A high-temperature ferromagnetic topological insulating phase by proximity coupling

Abstract

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Figure 1: XRD and high-resolution TEM of Bi2Se3–EuS bilayers.
Figure 2: SQUID magnetometry measurements for different Bi2Se3–EuS bilayers.
Figure 3: Polarized neutron reflectivity results for different Bi2Se3–EuS bilayers.
Figure 4: Ferromagnetic order in Bi2Se3–EuS bilayer samples.

References

  1. 1

    Hasan, M. Z. & Kane, C. L. Colloquium: Topological insulators. Rev. Mod. Phys. 82, 3045–3067 (2010)

    CAS  ADS  Article  Google Scholar 

  2. 2

    Qi, X.-L. & Zhang, S.-C. Topological insulators and superconductors. Rev. Mod. Phys. 83, 1057–1110 (2011)

    CAS  ADS  Article  Google Scholar 

  3. 3

    Fu, L. & Kane, C. L. Superconducting proximity effect and Majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008)

    ADS  Article  Google Scholar 

  4. 4

    Akhmerov, A., Nilsson, J. & Beenakker, C. Electrically detected interferometry of Majorana fermions in a topological insulator. Phys. Rev. Lett. 102, 216404 (2009)

    CAS  ADS  Article  Google Scholar 

  5. 5

    Ferreira, G. J. & Loss, D. Magnetically defined qubits on 3D topological insulators. Phys. Rev. Lett. 111, 106802 (2013)

    ADS  Article  Google Scholar 

  6. 6

    Chang, C. Z. et al. Experimental observation of the quantum anomalous Hall effect in a magnetic topological insulator. Science 340, 167–170 (2013)

    CAS  ADS  Article  Google Scholar 

  7. 7

    Checkelsky, J. G. et al. Trajectory of the anomalous Hall effect towards the quantized state in a ferromagnetic topological insulator. Nature Phys. 10, 731–736 (2014)

    CAS  ADS  Article  Google Scholar 

  8. 8

    Qi, X.-L., Hughes, T. L. & Zhang, S.-C. Topological field theory of time-reversal invariant insulators. Phys. Rev. B 78, 195424 (2008); erratum Phys. Rev. B 81, 159901 (2010)

    ADS  Article  Google Scholar 

  9. 9

    Essin, A., Moore, J. & Vanderbilt, D. Magnetoelectric polarizability and axion electrodynamics in crystalline insulators. Phys. Rev. Lett. 102, 146805 (2009); erratum Phys. Rev. Lett. 103, 259902 (2009)

    ADS  Article  Google Scholar 

  10. 10

    Nadj-Perge, S. et al. Observation of Majorona fermions in ferromagnetic atomic chains on a superconductor. Science 346, 602–607 (2014)

    CAS  ADS  Article  Google Scholar 

  11. 11

    Scholz, M. R. et al. Tolerance of topological surface states towards magnetic moments: Fe on Bi2Se3 . Phys. Rev. Lett. 108, 256810 (2012)

    CAS  ADS  Article  Google Scholar 

  12. 12

    Wei, P. et al. Exchange-coupling-induced symmetry breaking in topological insulators. Phys. Rev. Lett. 110, 186807 (2013)

    ADS  Article  Google Scholar 

  13. 13

    Chen, Y. L. et al. Massive Dirac fermion on the surface of a magnetically doped topological insulator. Science 329, 659–662 (2010)

    CAS  ADS  Article  Google Scholar 

  14. 14

    Vobornik, I. et al. Magnetic proximity effect as a pathway to spintronic applications of topological insulators. Nano Lett. 11, 4079–4082 (2011)

    CAS  ADS  Article  Google Scholar 

  15. 15

    Mellnik, A. R. et al. Spin-transfer torque generated by a topological insulator. Nature 511, 449–451 (2014)

    CAS  ADS  Article  Google Scholar 

  16. 16

    Miao, G.-X. & Moodera, J. S. Controlling magnetic switching properties of EuS for constructing double spin filter magnetic tunnel junctions. Appl. Phys. Lett. 94, 182504 (2009)

    ADS  Article  Google Scholar 

  17. 17

    Chappert, C. & Bruno, P. Magnetic anisotropy in metallic ultrathin films and related experiments on cobalt films. J. Appl. Phys. 64, 5736 (1988)

    CAS  ADS  Article  Google Scholar 

  18. 18

    Semenov, Y. G., Duan, X. & Kim, K. W. Electrically controlled magnetization in ferromagnet-topological insulator heterostructures. Phys. Rev. B 86, 161406 (2012)

    ADS  Article  Google Scholar 

  19. 19

    Stoehr, J. & Siegmann, H. C. Magnetism: From Fundamentals to Nanoscale Dynamics (Springer, 2006)

  20. 20

    Xu, S.-Y. et al. Hedgehog spin texture and Berry’s phase tuning in a magnetic topological insulator. Nature Phys. 8, 616–622 (2012)

    CAS  ADS  Article  Google Scholar 

  21. 21

    Yokoyama, T., Zang, J. & Nagaosa, N. Theoretical study of the dynamics of magnetization on the topological surface. Phys. Rev. B 81, 241410 (2010)

