Upload
noel-henry
View
215
Download
2
Embed Size (px)
Citation preview
Elshan Akhadov
Spin Electronics
QuarkNet, June 28, 2002
Peng Xiong
Department of Physics and MARTECHFlorida State University
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
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
Elshan Akhadov
Superparamagnetic Limit: thermal stability of magnetic media
Elshan Akhadov
Semiconductor Random Access Memory: alternatives?
High speed
Low density High power consumption Volatile
MO
S
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
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
Elshan Akhadov
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
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
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
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
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
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
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
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
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
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
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
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
Elshan Akhadov
-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
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
H
CrO2Co
AlOx
-1000 -500 0 500 100014.5
15.0
15.5
16.0
16.5
Res
ista
nce
()
Field (Oe)
Elshan Akhadov
Jeff ParkerJazcek BradenSteve WattsPavel Ivanov
Stephan von MolnárPedro SchlottmannDavid Lind
The People
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