Current-driven Skyrmion Research Articles

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.

Strain-mediated multistate skyrmion for neuron devices.

Magnetic skyrmions are potential candidates for neuromorphic computing because of their inherent topological stability, low drive current density and nanoscale size. However, an artificial neuron device based on current-driven skyrmion motion cannot satisfy the requirement of energy efficiency and integration density due to hundreds of millions of interconnected neurons and synapses present in the deep networks. Here, we present a compact and energy efficient skyrmion-based artificial neuron consisting of ferromagnetic/heavy metal/ferroelectric layers which uses strain-mediated voltage manipulation of skyrmion states to mimic the Integrate-and-Fire (IF) function of biological neurons. By implementation of a spiking neural network (SNN) based on the proposed skyrmionic neuronal devices, it can achieve a high accuracy of 95.08% on a modified National Institute of Standards and Technology (MNIST) handwritten digit dataset, as well as a low power consumption of ∼46.8 fJ per epoch per neuron. The present work suggests a novel way to realize energy-efficient and high-density neuromorphic computing.

Nontraditional Movement Behavior of Skyrmion in a Circular-Ring Nanotrack.

Magnetic skyrmions are considered promising candidates for use as information carriers in future spintronic devices. To achieve the development of skyrmion-based spintronic devices, a reasonable and feasible nanotrack is essential. In this paper, we conducted a study on the current-driven skyrmion movement in a circular-ring-shaped nanotrack. Our results suggest that the asymmetry of the inside and outside boundary of the circular ring changed the stable position of the skyrmion, causing it to move like the skyrmion Hall effect when driven by currents. Moreover, the asymmetric boundaries have advantages in enhancing or weakening the skyrmion Hall effect. Additionally, we also compared the skyrmion Hall effect from the asymmetric boundary of circular-ring nanotracks with that from the inhomogeneous Dzyaloshinskii-Moriya interaction. It was found that the skyrmion Hall effect in the circular ring is significantly greater than that caused by the inhomogeneous Dzyaloshinskii-Moriya interaction. These results contribute to our understanding of the skyrmion dynamics in confined geometries and offer an alternative method for controlling the skyrmion Hall effect of skyrmion-based devices.

A magnetic skyrmion diode based on potential well inducting effect

Magnetic skyrmions have great potential in the application of spintronic devices due to their stable topologically protected spin configuration. To meet the needs of spintronic device design, it is necessary to manipulate the movement of the magnetic skyrmions. Here we propose a skyrmion diode based on potential well induced skyrmion motion through theoretical calculations. The potential well is generated by the voltage-controlled magnetic anisotropy (VCMA) gradient. By utilizing the induction of the potential well as well as the skyrmion Hall effect (SkHE), the velocity and trajectory of the skyrmions can be controlled and the forward pass and reverse cutoff functions of diode-like devices have been realized. Furthermore, we report the dynamics of current-driven skyrmions in a racetrack with locally applied VCMA. Under the influence of the SkHE, the difference in dynamic behavior between forward and reverse motion of the skyrmions is obvious, and the potential well can produce different pinning, depinning and annihilating effects on forward and reverse moving skyrmions. Our results can be beneficial for the design and development of magnetic skyrmion diodes.

Realizing high-velocity skyrmions via anisotropy engineering

Skyrmion-based racetrack memories are expected to be operated by the motion of skyrmions through magnetic nano-wires made of metallic multi-layers with high spin–orbit coupling and Dzyaloshinskii–Moriya interaction. The current-driven skyrmions in a ferromagnetic system had limitations due to their higher Hall angles and lower velocity. We addressed this by interfacial anisotropy engineering, which helped us to stabilize skyrmions at a thickness of x = 0.65 nm of Cu in the HM–FM interface of perpendicularly magnetized Ta3nmPt3nmCuxCo0.5nmPt1nm. At x = 0.60 nm Cu, the system showed worm like structures, while at 0.70 nm of Cu, the system showed in-plane magnetization. This system with optimum magnetic anisotropy enabled one to achieve skyrmions with a velocity of 260 m/s and low skyrmion Hall effect at room temperature with driving current <3×1011A/m2.

