1. Spin Electronics Peng Xiong Department of Physics and MARTECH
Florida State University
QuarkNet, June 28, 2002

2. Moore’s Law… is the end in sight?
Speed: 100 Hz
Size: 10-2 m
Cost: $106/transistor
Speed: 109 Hz
Size: 10-7 m
Cost: $10-5/transistor
SOURCE
GATE
DRAIN
MOSFET

3. 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

4. Superparamagnetic Limit:
thermal stability of magnetic media

5. Semiconductor Random Access Memory: alternatives?
High speed
Low density
High power consumption
Volatile

6. Metal-based Spintronics: Spin valve and magnetic tunnel junctionEF E H EF
N(E) E H R H M
Applications: magnetic sensors, MRAM, NV-logic

7. Spintronics in Semiconductor: spin transistor
Dreams
High performance
opto-electronics
Single-chip computer
(instant on; low power)
Quantum computation
Datta and Das, APL, 1990
GATE H
SOURCE
DRAIN
GaAs
Issues
Spin polarized material
Spin injection
Spin coherence
Spin detection H

8. Spin Injection: the conductivity mismatch
Schmidt et.al., PRB, 2000 I I­
RF­
RN­ I¯ SC
mF­
RF¯
RN¯
mF¯
mN­
Solutions:
Use injector with 100%
spin polarization
Non-diffusive injection
Conductivity matching FM
mN¯

9. Measurement of spin polarization: using a superconductor
half-metallic
ferromagnet E
Uex
normal metal E
CrO2: a half metal
Tc = 400 K
m = 2mB/Cr
p = 100%
Schwarz, J. Phys. F, 1986
metallic ferromagnet E 4s 3d
Measurement of spin polarization: using a superconductor

10. Andreev reflection: normal metal/superconductor
Question:
What could happen to an electron with energy eV < D when it hits S from N?
bounce back;
go into S as an electron;
C. go into S in a Cooper pair.
A and B
B and C
C and A
A and B and C D eV EF -D
N(E) N S

11. clean metallic contact
Andreev reflection: normal metal/superconductor
p = 0
Z = 0
clean metallic contact
Z ~ 1
in-between
Z >> 1
tunnel junction
Blonder, Tinkham, and Klapwijk, PRB, 1982

12. Andreev reflection: ferromagnet/superconductor
Z = 0
metallic contact D eV EF -D
Z ~ 1
in-between
DOS
Z >> 1
tunnel junction V

13. Comparison: normal metal and ferromagnet
Z = 0
metallic contact
Z = 0
metallic contact
Z ~ 1
in-between
Z ~ 1
in-between
Z >> 1
tunnel junction
Z >> 1
tunnel junction V V

14. 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

15. 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

16. I ~ CrO2 TiO2 Junction Fabrication and Measurement Pb or Al Pb or Al
Grow CrO2 film
Pattern CrO2 stripe
Surface modification: Br etch
Deposit S cross stripes V ~
Lock-in
Pb or Al
Pb or Al I
CrO2
CrO2
TiO2
dV/dI vs V in He4 (1K) or He3 (0.3K) cryostats

17. w/o inelastic scattering
Results: CrO2/(I)/Pb junctions
Metallic contact
Z = 0 p = 97%
T = 1.2 K
= 1.44 meV
Tunnel junction
T = 400 mK
High quality barrier
w/o inelastic scattering

18. Measurement of spin polarization in high-Z junctions:
using Zeeman splitting E mH D eV EF -D
eV/D
N(E) H
Meservey and Tedrow,
Phys. Rep., 1994 F S

19. Zeeman splitting in an F/I/S junction
CrO2 H
In order to get high Hc:
Ultrathin S film
Parallel field
Negligible s-o interaction Al
CrO2 Al

20. Results: Zeeman splitting
T =400 mK

21. 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

22. CrO2/I/Co magnetic “tunnel” junctionH
CrO2 Co
AlOx

23. The People
Jeff Parker
Jazcek Braden
Steve Watts
Pavel Ivanov
Stephan von Molnár
Pedro Schlottmann
David Lind

24. 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