Hidden Interfaces and High-Temperature Magnetism in Intrinsic Topological Insulator - Ferromagnetic Insulator Heterostructures

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1 Hidden Interfaces and High-Temperature Magnetism in Intrinsic Topological Insulator - Ferromagnetic Insulator Heterostructures Valeria Lauter Quantum Condensed Matter Division, Oak Ridge National Laboratory, USA Ferhat Katmis, Jagadeesh S. Moodera Department of Physics and Francis Bitter Magnet Laboratory MIT Cambridge, USA Don Heiman Department of Physics, Northeastern University, Boston ORNL is managed by UT-Battelle for the US Department of Energy

2 Outline EuS Bi 2 Se 3 Topological Insulators - a new phase of matter with TRS Symmetry breaking in TI via magnetic proximity Induce ferromagnetism in Topological Insulator via exchange coupling in TI-FMI + - Polarized Neutron Reflectometry: depth resolved vector magnetometry for TI FMI heterostructures Managed by UT-Battelle 2 Presentation_name

3 Interface ferromagnetism in TI via magnetic proximity Introducing ferromagnetic order in TI: by doping with specific elements: - hard to separate the surface and the bulk phases. - introduces crystal defects, magnetic scattering centers, impurity states in the insulating gap are detrimental to mobility and the transport of spinmomentum locked surface electrons in TIs. by uniformly depositing magnetic atoms (Fe) over the TI surface: - the transport properties of a TI are influenced by the metallic ferromagnetic overlayer or atoms. by magnetic proximity with FI: - the spin-momentum locked helical electronic states in Tis and topological magneto-electric effect 3 Managed by UT-Battelle 3 Presentation_name

4 Topological insulator materials: Magnetic? 4 Mn-doped Bi2(TeSe)3 Checkelsky, et al., Nat. Phys. (2012) Cr-doped (BiSb)2Te3 Chang, et al., Science (2013) V-doped (BiSb)2Te3 Figures and Figure captions: Figure 1 The QAH effect in a 4QL (Bi0.29Sb0.71)1.89V0.11Te3 film (sample S1) measured at 25mK. a, Chang, et al., Nat. Mat. (2015) Magnetic field dependence of the longitudinal resistance curve) at charge neutral point Vg=Vg0. b, c, Expanded yx xx (red curve) and the Hall resistance (b) and xx magnetic field exhibits a value of ± h/e2, while (c) at low magnetic field. xx yx (blue yx at zero at zero magnetic field is only ~ ± h/e2 (~3.35±1.76 ). The dashed lines indicate the expected resistance value in ideal QAH regime. The error bars in (b) and (c) are estimated by the variability of the standard resistor, accuracy of the voltmeters, current source which is reflected in the averaged data. Kou, et al., Nano Lett. (2013) NiFe/Bi2Se3 Fan, et al., Nat. Mat. (2014) Kou, et al., PRL (2014) Bi2Se3/YIG EuS/Bi2Se3 13 Mellnik, et al., Nature (2014) Wei, et al., PRL (2013) by UT-Battelle Presentation_name 4Managed Lang, et al., Nano Lett. (2014)

5 Topological insulator materials: Magnetism via proximity FM TI Substrate TI FM Substrate Increasing Complexity FM TI FM Substrate Increasing Magnetization FM TI FM TI FM Substrate 5 Managed by UT-Battelle 5 Presentation_name

6 Interface ferromagnetism in TI via magnetic proximity The particular type of interface - between a topological insulator and a ferromagnet might become key to the computer industry's future ability. The goal is the ability to manipulate surface electron states. We introduce ferromagnetic order onto the surface of TI Bi 2 Se 3 thin films by using FI EuS. EuS 4f-5d energy gap 1.64 ev Fermi level inside the gap 6 Managed by UT-Battelle 6 Presentation_name

