Skyrmion Hall Research Articles

Controllable Generation of Antiferromagnetic Skyrmions in Synthetic Antiferromagnets with Thermal Effect

AbstractMagnetic skyrmions have gained unprecedented attention for their potential applications in spintronic devices, such as non‐volatile memory, energy‐efficient, and non‐von Neumann devices. In particular, skyrmions in synthetic antiferromagnets (SAF) are believed to overcome the fundamental limitations of the widely studied ferromagnetic skyrmions in terms of size and effective current‐induced manipulation. Recently, several methods have been proposed to stabilize SAF skyrmions, however, the controllable creation and manipulation of skyrmions in SAF remains a challenge. Here, the generation, manipulation, and current‐driven dynamics of SAF skyrmions are studied by a thermal induced integrated device. The thermal created SAF skyrmion provides a prerequisite for the subsequent current‐driven dynamics research where apparent inhibition of skyrmion Hall effect is observed due to the effectively compensated topological charge. In addition, by tuning the heating current, the density of skyrmions in SAF can be effectively manipulated, which is a key factor for utilizing SAF skyrmions toward practical spintronic devices.

Zero-field skyrmionic states and in-field edge-skyrmions induced by boundary tuning

When magnetic skyrmions are moved via currents, they do not strictly travel along the path of the current, instead their motion also gains a transverse component. This so-called skyrmion Hall effect can be detrimental in potential skyrmion devices because it drives skyrmions towards the edge of their hosting material where they face potential annihilation. Here we experimentally modify a skyrmion model system—an atomic Pd/Fe bilayer on Ir(111)—by decorating the film edge with ferromagnetic Co/Fe patches. Employing spin-polarized scanning tunneling microscopy, we demonstrate that this ferromagnetic rim prevents skyrmion annihilation at the film edge and stabilizes skyrmions and target states in zero field. Furthermore, in an external magnetic field the Co/Fe rim can give rise to skyrmions pinned to the film edge. Spin dynamics simulations reveal how a combination of different attractive and repulsive skyrmion-edge interactions can induce such an edge-pinning effect for skyrmions.

Dynamic transformation between a skyrmion string and a bimeron string in a layered frustrated system

Frustrated topological spin textures have unique properties that may enable novel spintronic applications, such as helicity-based information storage and computing. Here, we report the statics and current-induced dynamics of two-dimensional (2D) pancake skyrmions in a stack of weakly coupled frustrated magnetic monolayers, which form a three-dimensional (3D) skyrmion string. The Bloch-type skyrmion string is energetically more stable than its N\'eel-type counterpart. It can be driven into translational motion by the dampinglike spin-orbit torque and shows the damping-dependent skyrmion Hall effect. Most notably, the skyrmion string can be transformed to a dynamically stable bimeron string by the dampinglike spin-orbit torque. The current-induced bimeron string rotates stably with respect to its center, which can spontaneously transform back to a skyrmion string when the current is switched off. Our results reveal unusual physical properties of 3D frustrated spin textures, and may open up different possibilities for spintronic applications based on skyrmion and bimeron strings.

Magnetic Skyrmions: Recent advances and applications

Magnetic skyrmions are particle-like, nanometer-sized topological spin textures observed in several magnetic materials. They have emerged as an alternative to conventional spintronic memories and domain walls (DWs) and offer high storage density, more robust stability, low critical currents, and increased scalability. Recent advances have set the stage for their use in quantum computing, logic circuits, and neuromorphic computing. With the aid of electrical methods, it is possible to precisely create, manipulate, and destroy skyrmions in device-compatible materials. However, the maximum speed achievable by magnetic skyrmions and the reliable detection of data have been restricted by the skyrmion Hall effect (SkHE). Other issues include a low read margin and a lack of proper skyrmion motion control in nanowires. Most of these can be addressed by exploiting novel materials, such as antiferromagnets; employing specialized fabrication techniques; tuning driving current profiles; and circuit-level engineering. In this article, theoretical and experimental breakthroughs and challenges relevant to magnetic skyrmions and their applications in data storage, logic computing, and neuromorphic computing are highlighted.

