Disentangling the magnetic proximity effect in topological insulators with Mg(Al,Fe)₂O₄/Bi₂Se₃ thin films

Magnetic topological insulators (TIs) have attracted attention as platforms for hosting the quantum anomalous Hall effect, characterized by the emergence of a quantum Hall effect in zero magnetic field and dissipation-less conduction. Magnetic TIs have been achieved through the addition of magnetic dopants to TIs or by a magnetic proximity effect induced in TIs due to an adjacent ferromagnet. Anomalous Hall effect, magnetoresistance, and polarized neutron reflectometry (PNR) measurements have been used as evidence of a magnetic proximity effect in a TI. In this talk, we have studied heterostructures of the ferromagnetic insulator Mg(Al,Fe)₂O₄ and TI Bi₂Se₃ to explore a possible magnetic proximity effect. They exhibit a nonlinear Hall effect that can be interpreted in terms of an anomalous Hall effect or multiband carriers in the TI, especially when the ferromagnetic insulator has an in-plane easy axis. Polarized neutron reflectometry provides two possible magnetic profiles – a sharp interface with a proximity effect and a highly disordered interface with no proximity effect. Further measurements of interface quality are required to prove the existence of induced magnetism in TIs.