Abstract
An emergent topological particle in magnets, skyrmion, has several unique features distinct from the other magnetic textures such as domain wall, helical structure, and vortex. It is characterized by a topological integer called skyrmion number Nsk, which counts how many times the directions of the magnetic moments wrap the unit sphere. This Nsk gives the chiral nature of the skyrmion dynamics, and leads to the extremely small critical current density jc for the current-driven motion in terms of spin transfer torque effect. The finite jc indicates the pinning effect due to the disorder such as impurities and defects, and the behaviors of skyrmions under disorder have not been explored well theoretically although it is always relevant in real systems. Here we reveal by a numerical simulation of Landau-Lifshitz-Gilbert equation that there are four different skyrmion phases with the strong disorder, i.e., (A) pinned state, (B) depinned state, (C) skyrmion multiplication/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.
Highlights
An emergent topological particle in magnets, skyrmion, has several unique features distinct from the other magnetic textures such as domain wall, helical structure, and vortex
By comparing the results of Thiele equation[35,36,37,38] with ours presented below, one can identify the effects of the internal deformation and spin wave emission, which lead to the different behaviors in the strong disorder case while both are consistent with each other in the weak disorder case
We focus on the chiral magnet with Dzyaloshinskii-Moriya (DM) antisymmetric spin-orbit interaction[27,28,29], whose model Hamiltonian on the two-dimensional square lattice is given as
Summary
An emergent topological particle in magnets, skyrmion, has several unique features distinct from the other magnetic textures such as domain wall, helical structure, and vortex. It is characterized by a topological integer called skyrmion number Nsk, which counts how many times the directions of the magnetic moments wrap the unit sphere This Nsk gives the chiral nature of the skyrmion dynamics, and leads to the extremely small critical current density jc for the current-driven motion in terms of spin transfer torque effect. We reveal by a numerical simulation of LandauLifshitz-Gilbert equation that there are four different skyrmion phases with the strong disorder, i.e., (A) pinned state, (B) depinned state, (C) skyrmion multiplication/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 processes in each phases are analyzed as shown in Fig. 1, as well as the spectroscopic information
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