Configurable pixelated skyrmions on nanoscale magnetic grids

Topological spin textures can serve as non-volatile information carriers. Here we study the current-induced dynamics of an isolated magnetic skyrmion on a nanoscale square-grid pinning pattern formed by orthogonal defect lines with reduced magnetic anisotropy. The skyrmion on the square grid can be pixelated with a quantized size of the grid. We demonstrate that the position, size, and shape of skyrmion on the square grid are electrically configurable. The skyrmion center is quantized to be on the grid and the skyrmion may show a hopping motion instead of a continuous motion. We find that the skyrmion Hall effect can be perfectly prohibited due to the pinning effect of the grid. The pixelated skyrmion can be harnessed to build future programmable racetrack memory, multistate memory, and logic computing device. Our results will be a basis for digital information storage and computation based on pixelated topological spin textures on artificial pinning patterns.

Dynamic transition of current-driven single-skyrmion motion in a room-temperature chiral-lattice magnet

Driving and controlling single-skyrmion motion promises skyrmion-based spintronic applications. Recently progress has been made in moving skyrmionic bubbles in thin-film heterostructures and low-temperature chiral skyrmions in the FeGe helimagnet by electric current. Here, we report the motion tracking and control of a single skyrmion at room temperature in the chiral-lattice magnet Co9Zn9Mn2 using nanosecond current pulses. We have directly observed that the skyrmion Hall motion reverses its direction upon the reversal of skyrmion topological number using Lorentz transmission electron microscopy. Systematic measurements of the single-skyrmion trace as a function of electric current reveal a dynamic transition from the static pinned state to the linear flow motion via a creep event, in agreement with the theoretical prediction. We have clarified the role of skyrmion pinning and evaluated the intrinsic skyrmion Hall angle and the skyrmion velocity in the course of the dynamic transition. Our results pave a way to skyrmion applications in spintronic devices.

Dynamics of ferrimagnetic skyrmionium driven by spin-orbit torque

Magnetic skyrmionium is a skyrmionlike spin texture with nanoscale size and high mobility. It is a topologically trivial but dynamically stable structure, which can be used as a nonvolatile information carrier for next-generation spintronic storage and computing devices. Here we study the dynamics of a skyrmionium driven by the spin torque in a ferrimagnetic nanotrack. It is found that the direction of motion is jointly determined by the internal configuration of a skyrmionium and the spin polarization vector. Besides, the deformation of a skyrmionium induced by the intrinsic skyrmion Hall effect depends on both the magnitude of the driving force and the net angular momentum. The ferrimagnetic skyrmionium is most robust at the angular momentum compensation point, whose dynamics is quite similar to the skyrmionium in antiferromagnet. The skyrmion Hall effect is perfectly prohibited, where it is possible to observe the position of the skyrmionium by measuring the magnetization. Furthermore, the current-induced dynamics of a ferrimagnetic skyrmionium is compared with that of a ferromagnetic and antiferromagnetic skyrmionium. We also make a comparison between the motion of a ferrimagnetic skyrmionium and a skyrmion. Our results will open a new field of ferrimagnetic skyrmioniums for future development of ferrimagnetic spintronics devices.

Oblique drive tolerance of elliptical skyrmions moving in perpendicularly magnetized nanowire

A magnetic skyrmion is stabilized via the Dzyaloshinskii–Moriya interaction in a perpendicularly magnetized thin nanowire. When the skyrmion is driven by a spin-transfer torque due to spin currents flowing through the wire, the skyrmion approaches the wire edge owing to the skyrmion Hall effect. In other words, the skyrmion moves obliquely along the longitudinal direction of the wire. The skyrmion often breaks or disappears because of this oblique motion. In this study, we propose an elliptical skyrmion to prevent this disappearance. We simulated the current-induced motion of an elliptical skyrmion produced in a wire through a micromagnetic approach. The elliptical skyrmion was also moved obliquely to the longitudinal direction of the wire. When a small current flowed through the wire, the skyrmion moved in the longitudinal direction of the wire after it approached the wire edge. When a larger current flowed through the wire, the skyrmion disappeared after it approached the wire edge. The elliptical skyrmion can be driven over a long distance with a larger current compared to a circular skyrmion. The motion of the skyrmion approaching the wire edge was analyzed using Thiele’s equation, with an external force. We estimated the external force from the simulation results of the skyrmion motion. The external force was proportional to the distance between the skyrmion edge and the wire edge. The results of this study indicate that using the elliptical skyrmion as a binary digit in a magnetic memory, such as a skyrmion-based racetrack memory, can be advantageous in term of the stability of the binary digit.

