Surface state mediated ferromagnetism in Mn doped Bi₂Te₃ topological insulator thin films

Topological insulators with spontaneous ferromagnetic order are of scientific interest due to this moment breaking time reversal symmetry in the topological surface states and opening a gap at the Dirac point. These ferromagnetic topological insulators can exhibit quantum anomalous Hall effects and topological Hall effects associated with chiral magnetic phases. While ferromagnetism can be induced in Bi₂Te₃ by doping it with transition metal ions, the mechanism of long range order in these materials is not well understood. In this work, we present magnetic and electronic characterization of ferromagnetic Mn doped Bi₂Te₃ thin films. We present evidence that the magnetic moment of the n-type doped films increases as the carrier density decreases. We argue that this is due to a Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in the conducting surface state electrons. This is contrary to the expected behavior for ferromagnetism in metals where the magnetic moment tends to increase with increasing carrier density. This deviation from the expected behavior is indicative of the unique electronic band structure of the 3D topological insulators.