Elshan Akhadov
Spin Electronics
Peng Xiong
Department of Physics and MARTECH
Florida State University
QuarkNet, June 28, 2002
Elshan Akhadov
Moore’s Law… is the end in sight?
Speed: 100 Hz
Size: 10-2 m
Cost: $106/transistor
SOURCE
Speed: 109 Hz
Size: 10-7 m
Cost: $10-5/transistor
GATE
MOSFET
DRAIN
Elshan Akhadov
Magnetic Information Storage: superparamagnetic limit
Density: 20 Gb/in2
Speed: 200 Mb/s
Size: f2.5” x 2
Capacity: 50 Gb
Density: 2 kb/in2
Speed: 70 kb/s
Size: f24” x 50
Capacity: 5 Mb
Elshan Akhadov
Superparamagnetic Limit:
thermal stability of magnetic media
Elshan Akhadov
Semiconductor Random Access Memory: alternatives?
M
O
S
High speed
Low density
High power consumption
Volatile
Elshan Akhadov
Metal-based Spintronics:
Spin valve and magnetic tunnel junction
H
R
E
E
M
EFF
N(E)
N(E)
Applications: magnetic sensors, MRAM, NV-logic
H
Elshan Akhadov
Spintronics in Semiconductor: spin transistor
Datta and Das, APL, 1990
H
SOURCE
GATE
GaAs
H
DRAIN
Dreams
High performance
opto-electronics
Single-chip computer
(instant on; low power)
Quantum computation
Issues
Spin
Spin
Spin
Spin
polarized material
injection
coherence
detection
Elshan Akhadov
Spin Injection: the conductivity mismatch
Schmidt et.al., PRB, 2000
I
I
SC
mF
mF
FM
mN
mN
I
RF
RN
RF
RN
Solutions:
Use injector with 100%
spin polarization
Non-diffusive injection
Conductivity matching
Elshan Akhadov
E
E
CrO2: a half metal
Tc = 400 K
Uex
E
m = 2mB/Cr
p = 100%
4s
Schwarz, J. Phys. F, 1986
normal metal
3d
half-metallic
ferromagnet
metallic ferromagnet
Measurement of spin polarization: using a superconductor
Elshan Akhadov
Andreev reflection: normal metal/superconductor
N
E
S
Question:
D
eV
EF
-D
N(E)
What could happen to an
electron with energy eV < D
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 B
2. B and C
3. C and A
4. A and B and C
N
S
Elshan Akhadov
Andreev reflection: normal metal/superconductor
Z  H / vF
G(V)/Gn
p=0
2.0
2.0
1.5
1.5
1.5
1.0
1.0
1.0
0.5
0.5
0.5
0.0
-2 D - D
0
V
D
2D
Z=0
clean metallic contact
0.0
2.0
-2 D - D
0
V
D 2D
Z~1
in-between
0.0
-2 D - D
0
V
D
2D
Z >> 1
tunnel junction
Blonder, Tinkham, and Klapwijk, PRB, 1982
Elshan Akhadov
Andreev reflection: ferromagnet/superconductor
F
E
S
Z=0
metallic contact
0.5
D
eV
p = 75%
1.0
EF
-D
DOS
0.0
1.5
-2 D - D
0
D
2D
1.0
Z~1
in-between
0.5
0.0
2.0
-2 D - D
0
D
2D
1.5
1.0
Z >> 1
tunnel junction
0.5
0.0
V
Elshan Akhadov
Comparison: normal metal and ferromagnet
p=0
p = 75%
2.0
1.0
1.5
1.0
Z=0
0.5
metallic contact
0.0
2.0
Z=0
metallic contact
0.5
-2 D -1 D 0
1D 2D
0.0
1.5
-2 D - D
0
D
2D
1.5
1.0
Z~1
in-between
0.5
0.5
0.0
2.0
Z >> 1
tunnel junction
Z~1
in-between
1.0
-2 D -1 D
0
1D 2D
0.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
V
-2 D - D
0
D
2D
Z >> 1
tunnel junction
V
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
Elshan Akhadov
CrO2 Film Growth: Chemical Vapor Deposition
Furnace, T=280° C
O2 flow
Heater block, T=400°C substrate
Cr8O21 precursor
Ivanov, Watts, and Lind, JAP, 2001
Elshan Akhadov
Junction Fabrication and Measurement
~
Pb or Al
 Grow CrO2 film
 Pattern CrO2 stripe
 Surface modification: Br etch
 Deposit S cross stripes
Pb or Al
I
CrO2
Lock-in V
dV/dI vs V in He4 (1K) or He3 (0.3K) cryostats
dV
V ( I )  V ( I0 ) 
I 0  I ac cost  ...
dI
CrO2
TiO2
Elshan Akhadov
Results: CrO2/(I)/Pb junctions
Metallic contact
Z = 0 p = 97%
1.0
0.6
0.4
T = 1.2 K
D = 1.44 meV
0.2
0.0
-4
-2
0
2
V (mV)
4
2.0
1.5
G (V) / GN
G (V) / GN
0.8
1.0
0.5
Tunnel junction
High quality barrier
w/o inelastic scattering
T = 400 mK
0.0
-10
-5
0
V (mV)
5
10
Elshan Akhadov
Measurement of spin polarization in high-Z junctions:
using Zeeman splitting
mH
E
D
eV
EF
-D
eV/D
N(E)
F
H
S
Meservey and Tedrow,
Phys. Rep., 1994
Elshan Akhadov
Zeeman splitting in an F/I/S junction
CrO2
H
In order to get high Hc:
Ultrathin S film
Parallel field
Al
Negligible s-o interaction
CrO2
Al
Elshan Akhadov
Results: Zeeman splitting
100
80
D (meV)
1.6
1.4
40
20
1.2
0
0.0
1.0
0.5
1.0
1.5
2.0
2.5
H (T)
0.8
0.6
0.4
0.2
0.0
1.4
H = 0.0 T
= 0.5 T
= 1.0 T
= 1.5 T
= 2.0 T
= 2.5 T
-0.5
T =400 mK
1.2
1.0
G (V) / GN
G (V) / GN
60
0.8
0.6
+2.5T
-2.5T
0.4
0.0
V (mV)
0.5
0.2
0.0
-0.6
-0.4
-0.2
0.0
0.2
V (mV)
0.4
0.6
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
Elshan Akhadov
CrO2/I/Co magnetic “tunnel” junction
16.5
Resistance ()
H
Co
CrO2
AlOx
16.0
15.5
15.0
14.5
-1000
-500
0
Field (Oe)
500
1000
Elshan Akhadov
The People
Jeff Parker
Jazcek Braden
Steve Watts
Pavel Ivanov
Stephan von Molnár
Pedro Schlottmann
David Lind
Elshan Akhadov
Let’s build
“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