Magneto-optical detection of topological contributions to the anomalous Hall effect in a kagome ferromagnet

A single ferromagnetic kagome layer is predicted to realize a Chern insulator with quantized Hall conductance, which upon stacking can become a Weyl-semimetal with large anomalous Hall effect (AHE) and magneto-optical activity. Indeed, in the kagome bilayer material Fe$_3$Sn$_2$, a large AHE was detected, however, it still awaits the direct probing of the responsible band structure features by bulk sensitive methods. We measure the optical, both diagonal and Hall, conductivity spectra over a broad spectral range and identify the origin of the intrinsic AHE with the help of momentum- and band-decomposed first-principles calculations. We find that low-energy transitions, tracing "helical volumes" in momentum space reminiscent of the formerly predicted helical nodal lines, substantially contribute to the AHE, which is further increased by contributions from multiple higher-energy interband transitions. Our study also reveals that local Coulomb interactions lead to band reconstructions near the Fermi level.