Low-temperature magnetic proximity effects in Mg(Al,Fe)₂O₄/Bi₂Se₃ bilayers

Topological insulators (TIs) are of significant interest in spin current-based electronics due to their strong spin-orbit coupling from spin-momentum locking of surface states. Bi₂Se₃ is a 3D topological insulator with a semiconductor gap in the bulk and gapless surface states. However, conduction near room temperature is typically bulk-dominated due to Se vacancies. Bilayers of Mg(Al,Fe)₂O₄ (MAFO), a magnetic insulator, and Bi₂Se₃ show highly efficient spin pumping in the bulk-dominated regime at room temperature. At low temperature, below 45 K, we have found evidence of proximity-induced magnetism in the TI. To understand the underlying mechanism of this proximity-induced magnetism, we performed polarized neutron reflectometry which indicates a sharp interface and a magnetic moment in the TI up to at least 45 K. In transport measurements, we observed both the anomalous Hall effect (AHE) and a large unidirectional magnetoresistance in the TI. The AHE, around 1 Ω at 2 K, was observed up to ~40 K and was amplified by electrically gating Bi₂Se₃. These results suggest that the induced magnetism in Bi₂Se₃ is primarily due to the coupling between MAFO and the surface states of Bi₂Se₃.