  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.
