  To further investigate the interplay between ferromagnetism and topological
insulators, thin films of the low-carrier topological insulator
(Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ were deposited on the insulating ferromagnet EuS
(100) in situ. AC susceptibility indicates magnetic anomalies between
$T\approx30~\mathrm{K}$ and $T\approx60~\mathrm{K}$, well above the Curie
temperature $T_C \approx 15~\mathrm{K}$ of EuS. When the Fermi level is close
to the Dirac point and the surface state dominates the electric conduction,
sharp increases in resistance with decreasing temperatures were observed
concurrently with the magnetic anomalies. Positive-negative magnetoresistance
crossovers were observed at the Curie temperature, which seem only to appear
when the sheet resistance exceeds the Mott-Ioffe-Regel limit $h/e^2$. A
two-stage gap-opening process due to magnetic proximity is proposed.
