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Hidden Interfaces and

High-Temperature Magnetism

in Intrinsic Topological

Insulator - Ferromagnetic

Insulator Heterostructures

Ferhat Katmis, Jagadeesh S. Moodera Department of Physics and Francis Bitter Magnet Laboratory

MIT Cambridge, USA

Don Heiman Department of Physics, Northeastern University, Boston

Valeria Lauter Quantum Condensed Matter Division,

Oak Ridge National Laboratory, USA

2 Presentation_name 2 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 lauterv@ornl.gov

Outline

• Polarized Neutron Reflectometry:

depth resolved vector magnetometry

for TI – FMI heterostructures

• 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

EuS

Bi2Se3

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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 spin-

momentum 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

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Topological insulator materials:

Magnetic?

NiFe/Bi2Se3

Chang, et al., Science (2013) Chang, et al., Nat. Mat. (2015)

13

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,

Magnetic field dependence of the longitudinal resistance xx (red curve) and the Hall resistance yx (blue

curve) at charge neutral point Vg=Vg0. b, c, Expanded yx (b) and xx (c) at low magnetic field. yx at zero

magnetic field exhibits a value of 1.00019±0.00069h/e2, while xx at zero magnetic field is only

~0.00013±0.00007h/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.

V-doped (BiSb)2Te3

Kou, et al., Nano Lett. (2013)

Cr-doped (BiSb)2Te3

Fan, et al., Nat. Mat. (2014) Kou, et al., PRL (2014)

Mellnik, et al., Nature (2014)

Checkelsky, et al., Nat. Phys. (2012)

Mn-doped Bi2(TeSe)3

Lang, et al., Nano Lett. (2014)

Bi2Se3/YIG

Wei, et al., PRL (2013)

EuS/Bi2Se3

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Topological insulator materials:

Magnetism via proximity

Substrate

TI FM

FM

Substrate TI

FM

Substrate

TI FM

Substrate

TI FM

FM

TI

FM

Increasing

Magnetization

Increasing

Complexity

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• 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 Bi2Se3 thin films

by using FI EuS.

• EuS 4f-5d energy gap 1.64 eV

Fermi level inside the gap

Interface ferromagnetism in TI

via magnetic proximity

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Characterization Bi2Se3/EuS bilayers

HRTEM

(B) electron diffraction image with an hexagonal

symmetry of Bi2Se3

(C) HRTEM image for substrate and Bi2Se3

(D) HRTEM images for EuS and Bi2Se3 interface

Bi2Se3

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Epitaxial relationship between EuS & Bi2Se

3

• Bi2Se3 (0001) // Al2O3 (0001)

• EuS (111) // Bi2Se3 (0001)

30 QL Bi2Se3 with 10 nm EuS

10 QL Bi2Se3 with 3 nm EuS

5 QL Bi2Se3 with 1 nm EuS

5 QL Bi2Se3

• Heterostructures grown on

Al2O3 (0001)

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Strong interface magnetization

Bi2Se3 = 20 nm EuS = 1 nm

• 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

Out-of-plane

In-plane

Out-of-plane

In-plane

-1000 -500 0 500 1000

-200

0

200M

[em

u/c

m3]

Field [Oe]

H // surface

H surface

@ 5 K

SQUID

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Samples for PNR experiments

MBE with a base pressure of 10-10 Torr on substrates Al2O3

Al2O

3/EuS/Bi

2Se

3//Al

2O

3

Sample size

20 X 20 mm2

EuS (5 nm)Bi2Se3 (5nm)

EuS (5 nm)Bi2Se3 (10nm)

EuS (5 nm)Bi2Se3 (20nm)

EuS (5 nm) – reference sample

sapphire

Bi2Se3

EuS

Al2O3

sapphire

Bi2Se3

EuS

Al2O3

sapphire

EuS

Bi2Se3

Al2O3

11 Presentation_name 11 Managed by UT-Battelle for the U.S. Department of Energy Advances in Polarized Neutron Reflectometry, Bochum, July 3 – 4 , 2013 lauterv@ornl.gov