Giant tunability of microwave responses for current-driven skyrmions in a tapered nanostructure with notches

The investigation of the gigahertz dynamics of a skyrmion—a topologically protected chiral spin texture, offers several applications in high-frequency magnonic devices. In this work, we have investigated the motion and microwave resonant dynamics of skyrmions in an engineered nanostructure using micromagnetic simulations. The structure is a tapered ferromagnetic/heavy metal bilayer with notches to stabilize the skyrmion for dynamic studies. The skyrmion is moved along the nanostructure via current-induced spin–orbit torques. Multiple stable skyrmion states were demonstrated in the structure, where a remarkable tunability of 6 GHz is observed between the initial and the final state. This large shift in the resonant frequency has been attributed to the variation in the size of the skyrmion at various locations in the engineered nanostructure. The dependency of the skyrmion velocity and their microwave responses on the DzyalosnDzyaloshinskii–Moriya interaction strength and the perpendicular magnetic anisotropy is described. The calculation of the skyrmion Hall angle shed light on the motion and stability of the skyrmion in the structure.

Direct Observation of Magnetic Skyrmions and Their Current-Induced Dynamics in Epitaxial Single-Crystal Oxide Films.

Magnetic skyrmions are scarcely investigated for single-crystal quality films, for which skyrmions may have a remarkable performance. Even in the limited studies in this aspect, the skyrmions are usually probed by the topological Hall effect, missing important information on dynamic properties. Here, we present a comprehensive investigation on the generation/manipulation of magnetic skyrmions in La0.67Ba0.33MnO3 single-crystal films. Using the technique of magnetic force microscopy, the current-driven skyrmion dynamics are directly observed. Unlike isolated skyrmions produced by magnetic field alone, closely packed skyrmions can be generated by electric pulses in a magnetic background, with a high density (∼60/μm2) and a small size (dozens of nanometers). The threshold current moving skyrmions is ∼2.3 × 104 A/cm2, 2-3 orders of magnitude lower than that required by metallic multilayers or van der Waals ferromagnetic heterostructures. Our work demonstrates the great potential of single-crystal oxide films in developing skyrmion-based devices.

Current-driven magnetic skyrmion diodes controlled by voltage gates in synthetic antiferromagnets

Magnetic skyrmions, as promising candidates in various spintronic devices, have been widely studied owing to their particle-like properties, nanoscale size, and low driving current density. Here, we numerically and theoretically investigate the dynamics of current-driven skyrmion passing through a voltage gate in a synthetic antiferromagnetic racetrack. It is found that the critical current required for skyrmion to pass through the voltage gate positively is much less than that for skyrmion to pass through the gate negatively. Furthermore, we systematically study the linear dependence of the minimum velocity of skyrmion on the driving current density and perpendicular magnetic anisotropy (PMA) gradient, and the calculation results are quite consistent with the simulation results. Finally, we find that the variation of the PMA energy with the position of skyrmion can help us to compare the magnitude of resistance force when the skyrmion passes through different voltage gates. Our results can be beneficial for the design and development of skyrmion diodes.

Programmable skyrmion-based logic gates in a single nanotrack

Programmable logic devices with multiple functions are key ingredients in information technology. Here we propose a multifunction logic device based on the current-driven skyrmion motion in a single nanotrack, where the spatially modulated Dzyaloshinskii-Moriya interaction introduces a potential barrier to the skyrmion. Three stable stages of the skyrmion location under a driving current of different magnitudes are demonstrated at room temperature through micromagnetic simulations, according to which we implement all necessary logic gates for computing applications. Our proposal provides a feasible scheme to design skyrmion-based programmable logic devices.

