetic edge twists in such a geometrically confined system, introduces skyrmion inertia and ensures efficient motion with a vanishing skyrmion Hall angle. Our results showcase the steady motion of skyrmions in an ultrathin track, offering a practical pathway for implementing skyrmion-based spintronic devices.

Topological Complex Charge Conservation in Nontrivial Z2 × Z2 Domain Walls

Localized topological modes such as solitons, Majorana Fermions, and skyrmions are attracting great interest as robust information carriers for future devices. Here, a novel conserved quantity for topological domain wall networks of a Z2 × Z2 order generated with spin-polarized current in Sr2VO3FeAs is discovered. Domain walls are mobilized by the scanning tunneling current, which also observes in atomic scale active dynamics of domain wall vertices including merge, bifurcation, pair creation, and annihilation. Within this dynamics, the product of the topological complex charges defined for domain wall vertices is conserved with a novel boundary-charge correspondence rule. These results may open an avenue toward topological electronics based on domain wall vertices in generic Z2 × Z2 systems.

Topological Spin Textures in an Insulating van der Waals Ferromagnet

Generation and control of topological spin textures constitutes one of the most exciting challenges of modern spintronics given their potential applications in information storage technologies. Of particular interest are magnetic insulators, which due to low damping, absence of Joule heating and reduced dissipation can provide energy-efficient spin-textures platform. Here, it is demonstrated that the interplay between sample thickness, external magnetic fields, and optical excitations can generate a prolific paramount of spin textures, and their coexistence in insulating CrBr3 van der Waals (vdW) ferromagnets. Using high-resolution magnetic force microscopy and large-scale micromagnetic simulation methods, the existence of a large region in T-B phase diagram is demonstrated where different stripe domains, skyrmion crystals, and magnetic domains exist and can be intrinsically selected or transformed to each-other via a phase-switch mechanism. Lorentz transmission electron microscopy unveils the mixed chirality of the magnetic textures that are of Bloch-type at given conditions but can be further manipulated into Néel-type or hybrid-type via thickness-engineering. The topological phase transformation between the different magnetic objects can be further inspected by standard photoluminescence optical probes resolved by circular polarization indicative of an existence of exciton-skyrmion coupling mechanism. The findings identify vdW magnetic insulators as a promising framework of materials for the manipulation and generation of highly ordered skyrmion lattices relevant for device integration at the atomic level.

High-harmonic spin-shearing interferometry for spatially resolved EUV magneto-optical spectroscopy

We present a method for achieving hyperspectral magnetic imaging in the extreme ultraviolet (EUV) region based on high-harmonic generation (HHG). By interfering two mutually coherent orthogonally-polarized and laterally-sheared HHG sources, we create an EUV illumination beam with spatially-dependent ellipticity. By placing a magnetic sample in the beamline and sweeping the relative time delay between the two sources, we record a spatially resolved interferogram that is sensitive to the EUV magnetic circular dichroism of the sample. This image contains the spatially-resolved magneto-optical response of the sample at each harmonic order, and can be used to measure the magnetic properties of spatially inhomogeneous magnetic samples.

The role of non-uniform magnetization texture for magnon-magnon coupling in an antidot lattice

We numerically study the spin-wave dynamics in an antidot lattice based on a Co/Pd multilayer structure with reduced perpendicular magnetic anisotropy at the edges of the antidots. This structure forms a magnonic crystal with a periodic antidot pattern and a periodic magnetization configuration consisting of out-of-plane magnetized bulk and in-plane magnetized rims. Our results show a different behavior of spin waves in the bulk and in the rims under varying out-of-plane external magnetic field strength, revealing complex spin-wave spectra and hybridizations between the modes of these two subsystems. A particularly strong magnon-magnon coupling, due to exchange interactions, is found between the fundamental bulk spin-wave mode and the second-order radial rim modes. However, the dynamical coupling between the spin-wave modes at low frequencies, involving the first-order radial rim modes, is masked by the changes in the static magnetization at the bulk-rim interface with magnetic field changes. The study expands the horizons of magnonic-crystal research by combining periodic structural patterning and non-collinear magnetization texture to achieve strong magnon-magnon coupling, highlighting the significant role of exchange interactions in the hybridization.

