Theoretical study of current-driven skyrmions in disordered magnets

The magnetic skyrmion, an emergent topological particle in magnets, has several unique features distinct from the other spin textures such as domain wall, helical structure, and vortex. One of the features is the small critical current density for the current-driven motion in terms of spin transfer torque effect. The critical current density indicates the pinning effect due to the disorder such as impurities and defects, it is always relevant in real systems.
Here, we study the effect of impurities on current driven motion of skyrmions by a numerical simulation of Landau-Lifshitz-Gilbert equation. We find four different skyrmion phases with the strong disorder, i.e., (A) pinned state, (B) depinned state, (C) skyrmion creation/annihilation, and (D) segregation of skyrmions, as the current density increases, while only two phases (A) and (B) appear in the weak disorder case. The microscopic mechanisms of the new phases (C) and (D) are analyzed theoretically. These results offer a coherent understanding of the skyrmion dynamics under current with disorder.