Skyrmion Hall Effect Research Articles

Driving skyrmions in flow regime in synthetic ferrimagnets

The last decade has seen significant improvements in our understanding of skyrmions current induced dynamics, along with their room temperature stabilization, however, the impact of local material inhomogeneities still remains an issue that impedes reaching the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with the aim of achieving high mobility and reduced skyrmion Hall effect. We consider Pt|Co|Tb multilayers of various thicknesses with antiferromagnetic coupling between the Co and Tb magnetization. The increase of Tb thickness in the multilayers reduces the total magnetic moment and increases the spin-orbit torques allowing to reach velocities up to 400 ms−1 for skyrmions with diameters of about 160 nm. We demonstrate that due to reduced skyrmion Hall effect combined with the edge repulsion of the magnetic track, the skyrmions move along the track without any transverse deflection. Further, by comparing the field-induced domain wall motion and current-induced skyrmion motion, we demonstrate that the skyrmions at the largest current densities present all the characteristics of a dynamical flow regime.

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

Dynamic Behavior of Above-Room-Temperature Robust Skyrmions in 2D Van der Waals Magnet.

Magnetic skyrmions are swirl-like spin configurations that present topological properties, which have great potential as information carriers for future high-density and low-energy-consumption devices. The optimization of skyrmion-hosting materials that can be integrated with semiconductor-based circuits is the primary challenge for their industrialization. Two-dimensional van der Waals ferromagnets are emerging materials that have excellent carrier mobility and compatibility with integrated circuits, making them an ideal candidate for spintronic devices. Here, we report the realization of skyrmions at above room temperature in the 2D ferromagnet Fe3GaTe2. The thickness tunability of their skyrmion size and the formation of the skyrmion lattice are revealed. Furthermore, we demonstrate that the skyrmions can be moved by a low-density current at room temperature, together with an apparent skyrmion Hall effect, which is consistent with our quantitative micromagnetic simulation. Our work offers a promising 2D material platform for harnessing magnetic skyrmions in practical device applications.

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.

Energy-efficient synthetic antiferromagnetic skyrmion-based artificial neuronal device

Magnetic skyrmions offer unique characteristics such as nanoscale size, particle-like behavior, topological stability, and low depinning current density. These properties make them promising candidates for next-generation spintronics-based memory and neuromorphic computing. However, one of their distinctive features is their tendency to deviate from the direction of the applied driving force that may lead to the skyrmion annihilation at the edge of nanotrack during skyrmion motion, known as the skyrmion Hall effect (SkHE). To overcome this problem, synthetic antiferromagnetic (SAF) skyrmions that having bilayer coupling effect allows them to follow a straight path by nullifying SkHE making them alternative for ferromagnetic (FM) counterpart. This study proposes an integrate-and-fire (IF) artificial neuron model based on SAF skyrmions with asymmetric wedge-shaped nanotrack having self-sustainability of skyrmion numbers at the device window. The model leverages inter-skyrmion repulsion to replicate the IF mechanism of biological neuron. The device threshold, determined by the maximum number of pinned skyrmions at the device window, can be adjusted by tuning the current density applied to the nanotrack. Neuronal spikes occur when initial skyrmion reaches the detection unit after surpassing the device window by the accumulation of repulsive force that result in reduction of the device’s contriving current results to design of high energy efficient for neuromorphic computing. Furthermore, work implements a binarized neuronal network accelerator using proposed IF neuron and SAF-SOT-MRAM based synaptic devices for national institute of standards and technology database image classification. The presented approach achieves significantly higher energy efficiency compared to existing technologies like SRAM and STT-MRAM, with improvements of 2.31x and 1.36x, respectively. The presented accelerator achieves 1.42x and 1.07x higher throughput efficiency per Watt as compared to conventional SRAM and STT-MRAM based designs.

