  Topological insulators with intrinsic magnetic ordering, potentially hosting
rare quantum effects, recently attracted extensive attention. MnBi$_2$Te$_4$ is
the first established example. The Sb-doped variant
Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ shows a great variety of electronic properties
depending on the Sb content~$x$, such as shifts in the Fermi level and the Neel
temperature $T_{\mathrm N}$, and the change of the free charge carrier type
from $n$- to $p$-type at high Sb substitution ratios. Here, we investigate the
effect of magnetic ordering on the bulk electronic structure of
Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ with high Sb content $x=0.93$ by
temperature-dependent reflectivity measurements over a broad frequency range.
We observe anomalies in the optical response across $T_{\mathrm N}$ when the
antiferromagnetic order sets, which suggests a coupling between the magnetic
ordering and the electronic structure of the material.
