Within a decade, the field of magnetic skyrmionics has developed from a niche prediction to a huge and active research field. Not only do magnetic skyrmions—magnetic whirls with a unique topology—reveal fundamentally new physics, but they have also risen to prominence as up-and-coming candidates for next-generation high-density efficient information encoding. Within a few years, it has been possible to efficiently create, manipulate, and destroy nanometer-size skyrmions in device-compatible materials at room-temperature by all electrical means. Despite the incredibly rapid progress, several challenges still remain to obtain fully functional and competitive skyrmion devices, as discussed in this perspective article with a focus on recent results.

Perspective: Magnetic skyrmions—Overview of recent progress in an active research field

https://escholarship.org/content/qt1xm794rm/qt1xm794rm.pdf?t=p2njav

Skyrmions in magnetic multilayers

https://iopscience.iop.org/article/10.1088/0022-3727/49/42/423001/pdf

Magnetic skyrmions: from fundamental to applications

The field of magnetic skyrmions has been actively investigated across a wide range of topics during the last decades. In this topical review, we mainly review and discuss key results and findings in skyrmion research since the first experimental observation of magnetic skyrmions in 2009. We particularly focus on the theoretical, computational and experimental findings and advances that are directly relevant to the spintronic applications based on magnetic skyrmions, i.e. their writing, deleting, reading and processing driven by magnetic field, electric current and thermal energy. We then review several potential applications including information storage, logic computing gates and non-conventional devices such as neuromorphic computing devices. Finally, we discuss possible future research directions on magnetic skyrmions, which also cover rich topics on other topological textures such as antiskyrmions and bimerons in antiferromagnets and frustrated magnets.

https://iopscience.iop.org/article/10.1088/1361-648X/ab5488/pdf

Skyrmion-electronics: writing, deleting, reading and processing magnetic skyrmions toward spintronic applications.

Current-induced motion of skyrmions is attracting attention due to its low critical current density, however, its microscopic mechanisms have not been elucidated yet. Using numerical simulations, the authors demonstrate a universal current-velocity relation of skyrmion motion, independent of disorder or nonadiabatic effects.

http://absimage.aps.org/image/MAR13/MWS_MAR13-2012-004008.pdf

Universal current-velocity relation of skyrmion motion in chiral magnets

Spin transfer nano-oscillators (STNOs) are nanoscale devices which are promising candidates for on-chip microwave signal sources. For application purposes, they are expected to be nano-sized, to have broad working frequency, narrow spectral linewidth, high output power and low power consumption. In this paper, we demonstrate by micromagnetic simulation that magnetic skyrmions, topologically stable nanoscale magnetization configurations, can be excited into oscillation by a spin-polarized current. Thus, we propose a new kind of STNO using magnetic skyrmions. It is found that the working frequency of this oscillator can range from nearly 0 Hz to gigahertz. The linewidth can be smaller than 1 MHz. Furthermore, this device can work at a current density magnitude as small as 108 A m−2, and it is also expected to improve the output power. Our studies may contribute to the development of skyrmion-based microwave generators.

https://iopscience.iop.org/article/10.1088/1367-2630/17/2/023061/pdf

Current-induced magnetic skyrmions oscillator

By micromagnetic simulation, we investigated the interaction between propagating spin wave (or magnonic) and a 360° domain wall in a nanostrip. It is found that propagating spin wave can drive a 360° domain wall motion, and the velocity and direction are closely related to the transmission coefficient of the spin wave of the domain wall. When the spin wave passes through the domain wall completely, the 360° domain wall moves toward the spin wave source. When the spin wave is reflected by the domain wall, the 360° domain wall moves along the spin wave propagation direction. Moreover, when the frequency of the spin wave is coincident with that of the 360° domain wall normal mode, the 360° domain wall velocity will be resonantly enhanced no matter which direction the 360 DW moves along. On the other hand, when the spin wave is reflected from the moving 360° domain wall, we observed the Doppler effect clearly. After passing through a 360° domain wall, the phase of the spin wave is changed, and the phase shift is related to the frequency. Nevertheless, phase shift could be manipulated by the number of 360° domain walls that spin wave passing through.

Propagating and reflecting of spin wave in permalloy nanostrip with 360° domain wall

In this paper, phase locking dynamics of vortices induced by spin transfer torque in bridge-coupled nanodisks are studied by micromagnetic simulations. In the presence of the bridge coupling, the required time for the phase locking is dramatically reduced, and the phase difference between the two vortices keeps at a nonzero value after the phase locking. Moreover, the phase difference is affected significantly by bridge coupling, Oersted field distribution, nanodisk size, as well as in-plane bias magnetic field. In addition, the coupled gyrotropic frequency of vortices depends linearly on the perpendicular magnetic field. This systematic study of phase locking parameters, especially the phase difference, is important for the applications of vortex-based spin-torque nano-oscillators.

Phase locking of moving magnetic vortices in bridge-coupled nanodisks

Micromagnetic simulation for detection of magnetic nanobeads by spin torque oscillator

Transverse domain wall motion in strips with a series of nanobars placed perpendicularly on either side has been investigated systematically by micromagnetic simulation. Domain wall motion in strips is driven by electric current owing to the spin transfer torque effect. In a strip in contact with nanobars, a domain wall can withstand a high current density (above A/cm2) to maintain structural stability, but the domain wall velocity cannot be increased markedly. Domain wall motion in strips not in contact with nanobars can reach a higher speed under a relatively low current density. To examine the effects of nanobars on domain wall motion, the DW depinning energy and the Walker limit uw at different parameters are discussed systematically.

Qi Zheng

Papers

Current-induced magnetic skyrmions oscillator

Propagating and reflecting of spin wave in permalloy nanostrip with 360° domain wall

Phase locking of moving magnetic vortices in bridge-coupled nanodisks

Micromagnetic simulation for detection of magnetic nanobeads by spin torque oscillator

Current-induced domain wall motion in nanostrip–nanobars system