Skyrmion-based Devices Research Articles

Spin current pumped by confined breathing skyrmion

Spin pumping is a widely recognized method to generate the spin current in the spintronics, which can be found in varieties of magnetic materials and is acknowledged as a fundamentally dynamic process equivalent to the spin-transfer torque. In this work, we theoretically verified the oscillating spin current mixed by AC and DC components can be pumped from the microwave-motivated breathing skyrmion. The spin current can be pumped within a relatively low-frequency microwave compared with the in-plane ferromagnetic resonance (FMR). Although the amplitude of spin current density for a single confined skyrmion is several times lower than the FMR spin pumping, the pumped current would be improved to be the same order as the FMR through a tight skyrmion lattice. Based on the spin pumping of breathing skyrmion, we designed a high reading-speed racetrack memory model whose reading speed is much higher than the SOT (spin–orbit torque)/STT (spin-transfer torque) skyrmion racetrack. Our work focuses on the spin pumping phenomenon inside the breathing skyrmion, and it may contribute to the applications of the skyrmion-based device.

Theoretical study on stabilization and destabilization of magnetic skyrmions by uniaxial-strain-induced anisotropic Dzyaloshinskii-Moriya interactions

Magnetic skyrmions in chiral-lattice ferromagnets are currently attracting enormous research interest because of their potential applications in spintronic devices. However, they emerge in bulk specimens only in a narrow window of temperature and magnetic field. This limited stability regime is recognized as an obstacle to technical applications. Recent experiments demonstrated that the thermodynamic stability of magnetic skyrmions is enhanced or suppressed by the application of a uniaxial strain depending on its axial direction in bulk chiral-lattice ferromagnets MnSi [Y. Nii et al., Nat. Commun. 6, 8539 (2015), A. Chacon et al., Phys. Rev. Lett. 115, 267202 (2015)] and Cu2OSeO3 [S. Seki et al., Phys. Rev. B 96, 220404(R) (2017)]. Motivated by these experimental discoveries, we theoretically investigated the effects of anisotropic Dzyaloshinskii--Moriya interactions on the stability of magnetic skyrmions caused by this uniaxial strain. We find that magnetic skyrmions are significantly stabilized (destabilized) in the presence of anisotropic DM interactions when an external magnetic field lies perpendicular (parallel) to the anisotropy axis, along which the DM coupling is strengthened. Our results account completely for the experimentally observed strain-induced stabilization and destabilization of magnetic skyrmions and provide a firm ground for possible strain engineering of skyrmion-based electronic devices.

Controlled Individual Skyrmion Nucleation at Artificial Defects Formed by Ion Irradiation

Magnetic skyrmions are particle-like deformations in a magnetic texture. They have great potential as information carriers in spintronic devices because of their interesting topological properties and favorable motion under spin currents. A new method of nucleating skyrmions at nanoscale defect sites, created in a controlled manner with focused ion beam irradiation, in polycrystalline magnetic multilayer samples with an interfacial Dzyaloshinskii-Moriya interaction, is reported. This new method has three notable advantages: 1) localization of nucleation; 2) stability over a larger range of external field strengths, including stability at zero field; and 3) existence of skyrmions in material systems where, prior to defect fabrication, skyrmions were not previously obtained by field cycling. Additionally, it is observed that the size of defect nucleated skyrmions is uninfluenced by the defect itself-provided that the artificial defects are controlled to be smaller than the inherent skyrmion size. All of these characteristics are expected to be useful toward the goal of realizing a skyrmion-based spintronic device. This phenomenon is studied with a range of transmission electron microscopy techniques to probe quantitatively the magnetic behavior at the defects with applied field and correlate this with the structural impact of the defects.

Formation of skyrmion and skyrmionium in confined nanodisk with perpendicular magnetic anisotropy

We theoretically investigate the micromagnetic dynamics and spin configurations in a CoFeB/Pt nanodisk with varying effective anisotropy constant (K) and radius (R). It is found that skyrmion and skyrmionium could be stabilized well in a certain region of the phase diagram of topological structures, and their formations physically originated from the competition between exchange interactions and anisotropy contributions. Furthermore, we achieve the transition between the skyrmion and the skyrmionium through fine tuning the magnetic anisotropy. Our results reveal the specific conditions needed to form the equilibrium states of skyrmions and skyrmionium and have significance for the development of skyrmion-based devices.

