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).
The authors declare no competing financial interests.
Extended data figures and tables
a, Measurement of Eu M5 edge at 8 K and a field of 2 T. The spectra are consistent with a valence of 2+ and the corresponding large magnetic moment seen by XMCD. b, c, X-ray absorption at the Se L edge of Al2O3/Bi2Se3 (b) and EuS/Bi2Se3 (c), showing the good agreement of bulk- and interface-sensitive modes, affirming that the interface and bulk have identical electronic structure. TEY, total electron yield; TFY, total fluorescence yield.
Extended Data Figure 2 SQUID magnetometry measurements for a Bi2Se3–EuS bilayer with thicknesses of 7 QL for Bi2Se3 and 5 nm for EuS.
a, Magnetization versus temperature at various magnetic fields applied out-of-plane (H perpendicular to the surface). The arrows correspond to the direction of the local magnetization. The large decrease in M as T increases shows the EuS magnetism decreasing (plotted in logarithmic–logarithmic scale). However, at higher temperatures M(T) shows an increase that is much larger than expected from Eu paramagnetism alone, and this could be attributed to reoriented spins (perpendicular) at the interface in the absence of the large in-plane influence from EuS layers above. (Furthermore, control samples of 5-nm-thick EuS grown on sapphire (Al2O3(0001)) substrate did not show any hysteresis above ~50 K even with a 5-T applied field). The possible spin texture is schematically represented below the experimental M versus T results. For the in-plane applied magnetic field configuration, such an increase in magnetization at high temperatures does not show features such as are observed for the perpendicular configuration. The uncertainty in M from the subtraction of the substrate diamagnetism is smaller than the size of the data points. b, The low-field magnetic hysteresis at different temperatures, where the field is applied out-of-plane (H perpendicular to the surface). Insets show hysteresis at 5 K comparing data for in-plane (H parallel to the surface) and out-of-plane magnetic-field applications.
Extended Data Figure 3 Results from PNR for Bi2Se3/EuS bilayer samples with 5 QL, 10 QL of Bi2Se3 and pure EuS.
a and c, PNR nuclear (NSLD, in pink), magnetic (MSLD, in green) and absorption (ASLD, in blue) scattering length density (SLD) profiles, measured for samples with 5 QL (a) and 10 QL (c) at 5 K and with an in-plane magnetic field of 1 T and presented as a function of the distance from the surface. Magnetization measured inside the Bi2Se3 layer is marked with the red arrows. The scale on the right-hand side shows magnetization. b and d, SA as a function of the momentum transfer Q. Solid curves (dark pink) correspond to the best fits to the experimental data shown with filled circles with error bars (dark pink), with χ2 = 1.32 and 1.34, respectively; dashed curves (black) show a considerable deviation from the experimental data when the magnetization in the Bi2Se3 2 QL interfacial layer is set to zero with corresponding increased values of χ2 = 2.82 and 2.56. The error bars represent one standard deviation. e, PNR reflectivity data (logarithmic–linear scale) measured on a pure 5-nm-thick EuS film at 5 K, 50 K, 80 K, 120 K, 250 K and 300 K. f, Experimental data of the SA obtained from the measured reflectivities in e. The error bars represent one standard deviation.
a, The RHEED pattern for Bi2Se3 (2D-like), grown on an Al2O3 (0001) surface is shown, where the incident beam is along the -direction of the substrate. The RHEED pattern for the 2-nm-thick EuS surface is shown with the beam along the -direction in b, and along the -direction in c. The RHEED pattern for the 5-nm-thick EuS surface (3D-dominant) is shown along the -direction in d.
Extended Data Figure 5 Temperature-dependent Hall voltage for a bilayer sample of 7 QL and 5 QL Bi2Se3 with 5-nm-thick EuS measured with 10-μA direct current, with magnetic field applied perpendicular to the film plane.
A nonlinear contribution to the Hall voltage, ΔVyx, is seen in the 5 QL Bi2Se3/5 nm EuS (a–d) and 7 QL Bi2Se3/5 nm EuS (e, f) samples. Plot f is the zoom-out of e. The normalized remanent magnetization in the bilayer sample (7 QL Bi2Se3/5 nm EuS) versus temperature (g), shows a finite decrease as temperature increased, matching the Hall data behaviour coming from the interfacial exchange induced ferromagnetic state, as discussed in the main text.
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Katmis, F., Lauter, V., Nogueira, F. et al. A high-temperature ferromagnetic topological insulating phase by proximity coupling. Nature 533, 513–516 (2016). https://doi.org/10.1038/nature17635
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