Investigating the magnetoelastic properties in FeSn and Fe3Sn2 flat band metals

Topological quantum magnets FeSn and Fe3Sn2 were studied using neutron scattering and first-principles calculations. Both materials are metallic but host dispersionless flat bands with Dirac nodes at the K point in reciprocal space. The local structure determined from the pair density function analysis of the neutron-diffraction data provided no evidence for electron localization in both compounds, consistent with their metallic nature. At the same time, in FeSn, an anomalous suppression in the c-axis lattice constant coupled with changes in the phonon spectra were observed across TN indicating the presence of magnetoelastic coupling and spin-phonon interactions. In addition, it was observed that spin waves persisted well above TN, suggesting that the in-plane ferromagnetic spin correlations survive at high temperatures. In contrast, no lattice anomaly was observed in Fe3Sn2. The inelastic signal could be mostly accounted for by phonons, determined from density-functional theory, showing typical softening on warming.