Skyrmion-based artificial synapses for neuromorphic computing

Since the experimental discovery of magnetic skyrmions achieved one decade ago, there have been significant efforts to bring the virtual particles into all-electrical fully functional devices, inspired by their fascinating physical and topological properties suitable for future low-power electronics. Here, we experimentally demonstrate such a device: electrically-operating skyrmion-based artificial synaptic device designed for neuromorphic computing. We present that controlled current-induced creation, motion, detection and deletion of skyrmions in ferrimagnetic multilayers can be harnessed in a single device at room temperature to imitate the behaviors of biological synapses. Using simulations, we demonstrate that such skyrmion-based synapses could be used to perform neuromorphic pattern-recognition computing using handwritten recognition data set, reaching to the accuracy of ~89 percents, comparable to the software-based training accuracy of ~94 percents. Chip-level simulation then highlights the potential of skyrmion synapse compared to existing technologies. Our findings experimentally illustrate the basic concepts of skyrmion-based fully functional electronic devices while providing a new building block in the emerging field of spintronics-based bio-inspired computing.

Mapping skyrmion stability in uniaxial lacunar spinel magnets from first principles

The identification of general principles for stabilizing magnetic skyrmion phases in bulk materials over wide ranges of temperatures is a prerequisite to the development of skyrmion-based spintronic devices. Lacunar spinels with the formula GaM4X8 with M=V, Mo; X=S, Se are a convenient case study towards this goal as they are some of the first bulk systems suggested to host equilibrium chiral skyrmions far from the paramagnetic transition. We derive the magnetic phase diagrams likely to be observed in these materials, accounting for all possible magnetic interactions, and prove that skyrmion stability in the lacunar spinels is a general consequence of their crystal symmetry rather than the details of the material chemistry. Our results are consistent with all experimental reports in this space and demonstrate that the differences in the phase diagrams of particular spinel chemistries are determined by magnetocrystalline anisotropy, up to a normalization factor. We conclude that skyrmion formation over wide ranges of temperatures can be expected in all lacunar spinels, as well as in a wide range of uniaxial systems with low magnetocrystalline anisotropy.

Dynamics of an elliptical ferromagnetic skyrmion driven by the spin–orbit torque

Magnetic skyrmions are promising building blocks for developing information storage and computing devices. It can be stabilized in a ferromagnetic thin film with the Dzyaloshinskii-Moriya interaction (DMI). The moving ferromagnetic skyrmion may show the skyrmion Hall effect, that is, the skyrmion shows a transverse shift when it is driven by a spin current. Here, we numerically and theoretically study the current-driven dynamics of a ferromagnetic nanoscale skyrmion in the presence of the anisotropic DMI, where the skyrmion has an elliptical shape. The skyrmion Hall effect of the elliptical skyrmion is investigated. It is found that the skyrmion Hall angle can be controlled by tuning the profile of the elliptical skyrmion. Our results reveal the relation between the skyrmion shape and the skyrmion Hall effect, which could be useful for building skyrmion-based spintronic devices with a preferred skyrmion Hall angle. Also, our results provide a method for the minimization of the skyrmion Hall angle for applications based on in-line motion of skyrmions.

Skyrmion-based JK flip–flop with a wedge-shaped circular track

Magnetic skyrmion can be used as information carrier of logic devices because of its nano size and topological properties. Here, we introduce a wedge-shaped circular track structure, in which the cycle motion of skyrmion in the ferromagnetic track is induced by the gradient of voltage-controlled magnetic anisotropy. Compared with the straight track, the process of generation and annihilation of skyrmion is avoided due to the recycling of skyrmion. Furthermore, the logic of the JK flip–flops is successfully implemented in the wedge-shaped circular track, which provides guidance for the design of skyrmion-based sequential logic devices.