Electron-Assisted Generation and Straight Movement of Skyrmion Bubble in Kagome TbMn6Sn6

Topological magnetic textures are promising candidates as binary data units for the next-generation memory device. The precise generation and convenient control of nontrivial spin topology at zero field near room temperature endows the critical advantages in skyrmionic devices but is not simultaneously integrated into one material. Here, in the Kagome plane of quantum TbMn6Sn6, the expedient generation of the skyrmion bubbles in versatile forms of lattice, chain, and isolated one by converging the electron beam, where the electron intensity gradient contributes to the dynamic generation from local anisotropy variation near spin reorientation transition (SRT) is reported. Encouragingly, by utilizing the dynamic shift of the SRT domain interface, the straight movement is actualized with the skyrmion bubble slave to the SRT domain interface forming an elastic composite object, avoiding the usual deflection from the skyrmion Hall effect. The critical contribution of the SRT domain interface via conveniently electron-assisted heating is further theoretically validated in micromagnetic simulation, highlighting the compatible application possibility in advanced devices.

Direct Observation of Room-Temperature Magnetic Skyrmion Motion Driven by Ultra-Low Current Density in Van Der Waals Ferromagnets

The recent discovery of room-temperature ferromagnetism in 2D van der Waals (vdW) materials, such as Fe3GaTe2 (FGaT), has garnered significant interest in offering a robust platform for 2D spintronic applications. Various fundamental operations essential for the realization of 2D spintronics devices are experimentally confirmed using these materials at room temperature, such as current-induced magnetization switching or tunneling magnetoresistance. Nevertheless, the potential applications of magnetic skyrmions in FGaT systems at room temperature remain unexplored. In this work, the current-induced generation of magnetic skyrmions in FGaT flakes employing high-resolution magnetic transmission soft X-ray microscopy is introduced, supported by a feasible mechanism based on thermal effects. Furthermore, direct observation of the current-induced magnetic skyrmion motion at room temperature in FGaT flakes is presented with ultra-low threshold current density. This work highlights the potential of FGaT as a foundation for room-temperature-operating 2D skyrmion device applications.

Resilience of quantum spin fluctuations against Dzyaloshinskii-Moriya interaction

In low-dimensional systems, the lack of structural inversion symmetry combined with the spin-orbit coupling gives rise to an anisotropic antisymmetric superexchange known as the Dzyaloshinskii-Moriya interaction (DMI). Various features have been reported due to the presence of DMIs in quantum systems. We here study the one-dimensional spin-1/2 transverse field XY chains with a DMI at zero temperature. Our focus is on the quantum fluctuations of the spins measured by the spin squeezing and the entanglement entropy. We find that these fluctuations are resistant to the effect of the DMI in the system. This resistance will fail as soon as the system is placed in the chiral phase where its state behaves as a squeezed state, suggesting the merit of the chiral phase to be used for quantum metrology. Remarkably, we prove that the central charge vanishes on the critical lines between gapless chiral and ferromagnetic/paramagnetic phases where there is no critical scaling versus the system size for the spin squeezing parameter. Our phenomenal results provide a further understanding of the effects of the DMIs in the many-body quantum systems which may be testable in experiments.

Fast current-induced skyrmion motion in synthetic antiferromagnets

Magnetic skyrmions are topological magnetic textures that hold great promise as nanoscale bits of information in memory and logic devices. Although room-temperature ferromagnetic skyrmions and their current-induced manipulation have been demonstrated, their velocity has been limited to about 100 meters per second. In addition, their dynamics are perturbed by the skyrmion Hall effect, a motion transverse to the current direction caused by the skyrmion topological charge. Here, we show that skyrmions in compensated synthetic antiferromagnets can be moved by current along the current direction at velocities of up to 900 meters per second. This can be explained by the cancellation of the net topological charge leading to a vanishing skyrmion Hall effect. Our results open an important path toward the realization of logic and memory devices based on the fast manipulation of skyrmions in tracks.

Tunable Magnetic Domains in Ferrimagnetic MnSb2Te4

Highly tunable properties make Mn(Bi,Sb)2Te4 a rich playground for exploring the interplay between band topology and magnetism: On one end, MnBi2Te4 is an antiferromagnetic topological insulator, while the magnetic structure of MnSb2Te4 (MST) can be tuned between antiferromagnetic and ferrimagnetic. Motivated to control electronic properties through real-space magnetic textures, we use magnetic force microscopy (MFM) to image the domains of ferrimagnetic MST. We find that magnetic field tunes between stripe and bubble domain morphologies, raising the possibility of topological spin textures. Moreover, we combine in situ transport with domain manipulation and imaging to both write MST device properties and directly measure the scaling of the Hall response with the domain area. This work demonstrates measurement of the local anomalous Hall response using MFM and opens the door to reconfigurable domain-based devices in the M(B,S)T family.

