Stochastic dynamics and pattern formation of geometrically confined skyrmions

Alexander F Schäffer,Levente Rózsa,Jamal Berakdar,Elena Y Vedmedenko,Roland Wiesendanger

doi:10.1038/s42005-019-0176-y

Alexander F Schäffer, Levente Rózsa + Show 3 more

Open Access

https://doi.org/10.1038/s42005-019-0176-y

Copy DOI

Abstract

Ensembles of magnetic skyrmions in confined geometries are shown to exhibit thermally driven motion on two different time scales. The intrinsic fluctuating dynamics (t ∼ 1 ps) are governed by short-range symmetric and antisymmetric exchange interactions, whereas the long-time limit (t ≳ 10 ns) is determined by the coaction of skyrmion–skyrmion-repulsion and the system’s geometry. Micromagnetic simulations for realistic island shapes and sizes are performed and analyzed, indicating the special importance of skyrmion dynamics at finite temperatures. We demonstrate how the competition between skyrmion mobility and observation time directly affects the addressability of skyrmionic bits, which is a key challenge on the path of developing skyrmion-based room-temperature applications. The presented quasiparticle Monte Carlo approach offers a computationally efficient description of the diffusive motion of skyrmion ensembles in confined geometries, like racetrack memory setups.

Highlights

To connect experimental and theoretical model systems with technological applications, investigating the influence of finite temperatures is of crucial importance
As the number of skyrmions during the diffusive motion remains constant, quasiparticle models have been developed for their description in this limit[25,26,29,30], which are primarily based on the Thiele equation[31]
It is shown that the complex interplay of the repulsive interaction between the skyrmions[29,34] along with the confinement effect of the nanoisland and the thermally induced skyrmion diffusion leads to a pattern formation of the skyrmion probability distribution on the nanosecond time scale

Summary

Introduction

To connect experimental and theoretical model systems with technological applications, investigating the influence of finite temperatures is of crucial importance. Previous studies concentrated on diffusion in infinite or extended geometries, but a clarification of the role of the sample shape still seems to be missing in the case where the system size becomes comparable to that of the skyrmions Effects of this kind directly impede the addressability, which is indispensable when using skyrmions for storage technology, e.g., in racetrack memory devices. As the number of skyrmions during the diffusive motion remains constant, quasiparticle models have been developed for their description in this limit[25,26,29,30], which are primarily based on the Thiele equation[31] The advantage of such a collectivecoordinate description over micromagnetic or atomistic spin dynamics simulations is the significantly lower computation cost. The simple implementation and high speed of such a method may make it

Methods

Results

Conclusion