7 Characterization Bi2Se3/EuS bilayers Bi 2 Se 3 HRTEM (B) electron diffraction image with an hexagonal symmetry of Bi 2 Se 3 (C) HRTEM image for substrate and Bi 2 Se 3 (D) HRTEM images for EuS and Bi 2 Se 3 interface 7 Managed by UT-Battelle 7 Presentation_name for the U.S. Department of Energy APS meeting, Denver, March 3 7, 2014

8 Epitaxial relationship between EuS & Bi 2 Se 3 Heterostructures grown on Al 2 O 3 (0001) 30 QL Bi 2 Se 3 with 10 nm EuS 10 QL Bi 2 Se 3 with 3 nm EuS 5 QL Bi 2 Se 3 with 1 nm EuS 5 QL Bi 2 Se 3 Bi 2 Se 3 (0001) // Al 2 O 3 (0001) EuS (111) // Bi 2 Se 3 (0001) 8 Managed by UT-Battelle 8 Presentation_name

9 M [emu/cm3] Strong interface magnetization SQUID EuS = 1 nm Bi 2 Se 3 = 20 nm In-plane Out-of-plane 200 H // surface H surface In-plane Out-of-plane 5 K Field [Oe] All samples display ferromagnetism Out-of-plane remanance ratio does not depend on thicknesses > > evidence that the out-of-plane component is at the interface 9 Managed by UT-Battelle 9 Presentation_name for the U.S. Department of Energy APS meeting, Denver, March 3 7, 2014

10 Samples for PNR experiments Al 2 O 3 /EuS/Bi 2 Se 3 //Al 2 O 3 MBE with a base pressure of Torr on substrates Al 2 O 3 EuS (5 nm)bi 2 Se 3 (5nm) EuS (5 nm)bi 2 Se 3 (10nm) EuS (5 nm)bi 2 Se 3 (20nm) EuS (5 nm) reference sample Al 2 O 3 EuS Bi 2 Se 3 sapphire Al 2 O 3 EuS Bi 2 Se 3 sapphire Sample size 20 X 20 mm 2 Al 2 O 3 EuS Bi 2 Se 3 sapphire 10 Managed by UT-Battelle 10 Presentation_name

11 Spallation Neutron Source, ORNL,TN 11 Managed by UT-Battelle 11 Presentation_name for the U.S. Department of Energy Advances in Polarized Neutron Reflectometry, Bochum, July 3 4, 2013

12 Magnetism Reflectometer at SNS 3He analyzer SM FAN analyzer Magnet SM Polarizer Detector Displex 12 Managed by UT-Battelle 12 Presentation_name for the U.S. Department of Energy APS meeting, Denver, March 3 7, 2014

13 Magnetism Reflectometer at SNS High intensity sample size 5x5 mm 2 Low background 10-8 High polarization 98.5% Polarization analysis H Sample rotation 360 deg Polarized and unpolarized beam Fast laser pre-alignment Efficient thermal cycling (5K 750K) Sample environment new features: Displex: In-situ annealing Sample rotation Bias voltage Magnetic Field 1.15 Tesla Electromagnet 1.15 Tesla (50 mm gap) 1.24 Tesla (46 mm gap) 2.40 Tesla (15 mm gap) Sample Temperature from 5K to750k insulating Al 2 O 3 rods 13 Managed by UT-Battelle 13 Presentation_name for the U.S. Department of Energy APS meeting, Denver, March 3 7, 2014

14 Polarized Neutron Reflectometry experiment on Al 2 O 3 /EuS/Bi2Se3//Al 2 O 3 Fermi pseudopotential: V ± = 2pħ/m N(b n ± b m ) Momentum transfer Q = 4psin a i /l Nb n structural composition Nb m absolute magnetization vector profile M! NIm b - absorption profile Perpendicular M Al 2 O 3 /EuS/Bi 2 Se 3 //Al 2 O 3 structure profile Al 2 O 3 /EuS/Bi 2 Se 3 //Al 2 O 3 magnetization profile Al 2 O 3 /EuS/Bi 2 Se 3 //Al 2 O 3 absorbtion profile 14 Managed by UT-Battelle 14 Presentation_name for the U.S. Department of Energy APS meeting, Denver, March 3 7, 2014