Generation of skyrmions by combining thermal and spin-orbit torque: breaking half skyrmions into skyrmions

Skyrmions, swirling spin textures with topologically protected stability and low critical driven-current density, can be generated from the stripe domain with current pulses, bringing them closer to practical applications in racetrack memory. However, the mechanism of this topological transition from the stripe domain to the skyrmion remains unclear because the transition process occurs at a nanosecond timescale, giving rise to difficulties in observing this process using imaging tools. In this study, we controlled the domain wall - skyrmion transition by combining Joule heating with spin-orbit torque (SOT) and experimentally observed the details of this process, by which we confirmed the mechanism: the spatial variation of the topological charge density induces half skyrmions branching from the stripe domains, and these half skyrmions overcome the surface tension and break away from the stripe domain, resulting in the generation of skyrmions. The details were observed by employing Joule heating to overcome the pinning effect and manipulating the strength of the SOT to induce the branching and breaking of half skyrmions. These findings offer new insights into skyrmion generation and serve as an important step towards the development of highly efficient devices for processing and computing based on skyrmionics.

Observation of Topological Hall Effect in a Chemically Complex Alloy

The topological Hall effect (THE) is the transport response of chiral spin textures and thus can serve as a powerful probe for detecting and understanding these unconventional magnetic orders. So far, the THE is only observed in either noncentrosymmetric systems where spin chirality is stabilized by Dzyaloshinskii-Moriya interactions, or triangular-lattice magnets with Ruderman-Kittel-Kasuya-Yosida-type interactions. Here, a pronounced THE is observed in a Fe-Co-Ni-Mn chemically complex alloy with a simple face-centered cubic (fcc) structure across a wide range of temperatures and magnetic fields. The alloy is shown to have a strong magnetic frustration owing to the random occupation of magnetic atoms on the close-packed fcc lattice and the direct Heisenberg exchange interaction among atoms, as evidenced by the appearance of a reentrant spin glass state in the low-temperature regime and the first principles calculations. Consequently, THE is attributed to the nonvanishing spin chirality created by strong spin frustration under the external magnetic field, which is distinct from the mechanism responsible for the skyrmion systems, as well as geometrically frustrated magnets.

Electrical Detection and Nucleation of a Magnetic Skyrmion in a Magnetic Tunnel Junction Observed via Operando Magnetic Microscopy

Magnetic skyrmions are topological spin textures which are envisioned as nanometer scale information carriers in magnetic memory and logic devices. The recent demonstrations of room temperature skyrmions and their current induced manipulation in ultrathin films were first steps toward the realization of such devices. However, important challenges remain regarding the electrical detection and the low-power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction (MTJ) and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be manipulated by both gate voltages and external magnetic fields, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologies.

All-electrical skyrmionic magnetic tunnel junction

Topological whirls or 'textures' of spins such as magnetic skyrmions represent the smallest realizable emergent magnetic entities1-5. They hold considerable promise as robust, nanometre-scale, mobile bits for sustainable computing6-8. A longstanding roadblock to unleashing their potential is the absence of a device enabling deterministic electrical readout of individual spin textures9,10. Here we present the wafer-scale realization of a nanoscale chiral magnetic tunnel junction (MTJ) hosting a single, ambient skyrmion. Using a suite of electrical and multimodal imaging techniques, we show that the MTJ nucleates skyrmions of fixed polarity, whose large readout signal-20-70% relative to uniformly magnetized states-corresponds directly to skyrmion size. The MTJ exploits complementary nucleation mechanisms to stabilize distinctly sized skyrmions at zero field, thereby realizing three non-volatile electrical states. Crucially, it can electrically write and delete skyrmions to both uniform states with switching energies 1,000 times lower than the state of the art. Here, the applied voltage emulates a magnetic field and, in contrast to conventional MTJs, it reshapes both the energetics and kinetics of the switching transition, enabling deterministic bidirectional switching. Our stack platform enables large readout and efficient switching, and is compatible with lateral manipulation of skyrmionic bits, providing the much-anticipated backbone for all-electrical skyrmionic device architectures9,10. Its wafer-scale realizability provides a springboard to harness chiral spin textures for multibit memory and unconventional computing8,11.

