Dynamics of skyrmion bags driven by the spin–orbit torque

Abstract

Skyrmion bags are found in liquid crystals and predicted to exist in ferromagnetic films. They are expected to become the information carriers of the new generation racetrack memory due to their high-degree of freedom of topological charge number. Here, we numerically and theoretically investigate the dynamics of skyrmion bags driven by the spin–orbit torque in an infinite ferromagnetic film and a nanostrip with a finite length. It is found that the constant velocity of the skyrmion bag in the ferromagnetic film depends on its topological charge number, while in the case of motion in nanostrips with the skyrmion bag moving along the boundary, the velocity in the first approximation is independent of the topological charge number. The simulation results are in good agreement with the calculation results. Finally, we show that the skyrmion bag can pass through defects and impurities in the nanostrip due to topological protection. Our results reveal the dynamics of skyrmion bags driven by the spin–orbit torque and may also be beneficial for the development of racetrack memory based on skyrmion bags.