We investigate a microscopic origin of nonreciprocal magnons that is distinct from the Dzyaloshinskii-Moriya interaction in a honeycomb antiferromagnet. The key ingredient is a symmetric anisotropic exchange interaction depending on the bond direction, which results in valley-type nonreciprocal magnon excitations under staggered antiferromagnetic ordering. Furthermore, we find that this type of nonreciprocal magnon exhibits a peculiar magnetic-field response; the nonreciprocal direction can be manipulated by the in-plane rotating magnetic field. The obtained results can be accounted for by the emergence of magnetic toroidal multipoles.

• Received 12 December 2019
• Revised 25 May 2020
• Accepted 27 May 2020

DOI:https://doi.org/10.1103/PhysRevB.101.224419