    ADS  Article  Google Scholar 

  22. 22

    Tserkovnyak, Y. & Loss, D. Thin-film magnetization dynamics on the surface of a topological insulator. Phys. Rev. Lett. 108, 187201 (2012)

    ADS  Article  Google Scholar 

  23. 23

    Nogueira, F. S. & Eremin, I. Fluctuation-induced magnetization dynamics and criticality at the interface of a topological insulator with a magnetically ordered layer. Phys. Rev. Lett. 109, 237203 (2012)

    ADS  Article  Google Scholar 

  24. 24

    Lauter, V., Ambaye, H., Goyette, R., Hal Lee, W.-T. & Parizzi, A. Highlights from the magnetism reflectometer at the SNS. Physica B 404, 2543–2546 (2009)

    CAS  ADS  Article  Google Scholar 

  25. 25

    Zhu, T. et al. The study of perpendicular magnetic anisotropy in CoFeB sandwiched by MgO and tantalum layers using polarized neutron reflectometry. Appl. Phys. Lett . 100, 202406 (2012)

    ADS  Article  Google Scholar 

  26. 26

    Korneev, D. A. et al. Absorbing sublayers and their influence on the polarizing efficiency of magnetic neutron mirrors. Nucl. Instrum. Meth. Phys. Res. B 63, 328–332 (1992)

    ADS  Article  Google Scholar 

  27. 27

    Mauger, A. & Godart, C. The magnetic, optical, and transport properties of representatives of a class of magnetic semiconductors: the europium chalcogenides. Phys. Rep. 141, 51–176 (1986)

    CAS  ADS  Article  Google Scholar 

  28. 28

    Miyazaki, H. et al. La-doped EuO: a rare earth ferromagnetic semiconductor with the highest Curie temperature. Appl. Phys. Lett. 96, 232503 (2010)

    ADS  Article  Google Scholar 

  29. 29

    Ott, H. et al. Soft x-ray magnetic circular dichroism study on Gd-doped EuO thin films. Phys. Rev. B 73, 094407 (2006)

    ADS  Article  Google Scholar 

  30. 30

    von Molnár, S. & Kasuya, T. Evidence of band conduction and critical scattering in dilute Eu-chalcogenide alloys. Phys. Rev. Lett. 21, 1757–1761 (1968)

    ADS  Article  Google Scholar 

  31. 31

    Idzuchi, H. et al. Critical exponents and domain structures of magnetic semiconductor EuS and Gd-doped EuS films near Curie temperature. Appl. Phys. Expr. 7, 113002 (2014)

    ADS  Article  Google Scholar 

  32. 32

    Mamaev, Y. A., Petrov, V. N. & Starovoitov, S. A. Critical behavior at surfaces. Sov. Tech. Phys. Lett. 13, 642 (1987)

    Google Scholar 

  33. 33

    Weller, D., Alvarado, S., Gudat, W., Schröder, K. & Campagna, M. Observation of surface-enhanced magnetic order and magnetic surface reconstruction on Gd(0001). Phys. Rev. Lett. 54, 1555–1558 (1985)

    CAS  ADS  Article  Google Scholar 

  34. 34

    Kinoshita, T. et al. Spectroscopy studies of temperature-induced valence transition on EuNi2(Si1-x Ge x )2 around Eu 3d–4f, 4d–4f and Ni 2p–3d excitation regions. J. Phys. Soc. Jpn. 71, 148–155 (2002)

    CAS  ADS  Article  Google Scholar 

  35. 35

    Arenholz, E., Schmehl, A., Schlom, D. G. & van der Laan, G. Contribution of Eu 4f states to the magnetic anisotropy of EuO. J. Appl. Phys. 105, 07E101 (2009)

    Article  Google Scholar 

  36. 36

    Assaf, B. A. et al. Modified electrical transport probe design for standard magnetometer. Rev. Sci. Inst. 83, 033904 (2012)

    CAS  ADS  Article  Google Scholar 

Download references

Acknowledgements

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).

Author information

Affiliations

Authors

Contributions

The research was conceived and designed by F.K. and J.S.M. The samples were prepared and characterized by F.K. The XRD experiments and data analysis were carried out by F.K.; the high-resolution TEM experiments and data analysis were carried out by B.S.; the PNR experiments and data analysis were carried out by V.L.; the XAS/XMCD experiments and data analysis were carried out by F.K. and J.W.F.; the transport experiments and data analysis were carried out by F.K. and D.H.; and the SQUID experiments and data analysis were carried out by F.K., B.A.A., M.E.J. and D.H. The data was interpreted by F.K., V.L., F.S.N. and J.S.M. All authors discussed the results and commented on the manuscript. The manuscript was written by F.K., V.L. and F.S.N.

Corresponding authors

Correspondence to Ferhat Katmis or Jagadeesh S. Moodera.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Soft X-ray absorption spectroscopy.

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.

Extended Data Figure 4 RHEED for interface evolution.

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 (ad) 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.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

Further reading

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.