Current-driven skyrmion motion in granular films

TL;DR

This paper investigates how disorder affects current-driven skyrmion motion in granular films, revealing critical disorder thresholds, the roles of static and kinetic forces, and confirming the Thiele equation's applicability for describing skyrmion dynamics.

Contribution

It provides new insights into the influence of disorder on skyrmion motion, including the identification of critical disorder strength and the classification of disorder-induced forces, with validation of the Thiele equation.

Findings

Skyrmion transverse motion can be boosted or hindered depending on disorder and damping.

Above a critical disorder, skyrmions become pinned and cannot move.

The Thiele equation accurately describes skyrmion dynamics in granular films.

Abstract

Current-driven skyrmion motion in random granular films is investigated with interesting findings. For a given current, there exists a critical disorder strength below which its transverse motion could either be boosted below a critical damping or be hindered above the critical damping, resulting in current and disorder dependences of skyrmion Hall angle. The boosting comes mainly from the random force that is opposite to the driving force (current). The critical damping depends on the current density and disorder strength. However, the longitudinal motion of a skyrmion is always hindered by the disorder. Above the critical disorder strength, skyrmions are pinned. The disorder-induced random force on a skyrmion can be classified as static and kinetic ones, similar to the friction force in the Newtonian mechanics. In the pinning phase, the static (pinning) random force is transverse to the current density. The kinetic random force is opposite to the skyrmion velocity when skyrmions are in motion. Furthermore, we provide strong evidences that the Thiele equation can perfectly describe skyrmion dynamics in granular films. These findings provide insight to skyrmion motion and should be important for skyrmiontronics.