Weak-Localization-Like Magnetoresistance on a Topological Insulator - Ferromagnetic Insulator Interface*

In this talk, we will present measurements on the interface between a topological insulator (TI) and a ferromagnetic insulator (FI). The results provide a likely indication for gap opening in the TI surface states by its proximity to the FI. While above the Curie temperature ($T_C$) of the FI we observed weak-antilocalization-like positive magnetoresistance as ubiquitously in TIs, below $T_C$ an unusual weak-localization-like negative magnetoresistance was seen at low magnetic fields, which has never been observed in TIs without any magnetic doping. Such proximity introduced gap-opening and resulting massive Dirac fermions will hopefully lead to realization of many intriguing phenomena such as the quantum anomalous Hall effect and the inverse spin-galvanic effect.