Magnetic ordering at the topological insulator-magnetic insulator interface

The spin momentum locking property of topological insulators is the key to their practical utility. However, due to large bulk contribution and defects, it has been difficult to harness it. One of the possible solutions is to use heterostructures of topological insulators and magnetic insulators which can make the spin momentum locking robust at the interface. Here we report on the angular dependence of magnetoresistance on heterostructures of the topological insulator BiSbTeSe$_{\mathrm{2}}$ and the magnetic insulator YIG. We find that a four-fold symmetry arises in the in-plane angular dependence spectrum above a critical external field indicating the presence of magnetic ordering at the interface. We demonstrate that the interfacial magnetic ordering arises from a spin polarized interface state which is distinct from the topological surface state.