Elliptical skyrmion moving along a track without transverse speed

It was recently demonstrated that introducing the shape anisotropy of a skyrmion is an alternative strategy for controlling the skyrmion Hall effect. For example, an elliptical skyrmion shows anisotropic motion under the applied driving force. Here, we study the fundamental properties of an elliptical skyrmion induced by uniaxial in-plane magnetic anisotropy. The dynamics of a skyrmion under various parameters, such as uniaxial in-plane magnetic anisotropy, interfacial Dzyaloshinskii-Moriya interaction, and the damping constant, are investigated, and we employ Thiele's theory and derive expressions for the velocity of the skyrmion that match very well with micromagnetic simulation results. We find that the skyrmion Hall angle can reach ${90}^{\ensuremath{\circ}}$ with the optimized angle between the in-plane easy axis and the current. Our results offer a potential application of racetrack memory based on skyrmions, and a type of racetrack configuration was designed.

Suppression of Skyrmion Hall Motion in Antiferromagnets Driven by Circularly Polarized Spin Waves

An investigation of spin waves interacting with antiferromagnetic spin textures is meaningful for future spintronic and magnonic-based memory and logic applications. In this work, we numerically study the skyrmion dynamics driven by circularly polarized spin waves in antiferromagnets and propose a method of suppressing the Hall motion. It is demonstrated that the application of two circularly polarized spin waves with opposite chirality allows the skyrmion motion straightly along the intersection line of the two spin wave sources. The skyrmion speed depending on these parameters of the spin waves and system is estimated, and a comparison with other methods is provided. Furthermore, two depinning behaviors of the skyrmion related to the strengths of the defect are also observed in the simulations. Thus, the proposed method could be used in precisely modulating the skyrmion dynamics, contributing to skyrmion-based memory device design.

Electric Field Control of the Skyrmion Hall Effect in Piezoelectric-Magnetic Devices

Relying on both electromechanical and micromagnetic simulations, we propose a method to control the trajectory of current-driven skyrmions using an electric field in hybrid piezoelectric-magnetic systems. By applying a voltage between two lateral electrodes, a transverse strain gradient is created, as a result of the nonuniform electric field profile in the piezoelectric material. Due to magnetoelastic coupling, this transverse gradient leads to a lateral force on the skyrmions that can be used to suppress the skyrmion Hall angle for any given current density, if a proper voltage is applied. We show that this method works under realistic conditions, such as the presence of disorder in the ferromagnet, and that skyrmion trajectories can be controlled with moderate voltages. Moreover, our method allows the maximum current density that can be injected before the skyrmion is annihilated at the nanostrip edge to be increased, which leads to an increase in the maximum achievable velocities.

Ultrahigh frequency spin-torque nano-oscillator based on bilayer-skyrmions

A skyrmion in a nanodisk with a single free layer (Single-layer Skyrmion) can be excited by the spin-polarized current to a steady state of circular motion for high-frequency applications. However, limited by the threshold of the driving current owning to the skyrmion Hall effect (SkHE), the maximum frequency of the skyrmion oscillation is usually 1 ∼ 3 GHz. In this work, the dynamics of skyrmions in the nanodisk with two antiferromagnetic-coupled free layers (Bilayer-skyrmions) are investigated. Our simulations show that the maximum frequency of Bilayer-skyrmions can be increased up to above 10 GHz, about one order of magnitude larger than that of Single-layer Skyrmion because of the disappearance of the SkHE in the antiferromagnetic-coupled system. In addition, it is found that the oscillation frequency of Bilayer-skyrmions linearly increases with the current density. Finally, spin-torque nano-oscillators based on multiple pairs of Bilayer-skyrmions are further demonstrated to increase the frequency limit up to 36.5 GHz. Our results may be useful for the design and applications of current-induced skyrmion-based oscillators for ultrahigh-frequency applications.

