Topological Insulator/Magnetic Insulating Oxide: A Platform for Efficient Spin Current Transport*

Topological insulators (TIs) hold great promise for spintronic devices due to their large charge-to-spin conversion efficiency. It has been demonstrated that a TI can induce a spin-orbit torque to switch the magnetization of a magnetic metal. However, it is unclear if this is due to the topological surface state (TSS) because the electrons from the magnetic metal can suppress TSS. Here we discuss experiments that identified bona fide surface state-induced spin-orbit torques in topological insulator/magnetic oxide bilayers. In Bi₂Se₃/BaFe₁₂O₁₉, a large spin-orbit torque from Bi₂Se₃ switched the magnetization of BaFe₁₂O₁₉. When the magnetization was switched by a magnetic field, a current in Bi₂Se₃ can reduce the switching field by about 4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature. When BaFe₁₂O₁₉ is replaced with Mg(Al,Fe)₂O₄, efficient spin pumping in the bulk-dominated regime was found at room temperature. These results highlight the promise of topological insulator/ferromagnetic insulating oxide bilayers as a platform for studying topological surface states in the context of spin-to-charge interconversion.