15 Configuration of PNR experiment probing the magnetic moment distribution inside Bi2Se3/EuS interface 15 Managed by UT-Battelle 15 Presentation_name

16 Magnetization 5 K Q = 4psin a i /l Spin-Asymmetry : SA = (R + - R - )/(R + + R - ) 16 Managed by UT-Battelle 16 Presentation_name

17 Absorption effect R+ & R no absorbtion Absorption: Im part of the SLD provides additional information about Eu distribution No Eu atoms are determined in Bi 2 Se 3 Al 2 O 3 /EuS/Bi 2 Se 3 //Al 2 O 3 structure profile Al 2 O 3 /EuS/Bi 2 Se 3 //Al 2 O 3 magnetization profile Al 2 O 3 /EuS/Bi 2 Se 3 //Al 2 O 3 absorbtion profile 17 Managed by UT-Battelle 17 Presentation_name

18 Magnetization 50,75,120,300 K Magnetization behavior of bilayers (10 QL/ 5 nm) Non-zero magnetization present in the 2 QL Bi 2 Se 3 interfacial layer also penetrates into the EuS layer Magnetization reduced by an order of magnitude at higher temperatures No magnetization was detected above ~50 K in the pure EuS film. 18 Managed by UT-Battelle 18 Presentation_name

19 Reference Sample - EuS//Sapphire EuS (5nm) Sapphire 19 Managed by UT-Battelle 19 Presentation_name

20 PNR: Magnetic moment distribution EuS PNR measures In-plane projection of M 2 ML EuS 1 st QL Bi 2 Se 3 2 nd QL Bi 2 Se 3 Bi 2 Se 3 At the interface EuS/Bi 2 Se 3, the strong spin-orbit coupling affects anisotropy direction, leading to an out-of-plane magnetic moment and results in generating a gap in the TI surface state 20 Managed by UT-Battelle 20 Presentation_name

21 M [emu/cm 3 ] M [emu/cm 3 ] SQUID magnetometry measurement Magnetization versus Temperature at various perpendicular applied fields K 100 K 150 K 200 K 300 K 370 K H surface H [Oe] K 100 K 150 K 200 K 300 K 370 K At the interface H [Oe] Large S-O interaction, the spin-momentum locking at Dirac surface state creates strong anisotropy and stabilizes the ferromagnetic state! -0.5 H surface 21 Managed by UT-Battelle 21 Presentation_name

22 LETTER Nature 17635, May 9, 2016 /doi: A high-temperature ferromagnetic topological insulating phase by proximity coupling Ferhat Katmis1,2,3*, Valeria Lauter4*, Flavio S. Nogueira5,6, Badih A. Assaf7,8, Michelle E. Jamer7, Peng Wei1,2,3, Biswarup Satpati9, John W. Freeland10, Ilya Eremin5, Don Heiman7, Pablo Jarillo-Herrero1 & Jagadeesh S. Moodera1,2,3 * Authors contributed equally to this work 22 Managed by UT-Battelle 22 Presentation_name

23 Results PNR - depth resolved vector magnetometry- measures the spatial distribution of magnetization at the buried interfaces of Bi 2 Se 3 /EuS bilayers and the detailed chemical composition of the heterostructre The magnetization in the interfacial 2 ML EuS layer has an outof-plane component. PNR provides evidence that Bi 2 Se 3 /EuS heterostructures exhibit proximity-induced interfacial magnetization in 3QL layer of Bi 2 Se 3 This effect originates through exchange interaction without structural perturbation at the interface Magnetism persists up to high T above the Tc of EuS Acknowledgements The work at SNS was supported by the Scientific User Facilities Division, Office of Basic Energy Sciences and DOE