Current-Induced Dynamics of the Antiferromagnetic Skyrmion and Skyrmionium

Antiferromagnetic (AFM) skyrmionium composed of two topological AFM skyrmions shares the merits of an AFM skyrmion, for example, high mobility and no skyrmion Hall effect. Here, we analytically and numerically study the dynamics of the AFM skyrmion and skyrmionium induced by spin currents. Our calculations demonstrate that the current-induced spin-transfer torques can drive AFM skyrmion and skyrmionium with the same speed, while their steady motion speeds induced by spin-orbit torques are different. Furthermore, it is found that due to the existence of the effective AFM texture mass, the AFM skyrmion and skyrmionium obey the momentum theorem, and the time evolution of the position induced by alternating currents presents a phase. Besides, a spin torque nano-oscillator based on the AFM skyrmionium can produce high frequencies, similar to that based on the AFM skyrmion. Numerical simulations are in good agreement with the analytical solutions. Our results demonstrate the inertial dynamics of the AFM skyrmion and skyrmionium and may provide guidelines for building skyrmion-based spintronic devices.

Magnetic skyrmions in nanostructures of non-centrosymmetric materials

Magnetic skyrmions are a new form of magnetic ordering with whirlpool-like spin arrangements. These topologically protected particlelike spin textures were first discovered a decade ago in noncentrosymmetric magnetic materials. Confining magnetic skyrmions in nanostructures leads to interesting fundamental insights into skyrmion stability and could provide convenient platforms for potential practical applications of skyrmions in information storage technology. In this research update, we summarize the recent advances on studying magnetic skyrmions in nanostructures of skyrmion hosting noncentrosymmetric materials (especially the B20 materials) made via bottom-up synthesis or top-down fabrication methods. We discuss various real space imaging (such as Lorentz transmission electron microscopy or electron holography) or physical property measurement (such as magneto-transport) techniques that have been used to observe and detect these exotic magnetic domains in both nanostructure and bulk samples, which have proven to be critical to fully understanding them. We examine the importance of morphology and dimensionality of skyrmion hosting materials in stabilizing isolated magnetic skyrmions in confined geometry and their benefits for implementation in magnetic memory applications. We further highlight the need for experiments that allow the skyrmion research to move from the fundamental physics of skyrmion formation and dynamics to more applied device studies and eventual applications, such as the all-electrical writing and reading of skyrmions needed for skyrmion-based high density magnetic memory storage devices.

Formation and current-induced motion of synthetic antiferromagnetic skyrmion bubbles

Skyrmion, a topologically-protected soliton, is known to emerge via electron spin in various magnetic materials. The magnetic skyrmion can be driven by low current density and has a potential to be stabilized in nanoscale, offering new directions of spintronics. However, there remain some fundamental issues in widely-studied ferromagnetic systems, which include a difficulty to realize stable ultrasmall skyrmions at room temperature, presence of the skyrmion Hall effect, and limitation of velocity owing to the topological charge. Here we show skyrmion bubbles in a synthetic antiferromagnetic coupled multilayer that are free from the above issues. Additive Dzyaloshinskii-Moriya interaction and spin-orbit torque (SOT) of the tailored stack allow stable skyrmion bubbles at room temperature, significantly smaller threshold current density or higher speed for motion, and negligible skyrmion Hall effect, with a potential to be scaled down to nanometer dimensions. The results offer a promising pathway toward nanoscale and energy-efficient skyrmion-based devices.

Atomistic simulations of spin-lattice coupling effects on magnetomechanics in skyrmion materials

Skyrmion-based spintronic devices are considered one of the prime candidates for the next generation of functional devices. The mechanistic understanding of magnetomechanics arising from the intrinsic spin-lattice coupling in skyrmion materials becomes an attractive issue. In this paper, by taking MnSi as an example, we proposed an atomistic simulation model based on the developed spin-lattice dynamics scheme for skyrmion materials. The calculated magnetomechanical properties are examined to be qualitatively consistent with the experimental measurement, which suggests that our model is appropriate for the study of thermodynamics, magnetics, and mechanics of skyrmion materials. In addition, the microscopic understanding of the thermodynamic and kinetic aspects of magnetomechanics in skyrmion materials are presented. Through the intrinsic spin-lattice coupling, the transformation of magnetic ordering by external magnetic field results in the change of mechanical properties, for instance, the global mechanical anisotropy and local distortion of lattice structure. The calculation results indicate the magnetomechanics are the result of spin nonalignment induced by the synergistic effects of Dzyaloshinsky-Moriya interaction, Heisenberg exchange interaction, and Zeeman interaction of interspins and elastic interactions of interlattices. Further, the relaxation time of the lattice structure is calculated to characterize the speed of mechanical response against the magnetic ordering changes, which might provide hints of low-energy consumption and low applied electric current in driving skyrmions. The microscopic understanding provided by current study could help the research and design of skyrmion-based spintronic devices.