Thermally activated dynamics of current-driven skyrmions in random media

Based on the Landau-Lifshitz-Gilbert equation, we perform large-scale numerical simulations of the skyrmion dynamics with quench disorder at finite temperatures. Two universal creep behaviors are observed below the depinning transition current, and the creep exponents are accurately determined. The thermal rounding exponents at the depinning transition indicate an anisotropic temperature dependence in the perpendicular and parallel directions of the current. Further, the temperature-current phase diagram of the skyrmion Hall angle is builded, and the thermally activated forces are analyzed. Our results provide comprehensive understanding for the role of temperature in the current-driven skyrmion dynamics.

Suppression of the repulsion phenomenon of magnetic skyrmions at the end of synthetic antiferromagnetic racetracks

The synthetic antiferromagnetic (SAF) racetracks can perfectly suppress the skyrmion Hall effect, but the congestion phenomenon of the skyrmions at the end of the racetracks will seriously hinder the development of the skyrmion-based magnetic storage technology. In this paper, we have designed and investigated three racetrack structures of double-layer triangular notch, single-layer triangular notch (SLTN) and square hole-triangular notch (SHTN) in SAF racetracks by micromagnetic simulations. The critical current density that annihilates skyrmions in a racetrack with triangular notches is closely related to the angle of the triangular notches and decreases with the decrease of the angles. If a skyrmion in the top ferromagnetic (FM) layer of SLTN structure is annihilated, its counterpart in the bottom FM layer will also be annihilated due to the interlayer antiferromagnetic exchange coupling. Compared with the critical current density of 3.0 × 1012 A m−2 in a normal racetrack, the critical current density of SHTN structure can be dropped to 6.0 × 1010 A m−2. The results reveal that all the three structures can significantly reduce the critical current density, which can effectively solve the skyrmion congestion at the end of the racetracks.

Magnetic bibubble dynamics driven by magnetic field gradients

A magnetic bibubble is a composite structure involving two magnetic bubbles with topological skyrmion number Nsk=±2, which can be formed by merging two individual magnetic bubbles. Unlike the normal magnetic bubble, the axial symmetry of the bibubble is thus broken, which provide us an ideal platform to study the dynamics of topological structures with skyrmion number larger than unity. In this work, we studied the dynamics of the bibubble driven by a magnetic field gradient using micromagnetic simulations. We show that both the velocity and the skyrmion hall angle depend on the angle between the field gradient and the orientation of the bibubble. Moreover, the bibubble performs an orbital motion in the presence of circular field gradients accompanying with an oscillation of its orientation.

Dynamics and nonmonotonic drag for individually driven skyrmions

We examine the motion of an individual skyrmion driven through an assembly of other skyrmions in the absence of quenched disorder. The skyrmion behavior is determined by the ratio of the damping and Magnus terms, known as the intrinsic skyrmion Hall angle. For fixed drive in the damping dominated regime, the effective viscosity decreases monotonically with increasing skyrmion density. In contrast, in the Magnus dominated regime, the velocity varies nonmonotonically with density, and there is a regime in which the skyrmion moves faster with increasing density, as well as a pronounced speed-up effect in which a skyrmion traveling through a dense medium moves more rapidly than for low densities or the single-particle limit. The velocity-force curves in the Magnus-dominated regime exhibit an overshoot effect and other differences from the damping-dominated regime. We find a finite threshold force for skyrmion motion which increases with density as well as a drive-dependent skyrmion Hall angle. We map dynamic phase diagrams showing the threshold for motion, nonlinear flow, speed-up, and saturation regimes. In some cases, increasing the density can reduce the skyrmion Hall angle while producing a velocity boost, which could be valuable for applications.

Manipulation of Magnetic Skyrmion in a 2D van der Waals Heterostructure via Both Electric and Magnetic Fields

AbstractAs a promising candidate for the much‐desired low power consumption spintronic devices, 2D magnetic van der Waals material also provides a versatile platform for the design and control of topological spin textures. In this work on WTe2/CrCl3 bilayer van der Waals heterostructures, a complete Néel‐type skyrmion–bimeron–ferromagnet phase transition is demonstrated, accompanied by the evolution of the topological number. This cyclic transition, mediated by a perpendicular magnetic field, is largely driven by the competition between the out‐of‐plane magnetocrystalline anisotropy and magnetic dipole–dipole interaction. In the presence of a driving current, the Néel‐type skyrmion gains a higher velocity yet larger skyrmion Hall angle, in comparison to the bimeron. By incorporating a ferroelectric CuInP2S6 monolayer as a substrate, writing and erasing of skyrmions may be regulated using a ferroelectric polarization. This work sheds light on a novel approach to the design and control of magnetic skyrmions on 2D van der Waals materials.

