Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry1,2, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices3,4,5. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena6,7. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin–orbit interaction and the spin–momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator2,8 could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.
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F.K. thanks L. Fu, V. Madhavan, N. Gedik, B. Sinkovic, Y. Wang and H. Lin for discussions. V.L. thanks S. Nagler for discussions, and H. Ambaye, A. Glavic and the Spallation Neutron Source staff for support. The research conducted at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, and the US Department of Energy. F.K., P.J.-H., and J.S.M. thank the MIT MRSEC through the MRSEC Program of the National Science Foundation under award number DMR-0819762 (upgrade of the molecular beam epitaxy system) for support. J.S.M. thanks the National Science Foundation (DMR-1207469), Office of Naval Research (N00014-13-1-0301) and the STC Center for Integrated Quantum Materials under National Science Foundation grant DMR-1231319 for support, and the thin-film growth and characterization of the materials used. The hetero-structure characterization was supported by the US Department of Energy, Basic Energy Sciences Office, Division of Material Sciences and Engineering under award number DE-SC0006418 (F.K. and P.J.-H.). B.A.A., M.E.J. and D.H. thank the National Science Foundation under award numbers DMR-0907007 and ECCS-1402738 (for SQUID magnetometry characterization) for support. B.A.A. is also supported in part by the Agence Nationale de la Recherche LabEx grants ENS-ICFP (ANR-10-LABX-0010/ ANR-10-IDEX-0001-02 PSL). The use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract number DE-AC02-06CH11357. I.E. and F.S.N. acknowledge the German Research Council (DFG) for the financial support under the collaborative research centre SFB TR 12 and the priority programme SPP 1666 (grant number ER 463/9).