Tunable topological magnetism in superlattices of nonmagnetic B20 systems

We predict topological magnetic properties of B20 systems, that are organized in atomically thin multilayers. In particular, we focus on FeSi/CoSi and FeSi/FeGe superlattices with different numbers of layers and interface structures. We demonstrate that the absence of long-range magnetic order, previously observed in bulk FeSi and CoSi, is broken near the FeSi/CoSi interface, where a magnetic state with a nontrivial topological texture appears. Using the Heisenberg and Dzyaloshinskii-Moriya (DM) interactions calculated from first principles, we perform finite-temperature atomistic spin dynamics simulations for up to 2×106 spins to capture the complexity of noncollinear textures. Our simulations predict the formation of antiskyrmions in a [001]-oriented FeSi/CoSi multilayer, intermediate skyrmions in a [111]-oriented FeSi/CoSi system, and Bloch skyrmions in the FeSi/FeGe (001) system, with a size between 7 and 37 nm. These varieties of topological magnetic textures in the studied systems can be attributed to the complex asymmetric structure of the DM matrix, which is different from previously known magnetic materials. We demonstrate that through structural engineering both ferromagnetic and antiferromagnetic skyrmions can be stabilized, where the latter are especially appealing for applications due to the zero skyrmion Hall effect. The proposed B20 multilayers show a potential for further exploration and call for experimental confirmation. Published by the American Physical Society 2024

Circular motion of noncollinear spin textures in Corbino disks: Dynamics of Néel- versus Bloch-type skyrmions and skyrmioniums

Magnetic skyrmions are nanoscale magnetic whirls that can be driven by currents via spin torques. They are promising candidates for spintronic devices such as the racetrack memory, where a motion along the uniform current is typically desired. However, for spin torque nano-oscillators in Corbino disks, the goal is to achieve a circular motion, perpendicular to the radially applied current. As we show, based on analytical calculations and micromagnetic simulations, Bloch skyrmions engage in a circular motion with frequencies in the MHz range when driven by spin-orbit torques. In contrast, Néel skyrmions get stuck at the edges of the disk. Our analysis reveals that the antagonistic dynamics between Bloch- and Néel-type magnetic textures arise from their helicity. Furthermore, we find that skyrmioniums, which are topologically trivial variations of skyrmions, move even faster and allow an increase in the current density without being pushed toward the edges of the disk. When driven by spin-transfer torques instead, Bloch and Néel skyrmions no longer exhibit different dynamics. Instead, they move along a circular trajectory due to the skyrmion Hall effect caused by their topological charge. Consequently, the topologically trivial skyrmioniums inevitably become trapped at the disk edge in this scenario. To provide a comprehensive understanding, our paper also examines currents applied tangentially, further enriching our insights into skyrmion dynamics and appropriate current injection methods for skyrmion-based devices. Published by the American Physical Society 2024

Current-driven dynamics of antiferromagnetic skyrmions: from skyrmion Hall effects to hybrid inter-skyrmion scattering

Antiferromagnetic (AFM) skyrmions have emerged as a highly promising avenue in the realm of spintronics, particularly for the development of advanced racetrack memory devices. A distinguishing feature of AFM skyrmions is the cancellation of their net topological charge, leading to an anticipated absence of the skyrmion Hall effect (SkHE). Here, we unveil that the latter is finite under the influence of spin-transfer torque, depending on the direction of the injected current impinging on intrinsic AFM skyrmions emerging in Cr/Pd/Fe trilayer on Ir(111) surface. Hinging on first principles combined with atomistic spin dynamics simulations, we identify the origin of the SkHE, which is due to the ellipticity of the skyrmions, and we uncover that FM skyrmions in the underlying Fe layer act as effective traps for AFM skyrmions, confining them and affecting their velocity. These findings hold significant promise for spintronic applications, the design of multi-purpose skyrmion tracks while advancing our understanding of AFM–FM skyrmion interactions and hybrid soliton dynamics in heterostructures.

