Spin Electronics
description
Transcript of Spin Electronics
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Elshan Akhadov
Spin Electronics
QuarkNet, June 28, 2002
Peng Xiong
Department of Physics and MARTECHFlorida State University
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Elshan Akhadov
Speed: 100 Hz Size: 10-2 m Cost: $106/transistor
Speed: 109 Hz Size: 10-7 m Cost: $10-5/transistor
Moore’s Law… is the end in sight?
SOURCE GATE DRAIN
MOSFET
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Elshan Akhadov
Magnetic Information Storage: superparamagnetic limit
Density: 2 kb/in2
Speed: 70 kb/s Size: ” x 50 Capacity: 5 Mb
Density: 20 Gb/in2
Speed: 200 Mb/s Size: ” x 2 Capacity: 50 Gb
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Elshan Akhadov
Superparamagnetic Limit: thermal stability of magnetic media
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Elshan Akhadov
Semiconductor Random Access Memory: alternatives?
High speed
Low density High power consumption Volatile
MO
S
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Elshan Akhadov
Metal-based Spintronics:Spin valve and magnetic tunnel junction
EF
N(E)
E E
H R
HM
N(E)
EF
E E
H
Applications: magnetic sensors, MRAM, NV-logic
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Elshan Akhadov
Spintronics in Semiconductor: spin transistor
Datta and Das, APL, 1990
Issues
Spin polarized material
Spin injection
Spin coherence
Spin detection
SOURCE DRAINH
H
GaAs
Dreams
High performance
opto-electronics
Single-chip computer
(instant on; low power)
Quantum computation
GATE
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RF
Schmidt et.al., PRB, 2000
Solutions:
Use injector with 100%
spin polarization
Non-diffusive injection
Conductivity matching
SC
N
NFM
F
F
I
I
I
Spin Injection: the conductivity mismatch
RNRF
RN
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Elshan Akhadov
half-metallicferromagnet
E
Uex
Measurement of spin polarization: using a superconductor
CrO2: a half metal
Tc = 400 K
m = 2B/Cr
p = 100%Schwarz, J. Phys. F, 1986
normal metal
E
metallic ferromagnet
E4s
3d
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Elshan Akhadov
E
N(E)
EF
Andreev reflection: normal metal/superconductor
eV
N S Question:
What could happen to an electron with energy eV < when it hits S from N?
A. bounce back; B. go into S as an electron; C. go into S in a Cooper pair.
1. A and B2. B and C3. C and A4. A and B and CN S
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Elshan Akhadov
Andreev reflection: normal metal/superconductor
FvHZ /
Z = 0clean metallic contact
Z >> 1tunnel junction
Z ~ 1in-between
Blonder, Tinkham, and Klapwijk, PRB, 1982
-2 - 0 2 0.0
0.5
1.0
1.5
2.0
V-2 - 0 2 0.0
0.5
1.0
1.5
2.0
G(V
)/G
n
V-2 - 0 2 0.0
0.5
1.0
1.5
2.0
V
p = 0
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Elshan Akhadov
E
DOS
EF
eV
F S
Andreev reflection: ferromagnet/superconductor
p = 75%
Z = 0metallic contact
Z ~ 1in-between
Z >> 1tunnel junction
-2 - 0 2 0.0
0.5
1.0
-2 - 0 2 0.0
0.5
1.0
1.5
0.0
0.5
1.0
1.5
2.0
V
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Elshan Akhadov
p = 75%
Z = 0metallic contact
Z ~ 1in-between
Z >> 1tunnel junction
-2 - 0 2 0.0
0.5
1.0
-2 - 0 2 0.0
0.5
1.0
1.5
0.0
0.5
1.0
1.5
2.0
V
Z = 0metallic contact
Z >> 1tunnel junction
Z ~ 1in-between
p = 0
-2 -1 0 1 2 0.0
0.5
1.0
1.5
2.0
-2 -1 0 1 2 0.0
0.5
1.0
1.5
2.0
0.0
0.5
1.0
1.5
2.0
Comparison: normal metal and ferromagnet
V
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Elshan Akhadov
Spin Polarization of CrO2: our approach
Planar junction real device structure
Artificial barrier controlled interface
Preservation of spin polarization
at and across barrier
Key step: controlled surface modification of CrO2 via Br etch
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Furnace, T=280° CFurnace, T=280° C
substrateHeater block, T=400°C
O2 flow
Cr8O21 precursor
CrO2 Film Growth: Chemical Vapor Deposition
Ivanov, Watts, and Lind, JAP, 2001
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Elshan Akhadov
CrO2
TiO2
Pb or AlI
Grow CrO2 film Pattern CrO2 stripe Surface modification: Br etch Deposit S cross stripes
CrO2
Pb or Al
V
~
Lock-in
dV/dI vs V in He4 (1K) or He3 (0.3K) cryostats
...cos)()(00 tI
dI
dVIVIV acI
Junction Fabrication and Measurement
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Elshan Akhadov
-10 -5 0 5 100.0
0.5
1.0
1.5
2.0
G (V
) / G
N
V (mV)
-4 -2 0 2 40.0
0.2
0.4
0.6
0.8
1.0
G (V
) / G
N
V (mV)
Results: CrO2/(I)/Pb junctions
Metallic contactZ = 0 p = 97%
T = 1.2 K = 1.44 meV
T = 400 mKTunnel junction
High quality barrier w/o inelastic scattering
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Elshan Akhadov
Measurement of spin polarization in high-Z junctions: using Zeeman splitting
E
N(E)
EF
eV
F SMeservey and Tedrow, Phys. Rep., 1994
eV/
H
H
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Elshan Akhadov
Al
CrO2
H
CrO2 Al
In order to get high Hc:
Ultrathin S film
Parallel field
Negligible s-o interaction
Zeeman splitting in an F/I/S junction
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-0.5 0.0 0.50.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
H = 0.0 T = 0.5 T = 1.0 T = 1.5 T = 2.0 T = 2.5 T
G (
V)
/ GN
V (mV)
0.0 0.5 1.0 1.5 2.0 2.50
20
40
60
80
100
(e
V)
H (T)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.60.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
G (
V)
/ G
N
V (mV)T =400 mK
Results: Zeeman splitting
+2.5T-2.5T
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Elshan Akhadov
Summary (CrO2)
Verified half-metallicity of CrO2
Engineered an artificial barrier on CrO2 surface
Preserved complete spin polarization at interface
Achieved full spin injection from a half metal
Future
Apply the technique to other systems
Magnetic tunnel junction
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CrO2/I/Co magnetic “tunnel” junction
H
CrO2Co
AlOx
-1000 -500 0 500 100014.5
15.0
15.5
16.0
16.5
Res
ista
nce
()
Field (Oe)
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Elshan Akhadov
Jeff ParkerJazcek BradenSteve WattsPavel Ivanov
Stephan von MolnárPedro SchlottmannDavid Lind
The People
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Elshan Akhadov
“computers with wires no wider than 100 atoms, a microscope that could view individual atoms, machines that could manipulate atoms 1 by 1, and circuits involving quantized energy levels or the interactions of quantized spins.”
Richard Feynman –
“There’s Plenty of Room at the Bottom”
1959 APS Meeting
Let’s build