This document summarizes research on a thin film tri-layer structure of Fe/Bi2Te3/Fe, where Bi2Te3 is a topological insulator. Magnetron sputtering was used to deposit the layers on a MgO substrate at 100°C. Magneto-optic Kerr effect (MOKE) and vibrating sample magnetometry (VSM) measurements were performed to characterize the magnetic properties. MOKE showed the material does not have strong anisotropy, while VSM found the coercive field was maximum at 45° and Ms and Mr were highest at 0°. Tunneling magnetoresistance was observed, showing potential for applications in magnetic memory and sensors.

1. Fe/Bi₂Te₃/Fe Tunneling
Magneto-Resistance
with topological
insulator barrier
Vallery Salomon and Dereje Seifu,
Department of Physics,
Morgan State University, Baltimore, MD 21251

2. ABSTRACT
Thin film tri-layer structure Fe/Bi2Te3/Fe was synthesized using
magnetron DC / RF sputtering. This sample was synthesized at a
substrate temperature of 100C. It was studied using in-house built
magneto-optic Kerr effect (MOKE) instrument. The operating
principles of MOKE consist of measuring changes in polarization
of light reflected from a magnetic sample. The bulk magnetization
was measured using vibrating sample magnetometer (VSM).
Tunnel magneto-resistance (TMR) effect occurs in a structure that
is composed of two conductors separated by a thin insulator of the
order of few nanometers, the insulator barrier in this case is a well
known topological insulator, Bi2Te3. In this structure, electrons
tunnel from one of the conductors to the other through the
insulating barrier. This is a forbidden process in classical physics,
tunnel magneto-resistance is a purely quantum mechanical effect
which is key in developing magnetoresistive random access
memory (MRAM), magnetic sensors, and novel logic devices.

3. Sample Synthesis
• A DC / RF magnetron sputtering system was used to synthesize
multi-layered thin film of Fe/Bi2/Te3/Fe on MgO(100).
• Fe films with thickness 50 nm were deposited at 100 degrees
• A 5 nm layer of Bi2/Te3 was deposited at RT.

4. Magneto-Optic Kerr Effect
(MOKE) Schematic

5. Electronic Component of
MOKE
1.Computer display of hysteresis loop
during measurement.
2.PEM-100 Controller-photo elastic
modulator (PEM) modulates the
polarization of the laser light
passing through the PEM optical
head by50kH
3. SRS Preamplifier-converts current
signal from the photo diode to
voltage Signal and feeds it to the
Lock-in-Amplifier
4.SRS Lock-in amplifier (LIA) – is used
to increase signal to noise ratio
5.BOP Power supply-Supplies current
to the electromagnet in response
to a signal from LIA’s auxiliary
output.
6.Computer system interfaced with
electronic parts of MOKE.

6. Optical Part of MOKE
Wavelength and 5mW output power
8. Polarizer-linearly polarize light and allows linearly
polarized light at 45 from the vertical to pass
through
2b. PEM Optical head-modulates the polarization of the
laser light passing through. It is connected to the
lock-in amplifier reference input so only the
modulated light forms the signal.
9. Electromagnet-The sample is placed in a varying
magnetic field at the center of the electromagnet
10. Sample holder-used to hold the sample in place and
rotates in a plane perpendicular to the direction
of the light
11. Analyzer-crossed with the polarizer minimizing
light passing through it so that to only let through
the part of light whose polarization is rotated due
to reflection from a ferromagnetic sample in an
external magnetic field.
12. Photo diode- collects light waves and converts it to a
current signal which is fed to the pre-amplifier,
which then converts the current signal to a
voltage signal which is then fed to the Lock-in
amplifier

8. MOKE

9. MOKE (Azimuthal Plot)
100
200
300
30
210
60
240
90
270
120
300
150
330
180 0
MOKE Angle vs Hc
Angle
Hc

10. Hc (Oe) vs Angle using MOKE
266
268
270
272
274
276
278
280
282
284
0 50 100 150 200 250 300 350 400
Hc(Oe)
Angle
Angle vs Hc (Oe)

11. VSM of Fe/Bi2Te3/Fe on
MgO(100) grown at T=100C
-0.0025
-0.002
-0.0015
-0.001
-0.0005
0
0.0005
0.001
0.0015
0.002
0.0025
-1.50E+03 -1.00E+03 -5.00E+02 0.00E+00 5.00E+02 1.00E+03 1.50E+03
Magnetization(emu)
Applied Field (Oe)
90 Degrees
45 Degrees
0 Degrees

12. Hysteresis at Various Low
Temperature Measurements
-3.00E-03
-2.00E-03
-1.00E-03
0.00E+00
1.00E-03
2.00E-03
3.00E-03
-2.00E+03 -1.50E+03 -1.00E+03 -5.00E+02 0.00E+00 5.00E+02 1.00E+03 1.50E+03 2.00E+03
M(emu)
Hc (Oe)
-192.16 C
-153.41
-93.33 C
300k

13. Variation of Hc, Ms, & Mr using
VSM
0.00E+00
5.00E-04
1.00E-03
1.50E-03
2.00E-03
2.50E-03
0
50
100
150
200
250
300
0 10 20 30 40 50 60 70 80 90 100
Hc(Oe)
Angle
Hc(Oe)
Ms(emu)
Mr(emu)

14. Conclusion
• From VSM data, at 45 degrees the coercive field was
maximum, at value 243.9 Oe.
• The Mr and Ms values were maximum at 0 degrees.
• The Azimuthal graph of MOKE shows that the material does
not have substantial anisotropy.
• In MOKE the difference between the maximum and minimum
Hc values is 15 Oe, measured using longitudinal MOKE.
• The difference between the surface coercive field measured
using MOKE and bulk coercive field measure using MOKE is
found to be 38 Oe.
• In the near future AFM / MFM and MR measurements will be
performed on this system.

15. Acknowledgements
We would like to acknowledge support by
ARL W911NF-12-2-0041 and
by NSF MRI -DMR-1337339.
Thank you