Transformation of a cellular skyrmion to polyomino-like structures

Topological spin structures with transformable shapes may have potential implications for data storage and computation. Here, we demonstrate that a square cellular skyrmion on an artificial grid pinning pattern can be manipulated by programmed current pulses. We find that parallel short pulses could result in elongation of the skyrmion mainly in the current direction, while parallel long pulses are able to induce elongation in the direction perpendicular to the current due to the intrinsic skyrmion Hall effect. Consequently, a programmed sequence of parallel pulses could lead to transformation of the skyrmion to I-, L-, and Z-shaped polyomino-like structures without affecting the topological charge. In addition, we find that orthogonal pulses could lead to the transformation to more complex polyomino-like structures, including T-shaped and irregular ones. Particularly, when a small T-shaped structure is formed, the topological charge of the system is found to be non-integer due to incomplete compensation of local topological charge densities; however, the T-shaped structure is stable on the attractive pinning pattern. Our results offer an effective way to create polyomino-like spin structures toward functional applications.

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.

Skyrmion motion induced by spin-waves on magnetic nanotubes

Abstract We investigate the skyrmion motion driven by spin waves on magnetic nanotubes through micromagnetic simulations. Our key results include demonstrating the stability and enhanced mobility of skyrmions on the edgeless nanotube geometry, which prevents destruction at boundaries — a common issue in planar geometries. We explore the influence of the damping coefficient, amplitude, and frequency of microwaves on skyrmion dynamics, revealing a non-uniform velocity profile characterized by acceleration and deceleration phases. Our results show that the skyrmion Hall effect is significantly modulated on nanotubes compared to planar models, with specific dependencies on the spin-wave parameters. These findings provide insights into skyrmion manipulation for spintronic applications, highlighting the potential for high-speed and efficient information transport in magnonic devices.

Manipulation of hybrid skyrmion dynamics by step Dzyaloshinskii–Moriya interaction approach

The topological protected magnetic state, which plays a pivotal role against any continuous deformation of a magnetic skyrmion, comes with an unwanted skyrmion Hall effect (SkHE) that poses a significant challenge in practical applications. Here, we present a detailed micromagnetic simulation study that delves into the controlled manipulation of skyrmion dynamics through subtle engineering of the Dzyaloshinskii–Moriya interaction (DMI) in a hybrid skyrmion-based racetrack. In particular, we introduce a gradient variation of bulk and interfacial DMIs, which results in a parabolic trajectory of the skyrmion motion, thereby allowing us to find a critical DMI ratio with almost zero SkHE. Most importantly, we present a novel approach involving the engineering of a racetrack with strategically placed step DMI regions that gives us meticulous control over the size and speed of the hybrid skyrmions. The present study gives a new direction for the simultaneous realization of stable skyrmions without SkHE and an increased skyrmion speed with optimized DMI engineering.

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.

Bending skyrmion strings under two-dimensional thermal gradients

Magnetic skyrmions are topologically protected magnetization vortices that form three-dimensional strings in chiral magnets. With the manipulation of skyrmions being key to their application in devices, the focus has been on their dynamics within the vortex plane, while the dynamical control of skyrmion strings remained uncharted territory. Here, we report the effective bending of three-dimensional skyrmion strings in the chiral magnet MnSi in orthogonal thermal gradients using small angle neutron scattering. This dynamical behavior is achieved by exploiting the temperature-dependent skyrmion Hall effect, which is unexpected in the framework of skyrmion dynamics. We thus provide experimental evidence for the existence of magnon friction, which was recently proposed to be a key ingredient for capturing skyrmion dynamics, requiring a modification of Thiele’s equation. Our work therefore suggests the existence of an extra degree of freedom for the manipulation of three-dimensional skyrmions.

Suppression of the skyrmion Hall effect in synthetic ferrimagnets with gradient magnetization

Magnetic skyrmions are promising building blocks for future spintronic devices. However, the skyrmion Hall effect (SkHE) remains an obstacle for practical applications based on the in-line transport of skyrmions. Here, we numerically study the static properties and current-driven dynamics of synthetic ferrimagnetic skyrmions. Inspired by graded-index magnonics, we introduce a linear gradient of saturation magnetization (Ms) in the skyrmion-hosting sample, which effectively modulates the skyrmion Hall angle and suppresses the SkHE. Micromagnetic simulations reveal that ferrimagnetic skyrmions could exhibit greater susceptibility to the variation of Ms as compared to their ferromagnetic counterparts. The Thiele analysis is also applied to support the simulation results, which elucidates that the Ms gradient dynamically modifies the intrinsic normalized size of skyrmions, consequently impacting the SkHE. Our results pave the way to the graded-index skyrmionics, which offers novel insights for designing ferrimagnet-based skyrmionic devices. Published by the American Physical Society 2024