Chiral Induced Spin Selectivity

Since the initial landmark study on the chiral induced spin selectivity (CISS) effect in 1999, considerable experimental and theoretical efforts have been made to understand the physical underpinnings and mechanistic features of this interesting phenomenon. As first formulated, the CISS effect refers to the innate ability of chiral materials to act as spin filters for electron transport; however, more recent experiments demonstrate that displacement currents arising from charge polarization of chiral molecules lead to spin polarization without the need for net charge flow. With its identification of a fundamental connection between chiral symmetry and electron spin in molecules and materials, CISS promises profound and ubiquitous implications for existing technologies and new approaches to answering age old questions, such as the homochiral nature of life. This review begins with a discussion of the different methods for measuring CISS and then provides a comprehensive overview of molecules and materials known to exhibit CISS-based phenomena before proceeding to identify structure-property relations and to delineate the leading theoretical models for the CISS effect. Next, it identifies some implications of CISS in physics, chemistry, and biology. The discussion ends with a critical assessment of the CISS field and some comments on its future outlook.

Homochiral antiferromagnetic merons, antimerons and bimerons realized in synthetic antiferromagnets

The ever-growing demand for device miniaturization and energy efficiency in data storage and computing technology has prompted a shift towards antiferromagnetic topological spin textures as information carriers. This shift is primarily owing to their negligible stray fields, leading to higher possible device density and potentially ultrafast dynamics. We realize in this work such chiral in-plane topological antiferromagnetic spin textures namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. We demonstrate multimodal vector imaging of the three-dimensional Néel order parameter, revealing the topology of those spin textures and a globally well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Our analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role to significantly reduce the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as antiferromagnetic merons, in synthetic antiferromagnets, making them a promising platform for next-generation spintronics applications.

Mechanical Rippling for Diverse Ferroelectric Topologies in Otherwise Nonferroelectric SrTiO_{3} Nanofilms

Polar topological structures such as skyrmions and merons have become an emerging research field due to their rich functionalities and promising applications in information storage. Up to now, the obtained polar topological structures are restricted to a few limited ferroelectrics with complex heterostructures, limiting their large-scale practical applications. Here, we circumvent this limitation by utilizing a nanoscale ripple-generated flexoelectric field as a universal means to create rich polar topological configurations in nonpolar nanofilms in a controllable fashion. Our extensive phase-field simulations show that a rippled SrTiO_{3} nanofilm with a single bulge activates polarizations that are stabilized in meron configurations, which further undergo topological transitions to Néel-type and Bloch-type skyrmions upon varying the geometries. The formation of these topologies originates from the curvature-dependent flexoelectric field, which extends beyond the common mechanism of geometric confinement that requires harsh energy conditions and strict temperature ranges. We further demonstrate that the rippled nanofilm with three-dimensional ripple patterns can accommodate other unreported modulated phases of ferroelectric topologies, which provide ferroelectric analogs to the complex spin topologies in magnets. The present study not only unveils the intriguing nanoscale electromechanical properties but also opens exciting opportunities to design various functional topological phenomena in flexible materials.

Creation of Room-Temperature Sub-100 nm Antiferromagnetic Skyrmions in an Antiferromagnet IrMn through Interfacial Exchange Coupling

Antiferromagnetic (AFM) skyrmions are magnetic vortices composed of antiparallell-aligned neighboring spins. In stark contrast to conventional skyrmions based on ferromagnetic order, AFM skyrmions have vanished stray fields, higher response frequencies, and rectified translational motion driven by an external force. Therefore, AFM skyrmions promise highly efficient spintronics devices with high bit mobility and density. Nevertheless, the experimental realization of intrinsic AFM skyrmions remains elusive. Here, we show that AFM skyrmions can be nucleated via interfacial exchange coupling at the surface of a room-temperature AFM material, IrMn, exploiting the particular response from uncompensated moments to the thermal annealing and imprinting effects. Further systematic magnetic characterizations validate the existence of such an AFM order at the IrMn/CoFeB interfaces. Such AFM skyrmions have a typical size of 100 nm, which presents pronounced robustness against field and temperature. Our work opens new pathways for magnetic topological devices based on AFM skyrmions.

Regulation of interfacial Dzyaloshinskii-Moriya interaction in ferromagnetic multilayers

Interfacial Dzyaloshinskii-Moriya interaction (i-DMI) exists in the film materials with inversion symmetry breaking, which can stabilize a series of nonlinear spin structures and control their chirality, such as Néel-type domain wall, magnetic skyrmion and spin spiral. In addition, the strength and chirality of i-DMI are directly related to the dynamic behavior of these nonlinear spin structures. Therefore, regulating the strength and chirality of i-DMI not only has an important scientific significance for enriching spintronics and topological physics, but also has a significant practical value for constructing a new generation of memorizer, logic gate, and brain-like devices with low-power. This review summarizes the research progress on the regulation of i-DMI in ferromagnetic films and provides some prospects for future research.

