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ORNL is managed by UT-Battelle
for the US Department of Energy
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 [email protected]
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
3 Presentation_name 3 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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
4 Presentation_name 4 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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
5 Presentation_name 5 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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
6 Presentation_name 6 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
• 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
7 Presentation_name 7 Managed by UT-Battelle for the U.S. Department of Energy APS meeting, Denver, March 3 – 7 , 2014 [email protected]
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
8 Presentation_name 8 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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)
9 Presentation_name 9 Managed by UT-Battelle for the U.S. Department of Energy APS meeting, Denver, March 3 – 7 , 2014 [email protected]
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
10 Presentation_name 10 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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 [email protected]
Spallation Neutron Source, ORNL,TN
12 Presentation_name 12 Managed by UT-Battelle for the U.S. Department of Energy APS meeting, Denver, March 3 – 7 , 2014 [email protected]
Magnetism Reflectometer at SNS
Detector
Magnet
Displex
3He analyzer
SM FAN analyzer
SM Polarizer
13 Presentation_name 13 Managed by UT-Battelle for the U.S. Department of Energy APS meeting, Denver, March 3 – 7 , 2014 [email protected]
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
14 Presentation_name 14 Managed by UT-Battelle for the U.S. Department of Energy APS meeting, Denver, March 3 – 7 , 2014 [email protected]
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
15 Presentation_name 15 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
Configuration of PNR experiment
probing the magnetic moment
distribution inside Bi2Se3/EuS interface
16 Presentation_name 16 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
Magnetization – PNR @ 5 K
Q = 4psin ai /l
Spin-Asymmetry : SA = (R+ - R- )/(R+ + R- )
17 Presentation_name 17 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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
18 Presentation_name 18 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
• 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
19 Presentation_name 19 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
Reference Sample - EuS//Sapphire
Sapphire
EuS (5nm)
20 Presentation_name 20 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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
21 Presentation_name 21 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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]
22 Presentation_name 22 Managed by UT-Battelle for the U.S. Department of Energy TSD Workshop, University of Minnesota, May 12 – 13, 2016 [email protected]
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