Short-wave multipole magnons in topological skyrmion lattices resolved by cryogenic Brillouin light scattering microscopy

Chiral magnets provide an innovative framework to study non-collinear spin textures and their associated magnetization dynamics. They include helical and conical magnetic textures that are spatially modulated with a wavevector k_h, as well as the topologically non-trivial skyrmion lattice (SkL) phase. Their spin waves have been explored in the long-wavelength regime using resonance and spin wave spectroscopy [1], and in the short wavelength regime using inelastic neutron scattering [2]. Here, we show that Brillouin light scattering (BLS) is ideally suited to probe the complementary range of wavevectors k ≲ k_h. Generalizing the theory of Refs. [2,3], we provide parameter-free predictions for the BLS cross section and compute the resonances and their associated spectral weights. The theoretical results are compared to BLS experiments in the backscattering geometry that probe bulk spin waves in the SkL phase of Cu₂OSeO₃ with a wavevector k = 48 rad/μm < k_h= 105 rad/μm. The standard clockwise, counterclockwise and breathing modes are clearly resolved. Due to the finite magnon wavevector, non-zero spectral weight is theoretically predicted also for other resonances with multipole character. Experimentally, one additional magnon band with quadrupole character is identified, as well as the anti-crossing signature of the magnon-magnon hybridisation process.

[1] R. Takagi et al., Phys. Review B 104, 144410 (2021)

[2] T. Weber et al., Science 375, 1025-1030 (2022)

[3] N. Ogawa et al. PNAS 118 e2022927118 (2021)