Damped Dirac magnon in a metallic Kagome antiferromagnet FeSn*

FeSn is a prototype Kagome magnetic metal exhibiting novel electronic properties.  We report inelastic neutron scattering studies of the spin excitation spectrum of FeSn. The spectra display well-defined spin waves extending up to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin wave theory, we have determined an effective spin Hamiltonian that reproduces the measured dispersion at energies below the Stoner continuum. This analysis indicates that the Dirac magnon at the K-point remarkably occurs on the brink of a region where well-defined spin waves become damped. Our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic Kagome magnets