We report a study on the transition temperature $T_{\mathrm{C}}$ of Cr-doped topological insulator thin films, where an increase in the ferromagnetic onset can provide a pathway towards low-power spintronics in the future. Arrott plots, measured by surface-sensitive x-ray magnetic circular dichroism at the Cr $L_{2,3}$ edge as a function of field at various low temperatures, give a $T_{\mathrm{C}} \approx 7\mathrm{K}$ for the pristine surface. This is comparable to the bulk value of the film, which means that there is no indication that the spontaneous magnetization is different near the surface. Evaporation of a thin layer of Co onto the pristine surface of the in-situ cleaved sample increases the ordering temperature near the surface to $\sim 19\mathrm{K}$, while in the bulk it rises to $\sim 10\mathrm{K}$. X-ray absorption spectroscopy shows that Cr enters the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ host matrix in a divalent state, and is unchanged by the Co deposition. These results demonstrate a straightforward procedure to increase the transition temperature of doped topological insulators.

• Received 24 April 2015

DOI:https://doi.org/10.1103/PhysRevB.92.094420