Research: Propagating dynamics of spin excitations along skyrmion strings

Effective one-dimensional whirling structures are well-known from fluids where vortex filaments can form when the fluid is set into rotation, for example, when water is flowing in a rotating manner out of a bathtub. It is less known that such filaments also act as conducting channels for waves whose properties were studied by Lord Kelvin 140 years ago. 

A magnetic analogue of vortex filaments are skyrmion strings. Skyrmions are two-dimensional whirling textures of the magnetization that form in chiral magnets within the plane perpendicular to a suitably applied magnetic field. They extend along the field in the third direction forming effective one-dimensional strings. Magnetic excitations, so-called spin waves, can propagate along these skyrmion strings quite similar to the Kelvin waves on vortex filaments. Researchers of the SFB 1143 succeeded in collaboration with the experimental group of Prof. Dr. Seki from the University of Tokyo to study these spin waves on skyrmion strings in the chiral magnet Cu₂OSeO₃ . 

They found that, interestingly, these spin waves propagate in the two directions of the string with different velocities, i.e, they are non-reciprocal because of the intrinsic chirality of the material. Skyrmion strings therefore not only serve as spin wave channels that allow to steer their propagation but also act a bit like spin wave diodes.

S. Seki, M. Garst, J. Waizner, R. Takagi, N. D. Khanh, Y. Okamura, K. Kondou, F. Kagawa, Y. Otani, Y. Tokura,
Propagation dynamics of spin excitations along skyrmion strings,
Nat. Commun. 11, 256 (2020)