Spallation Neutron Source, ORNL,TN

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Magnetism Reflectometer at SNS

Detector

Magnet

Displex

3He analyzer

SM FAN analyzer

SM Polarizer

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Magnetism Reflectometer at SNS

High intensity sample size 5x5 mm2

Low background 10-8

High polarization 98.5%

Polarization analysis

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

Electromagnet 1.15 Tesla (50 mm gap)

1.24 Tesla (46 mm gap)

2.40 Tesla (15 mm gap)

insulating

Al2O3 rods

Sample Temperature

from 5K to750K

H

Magnetic Field

1.15 Tesla

Sample rotation

360 deg

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Polarized Neutron Reflectometry experiment

on Al2O

3/EuS/Bi2Se3//Al

2O

3

Momentum transfer

Q = 4psin ai /l

Nbn – structural composition

Nbm – absolute magnetization

vector profile M !

NImb- absorption profile

V± = 2pħ/m N(bn ± bm)

Fermi pseudopotential:

Al2O3/EuS/Bi2Se3//Al2O3 – structure profile

Al2O3/EuS/Bi2Se3//Al2O3 – magnetization profile

Al2O3/EuS/Bi2Se3//Al2O3 – absorbtion profile

Perpendicular M

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Configuration of PNR experiment

probing the magnetic moment

distribution inside Bi2Se3/EuS interface

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Magnetization – PNR @ 5 K

Q = 4psin ai /l

Spin-Asymmetry : SA = (R+ - R- )/(R+ + R- )

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Absorption effect

• Absorption: Im part of the SLD

provides additional information

about Eu distribution

• No Eu atoms are determined

in Bi2Se3

Al2O3/EuS/Bi2Se3//Al2O3 – structure profile

Al2O3/EuS/Bi2Se3//Al2O3 – magnetization profile

Al2O3/EuS/Bi2Se3//Al2O3 – absorbtion profile

R+ & R-

---- no absorbtion

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• Magnetization behavior of bilayers (10 QL/ 5 nm) • Non-zero magnetization present in the 2 QL

Bi2Se3 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.

Magnetization – PNR @ 50,75,120,300 K

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Reference Sample - EuS//Sapphire

Sapphire

EuS (5nm)

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EuS

Bi2Se3

PNR: Magnetic moment distribution

At the interface EuS/Bi2Se3, 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

2 ML EuS

1st QL Bi2Se3

2nd QL Bi2Se3

PNR measures

In-plane projection of M

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SQUID magnetometry measurement

Magnetization versus Temperature at

various perpendicular applied fields

At the interface

• Large S-O interaction, the spin-momentum locking at Dirac

surface state creates strong anisotropy and stabilizes the

ferromagnetic state!

-400 -200 0 200 400-15

-10

-5

0

5

10

15

50 K

100 K 300 K

370 K

150 K

200 K

Hsurface

M [em

u/c

m3]

H [Oe]

-200 -150 -100 -50 0 50 100 150 200-1.0

-0.5

0.0

0.5

1.0

50 K

100 K 300 K

370 K

150 K

200 K

Hsurface

M [em

u/c

m3]

H [Oe]

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LETTER Nature 17635, May 9, 2016 /doi:10.1038

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

Acknowledgements

The work at SNS was supported by the Scientific User Facilities Division,

Office of Basic Energy Sciences and DOE

Results

• PNR - depth resolved vector magnetometry- measures the

spatial distribution of magnetization at the buried interfaces

of Bi2Se

3/EuS bilayers and the detailed chemical composition

of the heterostructre

• The magnetization in the interfacial 2 ML EuS layer has an out-

of-plane component.

• PNR provides evidence that Bi2Se

3/EuS heterostructures

exhibit proximity-induced interfacial magnetization in 3QL

layer of Bi2Se

3

• This effect originates through exchange interaction without

structural perturbation at the interface

• Magnetism persists up to high T above the Tc of EuS