Interface induced states at the boundary between a 3D topological   insulator Bi$_2$Se$_3$ and a ferromagnetic insulator EuS
  By means of relativistic density functional theory (DFT) calculations we study electron band structure of the topological insulator (TI) Bi$_2$Se$_3$ thin films deposited on the ferromagnetic insulator (FMI) EuS substrate. In the Bi$_2$Se$_3$/EuS heterostructure, the gap opened in the spectrum of the topological state has a hybridization character and is shown to be controlled by the Bi$_2$Se$_3$ film thickness, while magnetic contribution to the gap is negligibly small. We also analyzed the effect of Eu doping on the magnetization of the Bi$_2$Se$_3$ film and demonstrated that the Eu impurity induces magnetic moments on neighboring Se and Bi atoms an order of magnitude larger than the substrate-induced moments. Recent magnetic and magneto-transport measurements in EuS/Bi$_2$Se$_3$ heterostructure are discussed. 
S.V. Eremeev, V.N. Men'shov, V.V. Tugushev, E.V. Chulkov
cond-mat.mtrl-sci