Raman scattering study of magnetism in the kagome materials Fe₃Sn₂ and Co₃Sn₂S₂

Fe₃Sn₂ and Co₃Sn₂S₂ have triangularly coordinated layers of 3d transition metal ions sitting on a kagome network. Both compounds have Dirac and Weyl nodes in the band structure. Fe₃Sn₂ is ferromagnetic below 670 K, and the spins start to reorient from perpendicular to parallel to the kagome layer below 150 K. This reorientation was first observed by Mössbauer spectroscopy but has in general little influence on other observables such as thermal expansion or magnetization. In our Raman study we find anomalies in both line width and energy of the lowest A_1g phonon where the Sn atoms vibrate perpendicular to the Fe plane. We interpret the anomaly in terms of an enhanced spin-phonon coupling below approximately 150 K. Co₃Sn₂S₂ starts ordering antiferromagnetically below 175 K and turns into a ferromagnet in the low-temperature limit. The spins are first in plane, and the order is nearly frustrated. Below 100 K the spins point along the c-axis and are parallel. The A_1g phonon couples strongly to a continuum as highlighted by the asymmetric Fano-type line shape. The asymmetry is maximal close to the magnetic transition. We argue that small changes of the lattice have an effect on the magnetism.