Observation by SANS and PNR of pure Néel-type domain wall profiles and skyrmion suppression below room temperature in magnetic [Pt/CoFeB/Ru]10 multilayers

We report investigations of the magnetic textures in periodic multilayers [Pt(1 nm)/(CoFeB(0.8 nm)/Ru(1.4 nm)]10 using polarised neutron reflectometry (PNR) and small-angle neutron scattering (SANS). The multilayers are known to host skyrmions stabilized by Dzyaloshinskii-Moriya interactions induced by broken inversion symmetry and spin-orbit coupling at the asymmetric interfaces. From depth-dependent PNR measurements, we observed well-defined structural features and obtained the layer-resolved magnetization profiles. The in-plane magnetization of the CoFeB layers calculated from fitting of the PNR profiles is found to be in excellent agreement with magnetometry data. Using SANS as a bulk probe of the entire multilayer, we observe long-period magnetic stripe domains and skyrmion ensembles with full orientational disorder at room temperature. No sign of skyrmions is found below 250 K, which we suggest is due to an increase of an effective magnetic anisotropy in the CoFeB layer on cooling that suppresses skyrmion stability. Using polarised SANS at room temperature, we prove the existence of pure Néel-type windings in both stripe domain and skyrmion regimes. No Bloch-type winding admixture, i.e. an indication for hybrid windings, is detected within the measurement sensitivity, in good agreement with expectations according to our micromagnetic modelling of the multilayers. Our findings using neutron techniques provide valuable microscopic insights into the rich magnetic behavior of skyrmion-hosting multilayers, which are essential for the advancement of future skyrmion-based spintronic devices.

Raman Shifts in Two-Dimensional van der Waals Magnets Reveal Magnetic Texture Evolution

Two-dimensional (2D) van der Waals magnets comprise rich physics that can be exploited for spintronic applications. We investigate the interplay between spin-phonon coupling and spin textures in a 2D van der Waals magnet by combining magneto-Raman spectroscopy with cryogenic Lorentz transmission electron microscopy. We find that when stable skyrmion bubbles are formed in the 2D magnet, a field-dependent Raman shift can be observed, and this shift is absent for the 2D magnet prepared in its ferromagnetic state. Correlating these observations with numerical simulations that take into account field-dependent magnetic textures and spin--phonon coupling in the 2D magnet, we associate the Raman shift to field-induced modulations of the skyrmion bubbles and derive the existence of inhomogeneity in the skyrmion textures over the film thickness.

Writing and Detecting Topological Charges in Exfoliated Fe5-xGeTe2

Fe5-xGeTe2 is a promising two-dimensional (2D) van der Waals (vdW) magnet for practical applications, given its magnetic properties. These include Curie temperatures above room temperature, and topological spin textures─TST (both merons and skyrmions), responsible for a pronounced anomalous Hall effect (AHE) and its topological counterpart (THE), which can be harvested for spintronics. Here, we show that both the AHE and THE can be amplified considerably by just adjusting the thickness of exfoliated Fe5-xGeTe2, with THE becoming observable even in zero magnetic field due to a field-induced unbalance in topological charges. Using a complementary suite of techniques, including electronic transport, Lorentz transmission electron microscopy, and micromagnetic simulations, we reveal the emergence of substantial coercive fields upon exfoliation, which are absent in the bulk, implying thickness-dependent magnetic interactions that affect the TST. We detected a "magic" thickness t ≈ 30 nm where the formation of TST is maximized, inducing large magnitudes for the topological charge density (∼6.45 × 1020 cm-2), and the concomitant anomalous (ρxyA,max ≃22.6 μΩ cm) and topological (ρxyu,T 1≃5 μΩ cm) Hall resistivities at T ≈ 120 K. These values for ρxyA,max and ρxyu,T are higher than those found in magnetic topological insulators and, so far, the largest reported for 2D magnets. The hitherto unobserved THE under zero magnetic field could provide a platform for the writing and electrical detection of TST aiming at energy-efficient devices based on vdW ferromagnets.

