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Nat Mater. 2014 Mar;13(3):241-6. doi: 10.1038/nmat3862. Epub 2014 Jan 26.

Thermally driven ratchet motion of a skyrmion microcrystal and topological magnon Hall effect.

Author information

1
1] Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 229-8558, Japan [2] PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan.
2
RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan.
3
1] PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan [2] RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan [3] Department of Applied Physics, Quantum-Phase Electronics Center, The University of Tokyo, Bunkyo-ku Tokyo 113-8656, Japan.
4
Department of Applied Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan.
5
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
6
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands.
7
1] RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan [2] Department of Applied Physics, Quantum-Phase Electronics Center, The University of Tokyo, Bunkyo-ku Tokyo 113-8656, Japan [3] Department of Applied Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan.

Abstract

Spontaneously emergent chirality is an issue of fundamental importance across the natural sciences. It has been argued that a unidirectional (chiral) rotation of a mechanical ratchet is forbidden in thermal equilibrium, but becomes possible in systems out of equilibrium. Here we report our finding that a topologically nontrivial spin texture known as a skyrmion--a particle-like object in which spins point in all directions to wrap a sphere--constitutes such a ratchet. By means of Lorentz transmission electron microscopy we show that micrometre-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi exhibit a unidirectional rotation motion. Our numerical simulations based on a stochastic Landau-Lifshitz-Gilbert equation suggest that this rotation is driven solely by thermal fluctuations in the presence of a temperature gradient, whereas in thermal equilibrium it is forbidden by the Bohr-van Leeuwen theorem. We show that the rotational flow of magnons driven by the effective magnetic field of skyrmions gives rise to the skyrmion rotation, therefore suggesting that magnons can be used to control the motion of these spin textures.

PMID:
24464244
DOI:
10.1038/nmat3862
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