Current-driven skyrmion motion in granular films

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 the 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 or kinetic, similar to the friction force in the Newtonian mechanics. In the pinning phase, the static (pinning) random force is against the tendency direction of skyrmion motion that is transverse to the current density for spin-transfer-torquedriven motion. 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 into skyrmion motion and should be important for skyrmiontronics.