Channeling skyrmions: Suppressing the skyrmion Hall effect in ferrimagnetic nanostripes

Channeling skyrmions: Suppressing the skyrmion Hall effect in ferrimagnetic nanostripes

Skyrmionium dynamics on a racetrack in the presence of a magnetic defect

Exotic topological textures known as skyrmioniums can stabilize in different types of ferromagnetic nanostructures. They possess a topological charge of Q=0 and can move in straight lines unaffected by the skyrmion Hall effect. In this work, using micromagnetic simulations, we study the behavior of a skyrmionium when a polarized spin current transports it along a racetrack in the presence of a semicircular magnetic defect with different Dzyaloshinskii–Moriya interaction values. As a first step, we obtained the perpendicular uniaxial anisotropy (Kz) and the Dzyaloshinskii–Moriya constants (Dint) for which it is possible to form a skyrmionium in the racetrack. Our results show that the higher the Kz value, the higher the Dint values necessary to stabilize the skyrmionium. Furthermore, interaction energies between the skyrmionium and the semicircular defect are presented. The results show that the interaction energy can be attractive or repulsive, depending on the Dzyaloshinskii–Moriya defect interaction constant (Dintdef). Finally, we conclude that using these types of defects and a spin-polarized current, the motion of the skyrmionium can be either modified in a specific way or even blocked.

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.

Quantum skyrmion Hall effect

We consider the problem of magnetic charges in (2+1) dimensions for a torus geometry in real space, subjected to an inverted Lorentz force due to an external electric field applied normal to the surface of the torus. We compute the Hall conductivity associated with transport of these charges for the case of finite energy gap between the ground state and excitations and global U(1) charge conservation symmetry, and find it is proportional to an integer-valued topological invariant Q, corresponding to the magnetic quantum Hall effect (MQHE). We identify a lattice model realizing this physics in the absence of an external electric field. Based on this, we identify a generalization of the MQHE to be quantized transport of magnetic skyrmions, the quantum skyrmion Hall effect (QSkHE), with a U(1) easy-plane anisotropy/spin rotation symmetry of magnetic skyrmions and effective conservation of charge associated with magnetic skyrmions yielding incompressibility, provided a hierarchy of energy scales is respected. As the lattice model may be characterized both by a total Chern number and the topological invariant Q, we furthermore outline a possible field theory for electric charges, magnetic charges, and correlations between magnetic and electric charges approximated as composite particles, on a two-torus, to handle the scenario of intermediate-strength correlations between electric and magnetic charges modeled as composite particles. We map this problem to a generalized (4+1)D theory of the quantum Hall effect for the composite particles. Published by the American Physical Society 2024

Transformation of a skyrmionium to a skyrmion through the thermal annihilation of the inner skyrmion

Skyrmioniums are doughnutlike topological spin textures in chiral magnets, which are able to move with zero skyrmion Hall effect. The annihilation of a skyrmionium involves the topological transition through singularity formation at the continuum level. Here we analytically and numerically study a type of thermal annihilation of a skyrmionium under the framework of micromagnetism. The skyrmionium initially exists in an ultrathin ferromagnetic film in the exchange-dominated regime and is stabilized under a uniform perpendicular magnetic field. The thermal annihilation of the “skyrmion” inside the skyrmionium results in the transformation of the skyrmionium to a skyrmion. We derive an analytical formula in the Arrhenius form describing the annihilation rate of this process. We find that the analytical solutions agree well with the micromagnetic simulation results. Besides, from the large deviation point of view, we derive the dynamical path for the collapse of a skyrmionium. Our results are useful for understanding the thermal stability of skyrmioniums and may lead to spintronic applications based on the thermal manipulation of skyrmioniums. Published by the American Physical Society 2024