Heterostructure of a Topological Insulator and a Ferromagnetic Insulator bi-layer*

The short-range nature of magnetic proximity coupling with a ferromagnetic insulator (FI) induces ferromagnetic interactions in the TI surface state with the symmetry breaking right at the interface. Here we investigate Bi$_{2}$Se$_{3}$/EuS heterostructures and the mechanism to induce ferromagnetic order at the surface of Bi$_{2}$Se$_{3}$ thin films by using the FI EuS. SQUID measurements demonstrated excessive magnetic moment for the EuS film alone, thus indicating that EuS might induce a magnetization in Bi$_{2}$Se$_{3}$. Using PNR we reveal that EuS induces a significant magnetic moment in the Bi$_{2}$Se$_{3}$ films. Thus, it creates broken time-reversal symmetry and should appear as magnetic signatures in electrical transport. The ferromagnetism of EuS and the coupling between EuS and Bi$_{2}$Se$_{3}$ have to be strong to signify surface magnetization effect. These findings may be used to investigate interesting emergent phenomena, because the local time-reversal symmetry breaking is essential for inducing a quantized topological magnetoelectric response.