Two-Stage Proximity-Induced Gap-Opening in Topological Insulator–Insulating Ferromagnet (Bi_xSb_1-x)₂Te₃–EuS Bilayers

To further investigate the interplay between ferromagnetism and topological insulators, the low-carrier topological insulator (Bi_xSb_1-x)₂Te₃ thin films were deposited on the insulating ferromagnet EuS (100) in situ. AC susceptibility indicates magnetic anomalies at T ≈ 30 K and T ≈ 60 K, above the Curie temperature T_C ≈ 15 K. When the Fermi level is close to the Dirac point and the surface state dominates the electric conduction, sharp increases in resistance were observed at the same temperature range. Together with the positive-negative magnetoresistance crossover at the Curie temperature, a two-stage gap-opening process due to magnetic proximity is proposed.