Origin of spin reorientation and intrinsic anomalous Hall effect in the kagome ferrimagnet ${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$

${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$ has attracted a lot of recent interest for a variety of reasons, most importantly, because of the hypothesis that it may support quantum-limit Chern topological magnetism, derived from the kagome geometry. Besides, ${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$ features a highly unusual magnetic reorientation transition about 100 K below the Curie point, whereby all spins in the system, remaining collinear, rotate by ${90}^{\circ }$. In this work, we address both issues combining experiment, mean-field theory, and first-principle calculations. Both magnetic reorientation and the unusual temperature dependence of the anomalous Hall conductivity (AHC) find quantitative explanation in the fact that Mn and Tb, by virtue of the Mermin-Wagner theorem, have very different spin dynamics, with Tb spins experiencing much more rapid fluctuation. We were able to cleanly extract the intrinsic AHC from our experiment, and calculated the same microscopically, with good semiquantitative agreement. We have identified the points in the band structure responsible for the AHC and showed that they are not the kagome-derived Dirac points at the K-corner of the Brillouin zone, as conjectured previously.