USING SPIN IN (FUTURE) ELECTRONIC DEVICESUSING SPIN IN (FUTURE) ELECTRONIC DEVICES

Tomas Jungwirth

IP ASCR, PragueJan Mašek,Alexander ShickJan Kučera, František Máca

University of Texas Allan MaDonald, Qian Niu, et

al.

University of WuerzburgLaurens Molenkamp, Charles Gould et al.

University of NottinghamBryan Gallagher, Kevin EdmondsTom Foxon, Richard Campion, et al.

Hitachi CambridgeJorg Wunderlich, Bernd Kaestner et al.

Texas A&MJairo Sinova, et al.

OUTLINEOUTLINE

- Current and future (???) spintronic devices- Current and future (???) spintronic devices

- Challenges for spintronics - Challenges for spintronics research topics research topics

- Electrical manipulation of spin in normal semiconductors- Electrical manipulation of spin in normal semiconductors (Spin Hall effect)(Spin Hall effect)

- Ferromagnetic semiconductors - materials and devices- Ferromagnetic semiconductors - materials and devices

Electron has a charge (electronics) and

spin (spintronics)

Electrons do not actually “spin”,they produce a magnetic moment that is equivalent to an electron spinning clockwise or anti-clockwise

CURRENT SPINTRONIC DEVICESCURRENT SPINTRONIC DEVICES

HARD DISKSHARD DISKS

HARD DISK DRIVE READ HEADSHARD DISK DRIVE READ HEADS

horse-shoe read/write heads

spintronic read heads

Anisotropic magnetoresistance (AMR) read headAnisotropic magnetoresistance (AMR) read head

1992 - dawn of spintronics1992 - dawn of spintronics

Ferromagnetism large response (many spins) to small magnetic fields

Spin-orbit coupling spin response detected electrically

Giant magnetoresistance (GMR) read headGiant magnetoresistance (GMR) read head

19971997

GMR

MEMORY CHIPSMEMORY CHIPS

.DRAMDRAM (capacitor) - high density, cheephigh density, cheep x slow,

high power, volatile

.SRAMSRAM (transistors) - low power, fastlow power, fast x low density,

expensive, volatile

.Flash (floating gate) - non-volatilenon-volatile x slow, limited life,

expensive

Operation through electron chargecharge manipulation

MRAM – universal memoryMRAM – universal memory (fast, small, non-volatile)

RAM chip that won't forget

↓

instant on-and-off computers

Tunneling magneto-resistance effect

MRAM – universal memoryMRAM – universal memory (fast, small, non-volatile)

RAM chip that won't forget

↓

instant on-and-off computers

Tunneling magneto-resistance effect

FUTURE (? or ???) SPINTRONIC DEVICESFUTURE (? or ???) SPINTRONIC DEVICES

Low-dissipation microelectronics

Where Does All the Power Go?United States Energy Consumption: An

Overview                         

April 24 — We have electronic gizmos for just about every part of our daily lives, from brushing our teeth to staying in touch no matter where we are. Our swollen houses are stuffed with TVs, computers, and ever-larger and more complicated appliances.

                                               

        The power we use at home and outside of work accounts for only about a fifth of the total energy consumed in the United States every year, according to the Department of Energy. (ABCNEWS.com)

Long spin-coherence times → information carried by spin-currents Instead of electrical currents. Functionality based on spin-dynamics,e.g., domain wall motion

PROCESSORS PROCESSORS

Allwood et al., Science ’02

NOT gate

QUANTUM COMPUTERSQUANTUM COMPUTERS

1 0

a + b

Classical bit

Q-bit massive quantummassive quantumparallelismparallelism

CHALLENGES FOR SPINTRONICSCHALLENGES FOR SPINTRONICS

FM

AFM

EXANGE-BIASEXANGE-BIAS

fails when scaled down to ~10 nm dimensions Look for other MR concepts

EXTERNAL MAGNETIC FIELDEXTERNAL MAGNETIC FIELD

problems with integration - extra wires, addressing neighboring bits

Current (insted of magnetic field) induced switching

Angular momentum conservation spin-torque

Buhrman & Ralph, NNUN ABSTRACTS '02Slonczewski, JMMM '96; Berger, PRB '96

current

magnetic field

local, reliable, but fairlylarge currents needed

Myers et al., Science '99; PRL '02

Likely the future of MRAMsLikely the future of MRAMs

INTEGRATION WITH SEMICONDUCTOR ELECTRONICSINTEGRATION WITH SEMICONDUCTOR ELECTRONICS

Spin-valve transistor

Metal ferromagnet to semiconductor spin-injector

All-semiconductor spintronicsAll-semiconductor spintronics

- electrical manipulation of spins (no external magnetic field)- electrical manipulation of spins (no external magnetic field) - making semiconductors ferromagnetic- making semiconductors ferromagnetic

