Shear Banding, Intermittency, Jamming and Dynamic Phases for Skyrmions in Inhomogeneous Pinning Arrays

TL;DR

This paper studies the complex dynamic phases of skyrmions in inhomogeneous pinning landscapes, revealing shear banding, intermittency, jamming, and phase transitions driven by external forces and system parameters.

Contribution

It provides a detailed mapping of skyrmion dynamic phases in inhomogeneous pinning arrays and identifies the effects of Magnus force and drive orientation on these phases.

Findings

Shear banding occurs at intermediate drives with zero skyrmion Hall angle.

High drive leads to a moving crystal phase with increased Hall angle.

Intermittent flow and jamming are observed depending on Magnus force and drive direction.

Abstract

We examine driven skyrmion dynamics in systems with inhomogeneous pinning where a strip of strong pinning coexists with a region containing no pinning. For driving parallel to the strip, we find that the initial skyrmion motion is confined to the unpinned region and the skyrmion Hall angle is zero. At larger drives, a transition occurs to a phase in which motion also appears within the pinned region, creating a shear band in the skyrmion velocity, while the skyrmion Hall angle is still zero. As the drive increases further, the flow becomes disordered and the skyrmion Hall angle increases with drive until saturating at the highest drives when the system transitions into a moving crystal phase. The different dynamic phases are associated with velocity and density gradients across the pinning boundaries. We map out the dynamic phases as a function of pinning strength, skyrmion density, and Magnus force strength, and correlate the phase boundaries with features in the velocity-force curves and changes in the local and global ordering of the skyrmion structure. For large Magnus forces, the shear banding instability is replaced by large scale intermittent flow in the pinned region accompanied by simultaneous motion perpendicular to the direction of the drive, which appears as oscillations in the transport curves. We also examine the case of a drive applied perpendicular to the strip, where we find a jamming effect in which the skyrmion flow is blocked by skyrmion-skyrmion interactions until the drive is large enough to induce plastic flow.