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Theory of current-driven skyrmions in disordered magnets
#MMPMID29679018
Koshibae W
; Nagaosa N
Sci Rep
2018[Apr]; 8
(1
): 6328
PMID29679018
show ga
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 N (sk) , which counts how many times the directions of the
magnetic moments wrap the unit sphere. This N (sk) gives the chiral nature of the
skyrmion dynamics, and leads to the extremely small critical current density j
(c) for the current-driven motion in terms of spin transfer torque effect. The
finite j (c) 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.
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