Antichiral spin order, its soft modes, and their hybridization with phonons in the topological semimetal .

We report the magnetic structure and spin excitations of , a breathing kagome antiferromagnet with transport anomalies attributed to Weyl nodes. Using polarized neutron diffraction, we show the magnetic order is a coplanar state belonging to a irreducible representation, which can be described as a perfect 120° antichiral structure with a moment of , superimposed with weak collinear ferromagnetism. Inelastic neutron scattering shows three collective excitations at , , and . A field theory of spin waves in triangular antiferromagnets with a 120° spin structure was used to classify these modes. The in-plane mode is gapless, is the gap to a doublet of out-of-plane spin excitations ( , ), and , result from hybridization of optical phonons with magnetic excitations. While a phenomenological spin Hamiltonian including exchange interactions, Dzyaloshinskii-Moriya interactions, and single-ion crystal field terms can describe aspects of the Mn-based magnetism, spin-wave damping [ ] and the extended range of magnetic interactions indicate itinerant magnetism consistent with the transport anomalies.