Size-dependent mobility of skyrmions beyond pinning in ferrimagnetic GdCo thin films

TL;DR

This study investigates how skyrmion size influences their mobility and depinning behavior in ferrimagnetic GdCo thin films, demonstrating a linear velocity-current relationship and the significance of size in skyrmion dynamics.

Contribution

It provides the first quantitative analysis of size-dependent skyrmion mobility beyond pinning effects using an analytical model validated by experiments.

Findings

Mobility increases with skyrmion size.

Depinning threshold current decreases with size.

Linear velocity-current relationship observed above depinning threshold.

Abstract

Magnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with low pinning. Above a sharp current depinning threshold, we observe a clearly linear velocity increase with current that extrapolates to zero and a constant deflection angle, reaching high velocities up to 200 m/s. The mobility increases and the depinning threshold current decreases with the skyrmion diameter, which we vary using an external magnetic field. An analytical model based on the Thiele equation quantitatively reproduces these findings with a single fitting parameter. This validates the linear flow regime description and shows, in particular, the important role of skyrmion size in its dynamics.