Nonreciprocal zone boundary magnon propagation in Cu₂OSeO₃

Inelastic neutron scattering in the chiral magnet Cu₂OSeO₃ reveals strong nonreciprocal effects on magnon propagation at the boundary of the nuclear Brillouin zone. The nonreciprocal response is strongest at a central position between the zone corner and the edge midpoint. We explain these results using an effective linear spin-wave model. While directional effects in chiral magnets have so far only been known to exist at low momenta close to the center of the Brillouin zone, the present study shows that nonreciprocity persists at the highest possible reduced momenta. The observed magnons show very little damping within the limits of our experimental resolution, making them of great interest for the fundamental research on compact, high-frequency magnonic applications.