Magnetization in Intrinsic Topological Insulators Induced by Exchange Interaction with Ferromagnetic Insulator*

We examine the magnetic proximity-induced symmetry breaking via the exchange interaction in heterostructures of the topological insulator (TI) Bi$_{2}$Se$_{3}$ and the ferromagnetic insulator (FMI) EuS [1]. We observed the emergence of a ferromagnetic phase in TI with the excess of magnetic moment at the interface using depth and element sensitive Polarized Neutron Reflectometry (PNR). We find that the magnetization, penetrating into the TI originates through exchange interaction, without structural perturbation at the interface. Due to the different interlayer exchange coupling as well as the properties of the bulk and surface magnetizations, we investigated several different heterostructures after cooling in zero field (ZFC) and in an external magnetic field (FC). The significantly enhanced magnetic properties of the heterostructures as revealed by the PNR studies, as well as the temperature and external magnetic field dependence will be presented.\\[4pt] [1] P. Wei, et al Phys. Rev. Lett. 110, 186807 (2013)