Surface State Magnetoresistance in Proximity Magnetized Topological Insulators*

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, which can be utilized for nano-electronic, spintronic, and quantum computing applications. While tremendous effort has been devoted to studying (quantum) anomalous Hall effects in magnetic proximity-coupled TIs, the inherent magnetoresistance (MR) properties of TI surface states remain largely unexplored. Here we present a strategy to fabricate TI-magnetic insulator (MI) heterostructure devices with high-quality interfaces, and experimentally measure the MR at various temperatures. We observe a plethora of hysteretic MR features including sharp, negative and positive MR switching, and broad, positive MR bumps, which we attribute to domain wall switching and out-of-plane anisotropic MR in the surface state, respectively. The out-of-plane magnetic anisotropy leads to gapping in the TI surface state, confirmed by an analytical diffusive transport model. These results provide insights into the fundamental mechanisms of magnetic proximity and spin exchange interactions in topological systems.