ELECTRICAL MANIPULATION OF SPINS IN NORMAL ELECTRICAL MANIPULATION OF SPINS IN NORMAL SEMICONDUCTORS - SPIN HALL EFFECTSEMICONDUCTORS - SPIN HALL EFFECT

B

V

I

_

+ + + + + + + + + + + + +

_ _ _ _ _ _ _ _ _ _ FL

Lorentz force deflect chargedcharged--particles towards the edge

Ordinary Hall effectOrdinary Hall effect

Detected by measuring transverse voltage

Spin Hall effectSpin Hall effect

Spin-orbit coupling “force” deflects like-spinlike-spin particles

I

_ FSO

FSO

_ __

V=0

non-magnetic

Spin-current generation in non-magnetic systems Spin-current generation in non-magnetic systems without applying external magnetic fieldswithout applying external magnetic fields

Spin accumulation without charge accumulationexcludes simple electrical detection

Kato, Myars, Gossard, Awschalom, Science Wunderlich, Kaestner, Sinova, Jungwirth, PRL '04

Ingredients: - potential V(r)

- motion of an electron

Producesan electric field

In the rest frame of an electronthe electric field generates and effective magnetic field

- gives an effective interaction with the electron’s magnetic moment

E

E

Beff

k

Spin-orbit coupling Spin-orbit coupling (relativistic effect)

effSO BμH

)(1

rVe

E

Ecm

kBeff

(r)Vkcm

sH imp22

2

SO

Skew scattering off impurity potentialSkew scattering off impurity potential

skewscattering

lsdr

rdV

err

mc

k

mc

seBH effSO

)(1

l=0 for electrons weak SO

l=1 for holes strong SO

SO-coupling from host atoms SO-coupling from host atoms in a perfect crystal

E

E

v

Enhanced in asymmetric QW

Classical dynamics in k-dependent (Rashba) field:

z-component of spin due to precession in effective "Zeeman" fieldz-component of spin due to precession in effective "Zeeman" field

xx eEdt

dk),kz(

LLG equations for small drift adiabatic solution:

x

yy

)t()t(n

dt

d

dt

dnn y

x

yzx

x2

x

z eEn

p -AlG a As

i-G a As

n- -d o p e d AlG a As

e tc he d

QW

I

Top Emission

Side Emission

Electrode

Spin polarization detected through circular polarization of emitted lightSpin polarization detected through circular polarization of emitted light

Conventional vertical spin-LED

Novel co-planar spin-LED

Y. Ohno et al.: Nature 402, 790 (1999)

R. Fiederling et al.: Nature 402, 787 (1999)

B. T. Jonker et al.: PRB 62, 8180 (2000)

X. Jiang et al.: PRL 90, 256603 (2003)

R. Wang et al.: APL 86, 052901 (2005)

…

● No hetero-interface along the LED current

● Spin detection directly in the 2DHG

● Light emission near edge of the 2DHG

● 2DHG with strong and tunable SO

2DHG2DHG

2DEG2DEG

EXPERIMENT

Spin Hall Effect

2DHG

2DEG VT

VD

np

10 µ

m

Spin Hall Effect Device

1 .5 mc h a n n e l

n

n

py

xz

L E D 1

L E D 2

I P

xy

zIp

-Ip

ILED 1

Experiment “A”

xy

zIpILED 1

ILED 2

Experiment “B”

Experiment “B”

1.505 1.510 1.515 1.520

-1

0

1

xy

zIpILED 1

ILED 2

CP

[%]

1.505 1.510 1.515 1.520

-1

0

1

xy

zIpILED 1

ILED 2

xy

zIpILED 1

ILED 2

CP

[%]

Experiment “A”

-1

0

1

xy

zIp

-Ip

ILED 1

CP

[%]

-1

0

1

xy

zIp

-Ip

ILED 1

-1

0

1

xy

zIp

-Ip

ILED 1

xy

zIp

-Ip

ILED 1

CP

[%]

Opposite perpendicular polarization for opposite Opposite perpendicular polarization for opposite IIpp currents currents

or opposite edges or opposite edges SPIN HALL EFFECT SPIN HALL EFFECT

FERROMAGNETIC SEMICONDUCTORSFERROMAGNETIC SEMICONDUCTORS

(Ga,Mn)As diluted magnetic semiconductor(Ga,Mn)As diluted magnetic semiconductor

MnGa

As

Ga

Low-T MBE - random but uniform Mn distribution up to ~ 10% doping

5 d5 d-electrons with -electrons with L=0, S=5/2L=0, S=5/2

moderately shallow moderately shallow acceptor acceptor

Effective magneticEffective magnetic: Coulomb correlation of d-electrons & hopping AF kinetic-exchange coupling

Jpd

= + 0.6 meV nm3

Theoretical descriptionsTheoretical descriptions

MicroscopicMicroscopic: atomic orbitals & Coulomb correlation of d-electrons & hopping

Jpd

SMn

.shole

As

GaMn

Mn Mn

Intrinsic properties of (Ga,Mn)AsIntrinsic properties of (Ga,Mn)As: Tc linear in MnGa local momentconcentration; falls rapidly with decreasing hole density in more than50% compensated samples; nearly independent of hole density for compensation < 50%.

