Ab initio study of spin fluctuations in the itinerant kagome magnet FeSn

The kagome antiferromagnetic metal FeSn has become an attractive platform for the exploration of novel electronic states such as topological Dirac states and the formation of flat bands by localized electrons. Apart from the electronic properties, Dirac magnons and flat magnon bands have also been proposed by applying simplified Heisenberg models to kagome magnetic systems. Inelastic neutron scattering studies on FeSn found well-defined magnon dispersions at low energies, but magnons at high energies are strongly damped, which cannot be explained by localized spin models. In this paper, we utilize both linear spin-wave theory and time-dependent density functional perturbation theory to investigate spin fluctuations of FeSn. Through the comparison of calculated spin-wave spectra and the Stoner continuum, we explicitly show that the damping of magnons at high energies is due to the Landau damping, and the appearance of high-energy optical-magnon-like branches at the $M$ and $K$ points is the result of the relatively low Stoner excitation intensity at those regions.