Imaging Spatially Varying Magnetic Order in Proximity Induced Magnetic Topological Insulator

Broken time-reversal symmetry on the surface states of a three dimensional topological insulator, such as Bi$_2$Se$_3$, results in quantized anomalous Hall conductance and is predicted to exhibit topological magneto-electric effects. We plan investigate how Dirac fermions interact with magnetism by imaging the magnetization of a topological insulator (Bi$_2$Se$_3$) sandwiched between two ferromagnetic insulator layers (EuS) with a scanning SQUID microscope. Cooling in an in-plane field leads to a magnetization that varies spatially on a micron scale, with 6-fold rotational symmetry. Understanding the origin of this magnetism may shed light on the exchange interaction and electronic properties of topological insulators.