Abstract
We report the magnetic structure and spin excitations of Mn3Ge, a breathing kagome antiferromagnet with transport anomalies attributed to Weyl nodes. Using polarized neutron diffraction, we show the magnetic order is a k=0 coplanar state belonging to a Γ9 irreducible representation, which can be described as a perfect 120∘ antichiral structure with a moment of 2.2(1) μB/Mn, superimposed with weak collinear ferromagnetism. Inelastic neutron scattering shows three collective Q=0 excitations at Δ1=2.9(6) meV, Δ2=14.6(3) meV, and Δ3=17.5(3) meV. A field theory of Q≈0 spin waves in triangular antiferromagnets with a 120∘ spin structure was used to classify these modes. The in-plane mode (α) is gapless, Δ1 is the gap to a doublet of out-of-plane spin excitations (βx,βy), and Δ2, Δ3 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 [Γ=25(8) meV] and the extended range of magnetic interactions indicate itinerant magnetism consistent with the transport anomalies.
