Fundamental concepts of spintronics

Jaroslav

Fabian

Institute for Theoretical PhysicsUniversity of Regensburg

Stara

Lesna, 24. 8. 2008 SFB 689

:outline:

•

what is spintronics? •

spin injection

•

spin-orbit coupling in solids (next lecture)•

spin devices

•

conclusions: challenges

I. Zutic, J. Fabian, and S. Das Sarma, Spintronics: Fundamentals and applications, Rev. Mod. Phys. 76, 323 (2004)

J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic,Semiconductor spintronics, Acta

Phys. Slov, 57, 566 (2007)

what

is

spintronics?

narrow (device): electronics

with spin

broad: umbrella for electron spin

phenomena in solids

spintronics drive

technology fundamental discoveries

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2007 jointly to

Albert Fert Unité

Mixte

de Physique CNRS/THALES, Université

Paris-Sud, Orsay, France

Peter Grünberg Forschungszentrum

Jülich, Germany,

"for the discovery of Giant Magnetoresistance".

The Nobel Prize in Physics 2007

Giant MagnetoResistance P. Grunberg et al. (1988), A. Fert et al. (1988)

multilayers

30 -

40% at RT

small resistance large resistance

GMR hard

disk

read

heads

From: IBM web site

SPINTRONICS GOALSspin

control

of electrical

properties

(I-V characteristics)

electrical

control

of spin(magnetization)

SPINTRONICS’

3 REQUIREMENTS

•

EFFICIENT SPIN INJECTION

•

SLOW SPIN RELAXATION @ SPIN CONTROL

•

RELIABLE SPIN DETECTION

Silsbee-Johnson spin-charge coupling

F N

:(electrical) spin injection:

Silsbee: emf

appears in the proximity of a ferromagnetic metal and spin-

polarized nonmagnetic metal (inverse of spin injection)

R. Silsbee, Bull. Mag. Reson. 2, 284 (1980)M. Johnson and R. H. Silsbee, Phys. Rev. Lett. 55, 1790 (1985).

spin injection spin detection

Johnson-Silsbee spin injection experiment

δMμ0μ0

E E

N (E) N (E) N (E) N (E)

visualizing spin injection

S. A. Crooker et al., JAP, 101,081716 (2007)

S. A. Crooker at al., Science 309, 2191 (2005)

spin injection into silicon

I. Appelbaum

et al, Nature 447, 295 (2007)I. Zutic

and J. Fabian, Nature (NW) 447, 269 (2007)

spin injection into graphene

N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van WeesElectronic spin transport and spin precession in single graphene layers at room temperature, Nature 448, 571 (2007)

N. Tombros, S. Tanabe, A. Veligura, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van WeesAnisotropic spin relaxation in graphene, arXiv:0802.2892

single-layer on a SiO2

substrate, room temperature

Zincblende

band structure (GaAs) optical

orientation

transitions

σ+σ+

mj

Eg

Δ

CB

SO

E

LH

HH

0 k

(a)

3/2P

1/2P

1/2S (b)

HH,LH

σ− −σ

1/2−1/2

−1/2 1/2

−3/2 3/2

−1/2 1/2

SO

CB

3 1 1 3

22Γ7

Γ8

6Γ

so

From: I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004)

:spin relaxation:

t=0, spin imbalance t=T1

, spin balance

B Fe

impurity

phonon spin-orbit coupling

:key concepts: spin relaxation and dephasing

:key concepts: spin relaxation and dephasing

Bloch eqs

Time-resolved Faraday rotation

Source: web site of Awschalom’s

group

ZnCdSe

QW

mechanisms of spin relaxation

J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic,Semiconductor spintronics, Acta

Physica

Slovaca, 57, 565 (2007)

Elliott-Yafet mechanismelemental metals and semiconductors

Dyakonov-Perel mechanismSemiconductors without center of inversion symmetry

Bir-Aronov-Pikus mechanismHeavily p-doped semiconductors

Hyperfine interactionElectrons bound on impurity sites or confinedIn quantum dots

spin

relaxation

in bulk

n-GaAs

τττττ

τ

relaxationtim

e(ns)

R. I. Dzhioev

et al., Phys. Rev. B 66, 245204 (2002)

spin

relaxation

in bulk

n-Si

0 50 100 150 200 250 300Temperature [K]

0

20

40

60

80

100sp

in r

elax

atio

n tim

e T

1 [ns

] 7.4 1014

3.7 1015

4.5 1015

7.8 1015

2.7 1016

8.0 1016

D. Lepine, Phys. Rev. B 6, 436 (1972)

J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic, Acta

Physica

Slovaca, 57, 565 (2007)

:spin devices: (spin detection)

:semiconductor

spintronics devices:

•

spin resonant diodes•

spin

field-effect

transistors•

magnetic

semiconductor

tunnel

junction

devices•

magnetic

bipolar junction

diodes and transistors•

spin

optoelectronic devices•

spin

galvanics devices•

spin Hall polarizeds•

spin-polarized

semiconductor

lasers•

spin pumping batteries•

spin-torque devices•

spin

quantum

computers•

...

