Magnetically doped nanoplate crystals of topological insulators Sb$_2$Te$_3$ and Bi$_2$Te$_3$

The surface states of topological insulators are robustly protected by time-reversal symmetry. Introducing magnetic impurities should 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 distinctly different from the conventional dilute magnetic semiconductors. In thin (quasi-2D) samples, this magnetic order gives rise to a topological electronic structure, with the quantized Hall conductance. Here we report synthesis and electrical and magnetic characterization of Fe and Cr doped \emph{thin} nanoplates of topological insulators Sb$_2$Te$_3$ and Bi$_2$Te$_3$. Nanoplate crystals were grown by catalyst-free vapor-liquid-solid method and were doped using the \textit{in situ} exchange of sources. Low-temperature magnetic, in-plane resistivity, and Hall measurements were performed in magnetic fields up to 9 T fields. The effects of magnetic dopant concentration on susceptibility and charge transport will be discussed.