Jungwirth, Wang, et al.cond-mat/0505215

Extrinsic effects: Interstitial Mn - a magnetism killer

Yu et al., PRB ’02:

~10-20% of total Mn concentration is incorporated as interstitials

Increased TC on annealing corresponds to removal of these defects.

Mn

As

Interstitial Mn is detrimental to magnetic order:

compensating double-donor – reduces carrier density

couples antiferromagnetically to substitutional Mn even in

low compensation samples smaller effective number of Mn momentsBlinowski PRB ‘03, Mašek,

Máca PRB '03

Tc as grown and annealed samplesTc as grown and annealed samples

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

140

160

180

T

C(K

)

Mntotal

(%)

-1 0 1-0.1

0.0

0.1

T = 172 K8% (Ga,Mn)As

M

[110](T

) / M S

at(5

K)

Magnetic Field [ Oe ]

Tc=173K

8% Mn

Open symbols as grown. Closed symbols annealed

Jungwirth, Wang, et al.cond-mat/0505215

Number of holes per Mneff

Tc/xTc/xeffeff vs p/Mn vs p/Mneffeff

High (>40%) compensatio

n

Jungwirth, Wang, et al.cond-mat/0505215

Theoretical linear dependence of Mnsub on total Mn confirmed experimentally

Generation of MnGeneration of Mnintint during growth during growth

Mnsub

MnInt

Jungwirth, Wang, et al.cond-mat/0505215

Prospects of (Ga,Mn)As based materials with room TProspects of (Ga,Mn)As based materials with room Tcc

- Concentration of uncompensated MnGa moments has to reach ~10% only 6.2% in the current record Tc=173K sample

- Charge compensation not so important unless > 40%

- No indication from theory or experiment that the problem is other than technological - better control of growth-T, stoichiometry; new growth or chemical composition strategies to incorporate more MnGa local moments or enhance p-d coupling

Tunneling anisotropic magnetoresistance Tunneling anisotropic magnetoresistance

Single magnetic layerSingle magnetic layersensor or memory sensor or memory

Gould, Ruster, Jungwirth, et al., PRL '04

Giant magneto-resistance

[100]

[010]

[100]

[010]

[100]

[010]

(Ga,Mn)As(Ga,Mn)As

AuAu

no exchange-bias needed

M || <111> M || <100>

Magnetization orientation dependences

- Hole total energy over Fermi volume → magnetic anisotropy

- Group velocities at the Fermi surface and density of states for scattering → in plane magneto-resistance anisotropy

- Density of states at the Fermi energyDensity of states at the Fermi energy → → anisotropic tunnel magneto-resistanceanisotropic tunnel magneto-resistance

(Abolfath, Jungwirth et al., PRB '01

spin-split bands at M≠0

Dietl et al., Science '00

Spin-orbit coupling and anisotropiesSpin-orbit coupling and anisotropies

GaMnAs Nanocontact TAMRGaMnAs Nanocontact TAMR

30nm constriction

Current [110]

5nm thick 2% Mn GaMnAs Hall bars & nanoconstrictions

Giddings, Khalid, Jungwirth, Sinova et al. PRL '05

-15 -10 -5 0 5 10 15-2.0

-1.0

0.0

1.0

2.0

10.0 K 4.2 K 1.5 K

V [mV]I [

nA]

Tunnelling conduction at low temperatures & voltages

30nm

Constriction

Landauer-BLandauer-Büttiker üttiker tunnelling probabilitestunnelling probabilites

Magnetisation in plane

y

x

jt

z

Wavevector dependent tunnelling probabilityT (ky, kz) Red high T; blue low T.

xz

y

jt

strong z-confinement (ultra-thin film)

less strong y –confinement (constriction)

constriction:

Magnetization perpendicular to plane

Magnetization in plane

30nm constriction

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6100

200

300

400

500

600

700

B || x

B || y

B || z

R (

MO

hm

s)

B (T)

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.60

1

2

3

4

5

6

R (

GO

hm

s)

B (T)

1400%

Very large TAMR in single nanocontacts

-0.2 -0.1 0.0 0.1 0.2

0.17

0.18

0.19

0.20

0.21

R [M]

B [T]-0.2 -0.1 0.0 0.1 0.2

2

4

6

8

10

B [T]

3m bar 30nm constriction

B|| y

B || x

B || y

B || x

B || z B || z

AMRAMR in unstructured bar TAMRTAMR in constriction

MR response of constricted device and bar are very similar

in character but largely enhanced in the tunnel constriction

AMR & TAMRAMR & TAMR

Spintronic nano-transistor Spintronic nano-transistor field-controlled MR device field-controlled MR device

Spintronic diode GMR, TMR, TAMR device

Spintronic wire AMR device

Final remark: spintronics in footsteps of electronicsFinal remark: spintronics in footsteps of electronics