Ferromagnetism in vanadium doped thin films of a topological insulator Bi$_2$Te$_3$

Recent first-principle calculations predict a new class of ferromagnetic systems that are distinctly different from the conventional dilute magnetic semiconductors. A novel ferromagnetic topological insulator (ferro-TI) state can be obtained when topological insulator are doped with certain transition metal elements. In the the quasi-2D limit these ferro-TIs are expected to support a quantized anomalous Hall effect. Here we report on electrical and magnetic characterization of vanadium doped thin ($\sim 50$ nm) films of a topological insulator Bi$_2$Te$_3$. Films were grown by \textit{rf} sputtering on S$_3$N$_4$/Si substrates with lithographically pre-patterned contact pads. Low-temperature in-plane and Hall resistivity measurements were performed in magnetic fields up to 5 T fields. We find that below 100 K, V-doped films display \textit{negative linear magnetoresistance}, which at lower temperatures becomes hysteretic. Hall resistivity is also hysteretic, suggesting an unusual ferromagnetic ordering below 10 K. Moreover, V-doping turns the \textit{p}-type conduction in as-grown films into \textit{n}-type. The doping and thickness dependence of these effects will be discussed.