Temperature dependence of current-driven and Brownian skyrmion dynamics in ferrimagnets with compensation point

Magnetic skyrmions are topological spin textures and promising candidates for novel spintronic applications. Recent studies on the current-driven dynamics of ferromagnetic (FM) skyrmions revealed that they exhibit an undesirable transverse motion, the skyrmion Hall effect. For antiferromagnetic (AFM) skyrmions, a vanishing skyrmion Hall effect was predicted, along with faster dynamics. However, their zero net magnetization obstructs efficient detection. Ferrimagnetic (FI) materials promise to combine both advantages: fast, AFM-like dynamics and easy read-out via stray fields. Here, we investigate the current-driven and Brownian dynamics of skyrmions in a FI with a compensation point. We perform atomistic spin dynamics simulations based on a model Hamiltonian and the stochastic Landau-Lifshitz-Gilbert equation supplemented with spin-orbit torques, accompanied by analytical calculations based on a collective coordinate approach. Our results unveil a nonmonotonic temperature dependence of the velocities and the diffusion coefficient with a strong enhancement at the angular momentum compensation temperature, due to scaling from FM- to AFM-like dynamics. These findings open up a new pathway for the efficient manipulation of skyrmion dynamics via temperature.

Stabilization and adiabatic control of antiferromagnetically coupled skyrmions without the topological Hall effect.

Synthetic antiferromagnetically coupled (SAF) multilayers provide different physics of stabilizing skyrmions while eliminating the topological Hall effect (THE), enabling efficient and stable control. The effects of material parameters, external current drive, and a magnetic field on the skyrmion equilibrium and propagation characteristics are largely unresolved. Here, we present a computational and theoretical demonstration of the large window of material parameters that stabilize SAF skyrmions determined by saturation magnetization, uniaxial anisotropy, and Dzyaloshinskii-Moriya interaction. Current-driven SAF skyrmion velocities reach ∼200 m s-1 without the THE. The SAF velocities are about 3-10 times greater than the typical ferromagnetic skyrmion velocities. The current densities needed for driving SAF skyrmions could be reduced to 108 A m-2, while 1011 A m-2 or above is needed for ferromagnetic skyrmions. By reducing the SAF skyrmion drive current by 3 orders, Joule heating is reduced by 6 orders of magnitude. These results pave the way for new SAF interfaces with improved equilibrium, dynamics, and power savings in THE-free skyrmionics.

Antiferromagnetic skyrmion based shape-configured leaky-integrate-fire neuron device

Spintronic devices based on antiferromagnetic (AFM) skyrmion motion on the nanotracks have gained significant interest as a key component of neuromorphic data processing systems. AFM skyrmions are favorable over the ferromagnetic (FM) skyrmions as they follow the straight trajectories and prevent its annihilation at the nanotrack edges. In this paper, the AFM skyrmion-based neuron device that exhibits the leaky-integrate-fire functionality is proposed for the first time. It exploits the current-driven skyrmion dynamics on the shape-configured nanotracks that are linearly decreasing and exponentially decaying. The device structure creates the regions from lower to higher energy states for the AFM skyrmions during its motion from the wider to narrower region. This causes the repulsion force from the nanotrack edges to act on the AFM skyrmion thereby, drifting it in the backward direction in order to minimize the system energy. This provides the leaking functionality to the neuron device without any external stimuli and additional hardware cost. The average velocities during the integration and leaky processes are in the order of 103 and 102 m s−1, respectively, for the linearly and exponentially tapered nanotracks. Moreover, the energy of the skyrmion is in the order 10−20 J. Hence, the suggested device opens up the path for the development of high-speed and energy-efficient devices in AFM spintronics for neuromorphic computing.

Asymmetric Motion of Magnetic Skyrmions in Ferromagnetic Nanotubes Induced by a Magnetic Field

Magnetic skyrmions, featuring topological stability and low driving current density, are believed to be a promising candidate of information carriers. One of the obstacles to application is the skyrmion Hall effect, which can lead to the annihilation of moving skyrmions at the lateral boundary of thin-film tracks. In order to resolve this issue, it was recently proposed to exploit ferromagnetic nanotubes as alternative skyrmion guides. In this work, we investigate the field-effect of current-driven skyrmion motion in nanotubes using micromagnetic simulations. It is found that, in the presence of an axial field, the skyrmion motion becomes asymmetric in tubes. This is fundamentally different from the flat strip, in which a field has little influence on the skyrmion dynamics. Based on the dissipation tensor determined by the spin texture of the skyrmions, the solution of the Thiele equation is obtained, yielding a perfect match with simulations. We argue that the asymmetry of the skyrmion dynamics originates from the curvature of the nanotube.

