Thermally Driven Ratchet Motion of Skyrmion Microcrystal and Topological Magnon Hall Effect

TL;DR

This paper demonstrates that skyrmion microcrystals can exhibit unidirectional rotation driven by thermal fluctuations and temperature gradients, revealing a topologically protected ratchet motion and a magnon Hall effect.

Contribution

It shows that skyrmions act as thermal ratchets with unidirectional rotation, driven by thermal fluctuations, a novel topological effect not observed in equilibrium.

Findings

Skyrmion crystals exhibit unidirectional rotation motion.

Thermal fluctuations and temperature gradients drive skyrmion rotation.

Magnon flow induced by skyrmions contributes to their rotation.

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 micron-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi display 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.