Electronic Structure Investigation of Magnetic Splitting and Gap Formation in Ferromagnetic Topological Insulator (Cr_0.35Sb_0.65)₂Te₃

We study the temperature (<i>T </i>) dependent electronic structure and magnetic properties of a high Curie temperature (<i>T_c</i>~ 185 K) ferromagnetic topological insulator (Cr_0.35Sb_0.65)₂Te₃ using Cr L-edge and Te M-edge x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). The <i>T</i>-dependent XAS and XMCD results show a systematic and spin-selective splitting of Cr 3<i>d</i> and Te 5<i>p</i> unoccupied density of states, resulting in a magnetic gap opening. The <i>T</i>-dependent XMCD signal and magnetic gap are consistent with bulk magnetization as well as mean-field theory. The full-multiplet charge-transfer cluster model calculations with a negative charge transfer energy show a good agreement with the experimental Cr L-edge XAS and XMCD spectra. The ARPES measurements show the Dirac-cone topological surface states (SS) below and above <i>T</i>_c, implying that the band inversion between the Sb and Te orbital is maintained even at high Cr doping level, and is consistent with a negative charge transfer energy. These results establish a direct link between Cr 3<i>d</i> dopant states and the Te 5<i>p</i> SS forming the magnetic gap in (Cr_0.35Sb_0.65)₂Te₃.