Magnon-driven skyrmion dynamics in antiferromagnets: Effect of magnon polarization

The controllable magnetic skyrmion motion represents a highly concerned issue in preparing advanced skyrmion-based spintronic devices. Specifically, magnon-driven skyrmion motion can be easily accessible in both metallic and insulating magnets, and thus is highly preferred over electric current control further for the ultra-low energy consumption. In this work, we investigate extensively the dynamics of skyrmion motion driven by magnon in an antiferromagnet using the collective coordinate theory, focusing on the effect of magnon polarization. It is revealed that the skyrmion Hall motion driven by circularly polarized magnon becomes inevitable generally, consistent with earlier report. Furthermore, the elastic scattering theory and numerical results unveil the strong inter-dependence between the linearly polarized magnon and skyrmion motion, suggesting the complicated dependence of the skyrmion motion on the polarization nature of driving magnon. On the reversal, the scattering from the moving skyrmion may lead to decomposition of the linearly polarized magnon into two elliptically polarized magnon bands. Consequently, a net transverse force acting on skyrmion is generated owing to the broken mirror symmetry, which in turn drives a skyrmion Hall motion. The Hall motion can be completely suppressed only in some specific condition where the mirror symmetry is preserved. The present work unveils non-trivial skyrmion-magnon scattering behavior in antiferromagnets, advancing the antiferromagnetic spintronics and benefiting to high-performance devices.

Regular and in-plane skyrmions and antiskyrmions from boundary instabilities

We formulate a theory of skyrmion and antiskyrmion generation using magnetic field and charge current pulses. We show that the topological defect can be created at an edge of a system with Dzyaloshinskii-Moriya interaction (DMI) as well as at a boundary between regions with different DMI. We consider both perpendicular and in-plane (also known as magnetic bimerons) versions of skyrmions and antiskyrmions. We show that the magnetization twist in the vicinity of an edge or a boundary is described by a kink solution, the presence of which can instigate the generation of topological defects. We study the collective excitations of magnetization analytically and numerically, and demonstrate that under application of magnetic field and charge current pulses the magnon modes localized near boundaries can develop instabilities leading to the formation of skyrmions or antiskyrmions. Due to the skyrmion and antiskyrmion Hall effects, a properly chosen current direction can push the topological defects away from the boundary, thus facilitating their generation.

Transformation from antiferromagnetic target skyrmion to antiferromagnetic skyrmion by unzipping process through a confined nanostructure

The manipulation of magnetic skyrmion has been attracting considerable attention for the fundamental physical perspective and promising applications in spintronics, ascribed to their nontrivial topology and emergent electrodynamics. However, there is a hindrance to the transmission of a skyrmion in the racetrack memory due to the skyrmion Hall effect (SHE). Antiferromagnetic (AFM) materials provide a possibility to overcome the SHE in high-velocity data writing. Herein, we systematically investigate the generation and motion of an AFM target skyrmion under the spin-polarized current. We found that the AFM target skyrmion can reach a velocity of 1088.4 m s−1 under the current density of 8 × 1012 A m−2, which is lower than 1269.8 m s−1 for the AFM skyrmion. This slowdown can be ascribed to the deformation of AFM target skyrmion in the process of motion on a nanotrack. In addition, we observed a transformation from AFM target skyrmion to AFM skyrmion by the unzipping process through a constricted nanostructure which is mediated by the formation of AFM domain wall. Two energy barriers need to be overcome in this dynamic process, i.e. 2.93 × 104 eV from AFM target skyrmion to AFM domain wall, and 7.625 × 103 eV from AFM domain wall to AFM skyrmion. Our results provide guidance for future target skyrmion-based devices.