Negative Te spin polarization responsible for ferromagnetic order in the doped topological insulator V0.04(Sb1−xBix)1.96Te3

Ferromagnetic topological insulators have emerged as a promising platform for quantum anomalous Hall (QAH) effect with a dissipationless edge transport. However, the observation of QAH effect has so far been restricted to extremely low temperatures. We investigate the microscopic origin of ferromagnetism coupled with topological insulators in vanadium-doped (Sb, Bi)(2)Te-3 employing the x-ray magnetic circular dichroism, angle-resolved two-photon photoemission spectroscopy, combined with first-principles calculations. We found a negative spin polarization, and thus an antiparallel magnetic moment at the Te site with respect to that of the vanadium dopants, which plays the key role in the ferromagnetic order. We ascribe it to the hybridization between Te p and V d majority spin states at the Fermi energy (E-F), being supported by a Zener-type p-d exchange interaction scenario. The substitution of Bi at the Sb site suppresses the bulk ferromagnetism by introducing extra electron carriers in the majority spin channel of Te p states that compensates the antiparallel magnetic moment on the Te site. Our findings reveal important clues to designing magnetic topological insulators with higher Curie temperature that work under ambient conditions.