Universal high-speed dynamics of distorted bubble skyrmions in an uncompensated amorphous ferrimagnet*

Magnetic skyrmions are topologically stabilized spin configurations that, like domain walls (DWs), can react to external stimuli by collective displacement, which is both physically intriguing and bears promises to realize next generation non-volatile data storage technologies.
Analytical equations of motion describe straight 180° DWs in the one-dimensional (1D) model while rigid, circular bubble domains and skyrmions move according to the Thiele equation. However, DWs and skyrmions are often not perfectly straight or circular.
Here, we study how strongly deformed DWs and bubble skyrmions move in uncompensated ferrimagnetic Pt/CoGd/W in response to current pulses. We find that all 1D spin textures exhibit velocity saturation at ~300 m/s in agreement with the 1D model, while all fully enclosed spin textures, even if significantly distorted, reach >500 m/s. We attribute this significant difference in the dynamics to the topology and the canting of spins in the skyrmion’s encircling DWs.