Deterministic Field-Free Skyrmion Nucleation at a Nanoengineered Injector Device.

Magnetic skyrmions are topological solitons promising for applications as encoders for digital information. A number of different skyrmion-based memory devices have been recently proposed. In order to demonstrate a viable skyrmion-based memory device, it is necessary to reliably and reproducibly nucleate, displace, detect, and delete the magnetic skyrmions, possibly in the absence of external applied magnetic fields, which would needlessly complicate the device design. While the skyrmion displacement and detection have both been thoroughly investigated, much less attention has been dedicated to the study of the skyrmion nucleation process and its sub-nanosecond dynamics. In this study, we investigate the nucleation of magnetic skyrmions from a dedicated nanoengineered injector, demonstrating the reliable magnetic skyrmion nucleation at the remnant state. The sub-nanosecond dynamics of the skyrmion nucleation process were also investigated, allowing us to shine light on the physical processes driving the nucleation.

Analytical modeling of the interaction between skyrmions and extended defects

The performance of skyrmion-based spintronic devices can be greatly affected by the presence of defects in the materials. Here we develop an analytic model for describing the interaction between skyrmions and defects in ultrathin films focusing on the case of extended defects. The dynamics of skyrmions under the driving of in-plane polarized current is studied, considering several types of torques. The system is modeled with Thiele's equation in the complex plane and described with a reduced number of parameters. We start by considering a Gaussian-like interaction with point defects and extend the treatment to segment, line, and grid defects. Conditions for dynamic regimes of pinning, guiding, accelerating, or arranging skyrmions by defects are established and discussed. In particular, expressions for the threshold driven current density to pin or depin skyrmions in such defects, the position of the critical points, as well as the guiding conditions along long defects are found, analytically in some cases. This enables a deeper understanding of the parameter dependence of the skyrmion dynamics under the influence of defects. Micromagnetic simulations show qualitative agreement with the analytical treatment.

Magnetic Skyrmion Spectrum Under Voltage Excitation and its Linear Modulation

Magnetic skyrmions are topological quasiparticles with great potential for applications in future information storage and processing devices because of their nanoscale size, high stability, and large velocity. Recently, the high-frequency properties of skyrmions have been explored for magnon nanodevices. Here we systematically study the dynamics of an isolated skyrmion under voltage excitation through the voltage-controlled magnetic anisotropy effect in a circular thin film. A theoretical model considering the demagnetization energy, which has often been neglected or treated superficially in previous skyrmion research but is demonstrated to have importance in determining the skyrmion dynamic state, is developed. With our model, the periodic oscillation of the skyrmion radius can be solved numerically with similar precision compared to micromgnetic simulations, and the characteristic frequency of the skyrmion breathing can be determined analytically with greater precision than previous studies. Furthermore, we find that the breathing skyrmion can be analogized as a modulator by investigating its linear modulation functionality under sinusoidal-form voltage excitation. Different from the conventional modulation system with complex CMOS circuits, this skyrmion modulator device integrates with both the modulation and carrier wave generation functionality, thus showing greater convenience and efficiency in applications. Our findings can provide useful guidance for both theoretical and experimental skyrmions research as well as the development of skyrmion-based magnonic devices with significant potential applicability in future communication system.

Directional spin-wave propagation in the skyrmion chain

The Dzyaloshinskii-Moriya interaction (DMI), favoring a chiral spin structure like the skyrmion, gives rise to the nonreciprocal propagation of spin waves. We investigate the propagation of spin waves in a nanostripe with the presence of a skyrmion chain by using micromagnetic simulations. Through applying a microwave locally, it is found that when the interval between skyrmions is large enough, the spin waves can be separated to the counter direction according to different frequencies. While for the tightly arranged skyrmions, the skyrmion chain with strong interactions between skyrmions becomes a channel for spin waves, which is around the frequency of skyrmion breathing and exhibit a characteristic of directional propagation. This work opens a vista for skyrmion-based spin wave devices.

