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