title: 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 symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. 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 insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Ferhat
authors.initials: F
authors.lastname: Katmis
authors.affiliation: Quantum Condensed Matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: Valeria
authors.initials: V
authors.lastname: Lauter
authors.affiliation: Institut fuer Theoretische Physik III, Ruhr-Universitaet Bochum, D-44801 Bochum, Germany.|Institute for Theoretical Solid State Physics, Institut fuer Festkoerper- und Werkstoffforschung, Dresden, D-01069 Dresden, Germany.
authors.forename: Flavio S
authors.initials: FS
authors.lastname: Nogueira
authors.affiliation: Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA.|Département de Physique, Ecole Normale Supérieure, Centre National de la Recherche Scientifique, Paris Sciences et Lettres Research University, Paris 75005, France.
authors.forename: Badih A
authors.initials: BA
authors.lastname: Assaf
authors.affiliation: Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA.
authors.forename: Michelle E
authors.initials: ME
authors.lastname: Jamer
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Peng
authors.initials: P
authors.lastname: Wei
authors.affiliation: Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 64, India.
authors.forename: Biswarup
authors.initials: B
authors.lastname: Satpati
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: John W
authors.initials: JW
authors.lastname: Freeland
authors.affiliation: Institut fuer Theoretische Physik III, Ruhr-Universitaet Bochum, D-44801 Bochum, Germany.
authors.forename: Ilya
authors.initials: I
authors.lastname: Eremin
authors.affiliation: Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA.
authors.forename: Don
authors.initials: D
authors.lastname: Heiman
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Pablo
authors.initials: P
authors.lastname: Jarillo-Herrero
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.|Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Jagadeesh S
authors.initials: JS
authors.lastname: Moodera