Topological spin Hall effects and tunable skyrmion Hall effects in uniaxial antiferromagnetic insulators

Recent advances in the physics of current-driven antiferromagnetic skyrmions have observed the absence of a Magnus force. We outline the symmetry reasons for this phenomenon, and show that this cancellation will fail in the case of spin polarized currents. Pairing micromagnetic simulations with semiclassical spin wave transport theory, we demonstrate that skyrmions produce a spin-polarized transverse magnon current, and that spin-polarized magnon currents can in turn produce transverse motion of antiferromagnetic skyrmions. We examine qualitative differences in the frequency dependence of the skyrmion Hall angle between ferromagnetic and antiferromagnetic cases, and close by proposing a simple skyrmion-based magnonic device for demultiplexing of spin channels.

Nanoscale magnetic skyrmions and target states in confined geometries

Research on magnetic systems with broken inversion symmetry has been stimulated by the experimental proof of particle-like configurations known as skyrmions, whose non-trivial topological properties make them ideal candidates for spintronic technology. This class of materials enables Dzyaloshinskii-Moriya interactions (DMI) which favor the stabilization of chiral configurations. Recent advances in material engineering have shown that in confined geometries it is possible to stabilize skyrmionic configurations at zero field. Moreover, it has been shown that in systems based on Pd/Fe bilayers on top of Ir(111) surfaces skyrmions can be as small as a few nanometres in diameter. In this work we present scanning tunneling microscopy measurements of small Pd/Fe and Pd$_2$/Fe islands on Ir(111) that exhibit a variety of different spin textures, which can be reproduced using discrete spin simulations. These configurations include skyrmions and skyrmion-like states with extra spin rotations such as the target state, which have been of interest due to their promising dynamic properties. Furthermore, using simulations we analyze the stability of these skyrmionic textures as a function of island size, applied field and boundary conditions of the system. An understanding of the parameters and conditions affecting the stability of these magnetic structures in confined geometries is crucial for the development of energetically efficient and optimally sized skyrmion-based devices.

Spin-wave mediated interactions for majority computation using Skyrmions and spin-torque nano-oscillators

Recent progress in all-electrical nucleation, detection and manipulation of magnetic skyrmions has unlocked the tremendous potential of skyrmion-based spintronic devices. Here, we show via micromagnetic simulations that the stable magnetic oscillations of STNO radiate spin waves (SWs) that can be scattered in the presence of skyrmions in the near vicinity. Interference between SWs emitted by the STNO and SWs scattered by the skyrmion gives rise to interesting dynamics that leads to amplification or attenuation of STNO’s magnetic oscillations. In the presence of strong Dzyaloshinskii-Moriya interaction (DMI), the amplified magnetic oscillations evolve into a new skyrmion. These interactions between skyrmions and STNOs are found to be identical for both Neel-type and Bloch-type skyrmions, and are not observed between domain walls and STNOs. These findings offer a novel perspective in processing information using single skyrmions and we propose a 3-bit majority gate for logic applications.

Current-Induced Nucleation and Dynamics of Skyrmions in a Co -based Heusler Alloy

We demonstrate room-temperature stabilization of dipolar magnetic skyrmions with diameters in the range of $100$ nm in a single ultrathin layer of the Heusler alloy Co$_2$FeAl (CFA) under moderate magnetic fields. Current-induced skyrmion dynamics in microwires is studied with a scanning Nitrogen-Vacancy magnetometer operating in the photoluminescence quenching mode. We first demonstrate skyrmion nucleation by spin-orbit torque and show that its efficiency can be significantly improved using tilted magnetic fields, an effect which is not specific to Heusler alloys and could be advantageous for future skyrmion-based devices. We then show that current-induced skyrmion motion remains limited by strong pinning effects, even though CFA is a magnetic material with a low magnetic damping parameter.