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Spin Electronics. Peng Xiong. Department of Physics and MARTECH Florida State University. QuarkNet, June 28, 2002. SOURCE. GATE. DRAIN. MOSFET. Moore’s Law… is the end in sight?. Speed: 10 0 Hz Size: 10 -2 m Cost: $10 6 /transistor. Speed: 10 9 Hz Size: 10 -7 m

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Spin Electronics

Peng Xiong

Department of Physics and MARTECH

Florida State University

QuarkNet, June 28, 2002


SOURCE

GATE

DRAIN

MOSFET

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


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


Superparamagnetic Limit:

thermal stability of magnetic media


Semiconductor Random Access Memory: alternatives?

M

O

S

  • High speed

  • Low density

  • High power consumption

  • Volatile


H

R

H

E

E

E

E

H

M

EF

EF

N(E)

N(E)

Metal-based Spintronics:

Spin valve and magnetic tunnel junction

Applications: magnetic sensors, MRAM, NV-logic


GATE

Spintronics in Semiconductor: spin transistor

  • Dreams

  • High performance

  • opto-electronics

  • Single-chip computer

  • (instant on; low power)

  • Quantum computation

Datta and Das, APL, 1990

H

SOURCE

DRAIN

GaAs

  • Issues

  • Spin polarized material

  • Spin injection

  • Spin coherence

  • Spin detection

H


  • Solutions:

  • Use injector with 100%

  • spin polarization

  • Non-diffusive injection

  • Conductivity matching

Spin Injection: the conductivity mismatch

I

Schmidt et.al., PRB, 2000

RN­

RF­

SC

mF­

RN¯

RF¯

mN­

mF¯

mN¯

FM


E

E

CrO2: a half metal

Tc = 400 K

m = 2mB/Cr

p = 100%

Uex

E

4s

Schwarz, J. Phys. F, 1986

normal metal

half-metallic

ferromagnet

3d

metallic ferromagnet

Measurement of spin polarization: using a 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

Andreev reflection: normal metal/superconductor

E

S

N

D

eV

EF

-D

N(E)

N

S


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


Andreev reflection: ferromagnet/superconductor

p = 75%

E

F

S

Z = 0

metallic contact

D

eV

EF

-D

Z ~ 1

in-between

DOS

Z >> 1

tunnel junction

V


Comparison: normal metal and ferromagnet

p = 75%

p = 0

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


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


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


~

V

Lock-in

dV/dI vs V in He4 (1K) or He3 (0.3K) cryostats

Junction Fabrication and Measurement

  • Grow CrO2 film

  • Pattern CrO2 stripe

  • Surface modification: Br etch

  • Deposit S cross stripes

Pb or Al

Pb or Al

I

CrO2

CrO2

TiO2


Results: CrO2/(I)/Pb junctions

Metallic contact

Z = 0p = 97%

T = 1.2 K

  • = 1.44 meV

Tunnel junction

T = 400 mK

High quality barrier

w/o inelastic scattering


mH

H

Measurement of spin polarization in high-Z junctions:

using Zeeman splitting

E

D

eV

EF

-D

eV/D

N(E)

Meservey and Tedrow,

Phys. Rep., 1994

S

F


Zeeman splitting in an F/I/S junction

CrO2

In order to get high Hc:

Ultrathin S film

Parallel field

Negligible s-o interaction

H

Al

Al

CrO2


Results: Zeeman splitting

+2.5T

-2.5T

T =400 mK


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


CrO2/I/Co magnetic “tunnel” junction

H

Co

CrO2

AlOx


The People

Jeff Parker

Jazcek Braden

Steve Watts

Pavel Ivanov

Stephan von Molnár

Pedro Schlottmann

David Lind


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


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