Writing skyrmion at a specific position in synthetic antiferromagnetic racetrack by voltage

Site-specific writing and directional movement of skyrmion in a magnetic stripe are very important for skyrmion-based racetrack memory. In this paper, we propose a system composed of circular heterostructure and a synthetic antiferromagnet racetrack where the circular heterostructure is fabricated at one end of the racetrack. The skyrmion can be firstly written by the voltage at the fixed-point position and then be driven by the current induced spin–orbit torques. A skyrmion is first nucleated in the ferromagnetic layer of the circular heterostructure by using voltage control magnetic anisotropy effect. In the meanwhile, an antiferromagnetic skyrmion is generated by antiferromagnetic exchange coupling from the magnetic layer of the circular heterostructure. Subsequently, the antiferromagnetic skyrmion is shifted straightly in the synthetic antiferromagnetic racetrack by the current-generated spin–orbit torques. This work provides a promising method for the fixed-point writing skyrmions in the current-driven skyrmion racetrack. Meanwhile, it provides theoretical guidance for the experimental research of skyrmionic storage and logic device.

Skyrmion Dynamics in the Presence of Deformation

The nonlinear dynamics of current-driven skyrmions are essential for designing skyrmion-based spintronic devices. However, Thiele's approach uses a rigid-body assumption that is not suitable here, as skyrmions may change shape at high speeds. This study extends Thiele's equation by including the radial and tangential force components. With this approach, one may depict the inner structure of a current-driven skyrmion and propose a way to suppress deformation with an in-plane magnetic field. This extended analytic formulation may help to describe the nonlinear dynamics of other spin textures as well, and to promote applications based on such dynamics.

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.

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.

Zeeman coupling and Dzyaloshinskii-Moriya interaction driven by electric current vorticity

The Dzyaloshinskii-Moriya interaction, i.e., antisymmetric exchange interaction, combined with a Zeeman magnetic field gives rise to various magnetic states such as chiral, helical, and skyrmionic states. This interaction conventionally originates from spin-orbit coupling and thus needs somewhat heavy elements. In contrast, we here show a Dzyaloshinskii-Moriya interaction which is driven by electric current vorticity. We also find that the vorticity acts on localized spins as a Zeeman field, which may explain a recent experiment on current-driven magnetic skyrmion creation and annihilation without an external magnetic field in a device with a notch structure in FeGe. theory explains the control of skyrmion creation and annihilation by current direction and opens different possibilities for studies of magnetic textures by using structural settings.

Random walk of antiferromagnetic skyrmions in granular films

The impressive absence of the skyrmion Hall effect of antiferromagnetic (AFM) skyrmions in clean systems has raised high expectations of AFM skyrmions as information carriers and the potential applications in data storage technology. Here, we report an undesired behavior of current-driven AFM skyrmion dynamics in granular thin films. Understanding the motion of AFM skyrmions in disordered systems is crucial because disorders are inevitable in all materials. Micromagnetic simulations show that AFM skyrmions deviate from their trajectory in the clean limit. Interestingly, the transverse (to the current direction) motion is diffusive, i.e., the mean square of transverse displacement is proportional to the traveling time. The longitudinal (along with the current direction) motion is always hindered by disorders. An effective theory based on the stochastic Thiele equation can account for the observed phenomena. At a very strong disorder above a critical value that depends on the current density, AFM skyrmions are pinned. Our findings should be important for future spintronic devices based on AFM skyrmions.

Helium Ions Put Magnetic Skyrmions on the Track.

Magnetic skyrmions are deemed to be the forerunners of novel spintronic memory and logic devices. While their observation and their current-driven motion at room temperature have been demonstrated, certain issues regarding their nucleation, stability, pinning, and skyrmion Hall effect still need to be overcome to realize functional devices. Here, we demonstrate that focused He+-ion-irradiation can be used to create and guide skyrmions in racetracks. We show that the reduction of the perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction in the track defined by ion-irradiation leads to the formation of stable isolated skyrmions. Current-driven skyrmion motion experiments and simulations reveal that the skyrmions move along the irradiated track, resulting in the suppression of the skyrmion Hall effect, and that the maximum skyrmion velocity can be enhanced by tuning the magnetic properties. These results open up a new path to nucleate and guide magnetic skyrmions in racetrack devices.