Acoustic-driven magnetic skyrmion motion

Magnetic skyrmions have great potential for developing novel spintronic devices. The electrical manipulation of skyrmions has mainly relied on current-induced spin-orbit torques. Recently, it was suggested that the skyrmions could be more efficiently manipulated by surface acoustic waves (SAWs), an elastic wave that can couple with magnetic moment via the magnetoelastic effect. Here, by designing on-chip piezoelectric transducers that produce propagating SAW pulses, we experimentally demonstrate the directional motion of Néel-type skyrmions in Ta/CoFeB/MgO/Ta multilayers. We find that the shear horizontal wave effectively drives the motion of skyrmions, whereas the elastic wave with longitudinal and shear vertical displacements (Rayleigh wave) cannot produce the motion of skyrmions. A longitudinal motion along the SAW propagation direction and a transverse motion due to topological charge are simultaneously observed and further confirmed by our micromagnetic simulations. This work demonstrates that acoustic waves could be another promising approach for manipulating skyrmions, which could offer new opportunities for ultra-low power skyrmionics.

Optimizing skyrmionium movement and stability via stray magnetic fields in trilayer nanowire constructs

Skyrmioniums, known for their unique transport and regulatory properties, are emerging as potential cornerstones for future data storage systems. However, the stability of skyrmionium movement faces considerable challenges due to the skyrmion Hall effect, which is induced by deformation. In response, our research introduces an innovative solution: we utilized micro-magnetic simulations to create a sandwiched trilayer nanowire structure augmented with a stray magnetic field. This combination effectively guides the skyrmionium within the ferromagnetic (FM) layer. Our empirical investigations reveal that the use of a stray magnetic field not only reduces the size of the skyrmionium but also amplifies its stability. This dual-effect proficiently mitigates the deformation of skyrmionium movement and boosts their thermal stability. We find these positive outcomes are most pronounced at a particular intensity of the stray magnetic field. Importantly, the required stray magnetic field can be generated using a heavy metal (HM1) layer of suitable thickness, rendering the practical application of this approach plausible in real-world experiments. Additionally, we analyze the functioning mechanism based on the Landau-Lifshitz-Gilbert (LLG) equation and energy variation. We also develop a deep spiking neural network (DSNN), which achieves a remarkable recognition accuracy of 97%. This achievement is realized through supervised learning via the spike timing dependent plasticity rule (STDP), considering the nanostructure as an artificial synapse device that corresponds to the electrical properties of the nanostructure. In conclusion, our study provides invaluable insights for the design of innovative information storage devices utilizing skyrmionium technology. By tackling the issues presented by the skyrmion Hall effect, we outline a feasible route for the practical application of this advanced technology. Our research, therefore, serves as a robust platform for continued investigations in this field.

Dipolar skyrmions and antiskyrmions of arbitrary topological charge at room temperature

Magnetic skyrmions are localized, stable topological magnetic textures that can move and interact with each other like ordinary particles when an external stimulus is applied. The efficient control of the motion of spin textures using spin-polarized currents opened an opportunity for skyrmionic devices such as racetrack memory and neuromorphic or reservoir computing. The coexistence of skyrmions with high topological charge in the same system promises further possibilities for efficient technological applications. In this work, we directly observe dipolar skyrmions and antiskyrmions with arbitrary topological charge in Co/Ni multilayers at room temperature. We explore the dipolar-stabilized spin objects with topological charges of up to 10 and characterize their nucleation process, their energy dependence on the topological charge and the effect of the material parameters on their stability. Furthermore, our micromagnetic simulations demonstrate spin-transfer-induced motion of these spin objects, which is important for their potential device application.

Voltage-controlled magnetic anisotropy gradient-driven skyrmion-based half-adder and full-adder

Spintronic devices have revolutionized the way we process or store information compared to dissipative charge-based electronics. Among various spin-based technologies, skyrmions - topologically protected nano-size spin textures - have emerged as the most promising alternative for future data processing. Here, we have proposed binary adder circuits - central to most digital logic circuits - based on skyrmions. Using micromagnetic simulations, we have demonstrated half-adder and full-adder logic functionalities by precisely driving the skyrmions through voltage-controlled magnetic anisotropy gradient, besides taking advantage of the physical effects such as the skyrmion Hall effect, skyrmion-skyrmion topological repulsion and skyrmion-edge repulsions. The proposed voltage-control-based method of driving the skyrmions is energy efficient compared to the electrical current-driven approach, and it also overcomes the issue of Joule heating. A reliable operation in a wide range of Dzyaloshinskii-Moriya interaction strengths, magnetic anisotropy gradient, and dimensional parameters has been shown, which offers robustness to the device design. The results pave the way for the skyrmion-based computational architecture, which is significant for next-generation non-volatile data processing.