Ferromagnetism in chromium doped topological insulator thin films and nanoplate crystals*

The surface states of topological insulators are protected by time-reversal symmetry. Introducing magnetic impurities should break this symmetry and open a gap in the otherwise gapless surface states. Recent first-principle calculations predict that when topological insulators are doped with transition metal elements, such as Cr or Fe, a \emph{magnetically ordered} insulating state will form -- a state that in thin (quasi-2D) samples may support a quantized Hall conductance. Here we report on electrical and magnetic characterization of thin Cr doped topological insulators: Sb$_2$Te$_3$ nanoplate crystals and $\sim 50$ nm thin films of Bi$_2$Te$_3$. Electrical contacts to samples were lithographically defined, with \textit{rf} sputtered films grown on pre-patterned substrates. Low-temperature in-plane resistivity, Hall, and magnetization measurements were performed in up to 5 T magnetic fields. For 5 at\% Cr content, a distinct ferromagnetic hysteretic response is observed at temperatures below 10 K. Hysteretic loops, also observed in Hall resistivity, indicate low-$T$ coercive fields of the order of 0.5 T. Correlation of transport and magnetic measurements indicating anomalous Hall effect, and strong dependence on dopant concentration and sample thickness will be presented.