J. Fabian, A. Matos-Abiague, C. Ertler, and P. Stano,Semiconductor spintronics, Acta

Phys. Slov, 57, 566 (2007)

International Technology Roadmap for Semiconductors:

Emerging Research Logic Devices

risk

RSFQ1-D

structuresresonanttunneling SET molecular QCA

spin transistor

2004

2005, 2006

International Technology Roadmap for Semiconductors:

Emerging Research Logic Devices

risk

RSFQ1-D

structuresresonanttunneling SET molecular QCA

spin transistor

2004

2007

detour: material case study: GaMnAs

• 5-15 % Mn• p-doped (Mn

replaces Ga) • degenerate: p = 1020

- 1021/cm3

• Tc

= 170 K• ferromagnetism and carrier density coupled• kλ

about 3 (localization?)• impurity or valence band?• quantum coherence effects observed

GaMnAs, from Jungwirth et al, Rev. Mod. Phys. 78, 809 (2006)

Where does GaMnAs

fit? No good answer yet

b)

BeZnSe

ZnMnSeZnSe

ZnSe

BeZnSeBeZnSe

ZnMnSeZnSe

ZnSe

BeZnSe

magnetic

Resonant

Tunnel

Diodes A. Slobodskyy

et al, Phys. Rev. Lett. 90, 246601 (2003)

C. Ertler

and J. Fabian, Appl. Phys. Lett. 89, 193507 (2006)

C. Ertler

and J. Fabian, Phys. Rev. B 75 195323 (2007)

• efficient spin filtering• spin detection• fast switching times• coherence issues• RT operation?

6T

3T

0T

8% Mn

T=1.3K

a)

ZnSeZnMnSe

ZnSe

B

1.3

K

Voltage (0-0.2 V)

Cur

rent

(0-1

50 μ

A)

0 0.05 0.1 0.15 0.2 0.250

0.5

1

1.5

2

2.5

3x 10

5

Voltage (V)

Cur

rent

Den

sity

(A

/cm

2 )

0 10 20 300

50

100

z (nm)

Ene

rgy

(meV

)

Δ E = 0

Δ E = 5 meV

Δ E = 10 meV

Δ E = 15 meV

Δ E = 20 meV

Δ E = 25 meV

Δ E = 40 meV

T = 4.2 K

ΔV2out

ΔV3out

ΔV1out

:selfsustained magneto-electric oscillations in MRTDs:

C. Ertler

and J. Fabian, Phys. Rev. Lett. 101, 077202 (2008)

0 10 20 300

5

10

x 1015

Voltage (mV)j (

a.u.

)

jmax

jmin

I

II

(a)

0 10 20 300

5

10

15

20

Voltage (mV)

Δ (m

eV)

Δmax

Δmin

(b)

II

I

50 100 150 2000

5

10

x 1015

Time (t*)

j (a.

u.)

jtot

j↑j↓

(c)

50 100 150 2002

4

6

8

10

12

14x 1011

Time (t*)

n (1

/cm

2 )

ntot

n↑

n↓

(d)

Intrinsic bistability

leads to temporal oscillations in the current, magnetizaion, and particle density

:nanospintronics:spin-based quantum information processing

D. Loss and D. P. DiVincenzo, PRA 57, 120 (1998)

• single and few spins manipulation and detection• spin relaxation and decoherence• entanglement control (EDAP: Fabian and Hohenester, PRB 72, 201304 (R) 2005)

closing: challenges in spintronics

•

room-temperature ferromagnetic semiconductors, n and p type, identification of mechanisms for ferromagnetic long-range order

•

magnetic heterostructures: ferromagnetic quantum wells and quantum dots

•

spin-polarized transport through magnetic interfaces and inhomogeneities, accurate determination of spin polarization of ferromagnets

•

development of silicon (Si, Si:Ge) spintronics: spin injection, spin relaxation, magnetism (?), quantum dots

•

demonstration of semiconductor spin transistors--power gain and magnetologic:spin FETs, bipolar spin transistors

•

niche devices for GaMnAs

or other dilute magnetic semiconductors, specific functionalities

closing: challenges in spintronics

•

control of ferromagnetism by gating or current injection, spin-transfer torque

•

spin dynamics and spin pumping phenomena in spin transport

•

control of spin-orbit coupling by gate and doping, interface properties

•

single channel devices

•

Spin transport in carbon nanotubes, graphene

•

spin quantum information processing:single and few spin manipulation, relaxation and decoherence, spin entanglement control