Spintronics in metals and semiconductors

Tomas Jungwirth

University of Nottingham Bryan Gallagher, Tom Foxon,

Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al.

Hitachi Cambridge Jorg Wunderlich, Andrew Irvine, David Williams,

Elisa de Ranieri, Byonguk Park, Sam Owen, et al.

Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák, Kamil Olejník, et al.

University of Texas Allan MaDonald, et al.

Texas A&MJairo Sinova, et al.

OutlineOutline

11.. Tunneling anisotropic magnetoresistance in transition metals Tunneling anisotropic magnetoresistance in transition metals

2. Ferromagnetism in (Ga,Mn)As and related semiconductors2. Ferromagnetism in (Ga,Mn)As and related semiconductors

3. Spintronic transistors3. Spintronic transistors

Spintronics: Spin-orbit & exchange interactions

nucleus rest frame electron rest frame

vI Q rE3

04 r

Q

3

0

4 r

rIB

EvEvB 200

1

c

EvSS 22

B2 mc

egH B

SO

Thomas precession

Coulomb repulsion & Pauli exclusion principle exchange interaction

ferromagnetism

spin-orbit interaction

DOS

AMRAMR~ 1% MR effect~ 1% MR effect

TMRTMR~ 100% MR effect~ 100% MR effect

TAMRTAMR

) vs.( ~ IMvg

)(BM

M

Exchange int.:

Spin-orbit int.:

magnetic anisotropy

Exchange int.:

)()( TDOSTDOSAFM-FM exchange bias

)(MTDOS

Au

ab intio theory Shick, et al, PRB '06, Park, et al, PRL '08

experiment Park, et al, PRL '08

TAMR in CoPt structures

spontaneous momentmag

netic su

sceptib

ility

Consider uncommon TM combinationsMn/W ~100% TAMR

Consider both Mn-TM FMs & AFMs

exchange-spring rotation of the AFMScholl et al. PRL ‘04

Proposal for AFM-TAMR: first microelectronic device with active AFM component

spin

-orb

it cou

plin

g

TAMR in TM structures

Shick, et al,unpublished

Shick, et al,unpublished

OutlineOutline

11.. Tunneling anisotropic magnetoresistance in transition metals Tunneling anisotropic magnetoresistance in transition metals

2. Ferromagnetism in (Ga,Mn)As and related semiconductors2. Ferromagnetism in (Ga,Mn)As and related semiconductors

3. Spintronic transistors3. Spintronic transistors

Magnetic materials

Ferroelectrics/piezoelectrics Semiconductors

spintronic magneto-sensors, memories

electro-mechanical transducors, large & persistent el. fields

transistors, logic,sensitive to doping and electrical gating

TM-based semiconducting multiferroic spintronicssensors & memories transistors & logic

Ferromagnetic semiconductors

GaAs - GaAs - standard III-V semiconductorstandard III-V semiconductor

Group-II Group-II Mn - Mn - dilute dilute magneticmagnetic moments moments & holes& holes

(Ga,Mn)As - fe(Ga,Mn)As - ferrromagneticromagnetic semiconductorsemiconductor

Need true FSs not FM inclusions in SCs

Mn

Ga

AsMn

Mn-d-like localmoments

As-p-like holes

Mn

Ga

AsMn

EF

DO

S

Energy

spin

spin

GaAs:Mn – extrinsic p-type semiconductor

FM due to p-d hybridization

(Zener local-itinerant kinetic-exchange)

valence band As-p-like holes

As-p-like holes localized on Mn acceptors

<< 1% Mn ~1% Mn >2% Mn

onset of ferromagnetism near MIT

(Ga,Mn)As synthesis

Low-T MBE to avoid precipitation

High enough T to maintain 2D growth

need to optimize T & stoichiometry for each Mn-doping

Inevitable formation of interstitial Mn-donorscompensating holes and moments need to anneal out

high-T growth

optimal-T growth

Interstitial Mn out-diffusion limited by surface-oxide

GaMnAs

GaMnAs-oxide

Polyscrystalline20% shorter bonds

MnI++

O

Optimizing annealing time & temperature (removing int. Mn & keeping MnGa in place) is essential Rushforth et al, unpublished

x-ray photoemission

Olejnik et al, ‘08

10x shorther annealing with etch

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

140

160

180

TC(K

)

Mntotal

(%)

Indiana & California (‘03): “ .. Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..” Yu et al. ‘03

California (‘08): “…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...”

Nottingham & Prague (’08): Tc up to 188 Kso far

“Combinatorial” approach to growthwith fixed growth and annealing cond.

?Mack et al. ‘08

Tc limit in (Ga,Mn)As remains open

Weak hybrid.Delocalized holeslong-range coupl.

Strong hybrid.Impurity-band holesshort-range coupl.

InSb

GaP

d5

(Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host

Other (III,Mn)V’s DMSs

Mean-field butlow Tc

MF

Large TcMF but

low stiffness

Kudrnovsky et al. PRB 07

III = I + II Ga = Li + Zn

Other DMS candidates

Masek et al. PRL 07But Mn isovalent in Li(Zn,Mn)As

no Mn concentration limit and self-compensation

possibly both p-type and n-type ferromagnetic SC

(Li / Zn stoichiometry)

GaAs and LiZnAs are twin SC

(Ga,Mn)As and Li(Zn,Mn)As

should be twin ferromagnetic SC

Towards spintronics in (Ga,Mn)As: FM & transport

Dense-moment MSF<< d-

Eu - chalcogenides

Dilute-moment MSF~ d-

Critical contribution to resistivity at Tc

~ magnetic susceptibility

Broad peak near Tc disappeares with annealing (higher uniformity)???

Ni

(Ga,Mn)As (Prague Nottingham)

Fe

Critical contribution at Tc to d/dT like TM FMs

d/dT ~ cv

F ~ d-

Fisher & Langer ’68Novak et al., ‘08

)/1~~( dkk F

2)(~ Suncor

~)0~~( Fkk

smalluncor

vcdTddTd ~/~/

Tc

Tc

EuCdSe Ni

MF

][~),(~)( 002 SSSSJTRT iipdi

0k1kd

1~kd

As-p-like holes

Ferromagnetism & strong spin-orbit coupling

LSdr

rdV

err

mc

p

mc

SeBH effSO

)(1

Strong SO due to the As p-shell (L=1) character of the top of the valence band

V

BBeffeff

pss

Beff Bex + Beff TAMR discovered in (Ga,Mn)As Gold et al. PRL’04

Mn

Ga

AsMn

SO couped carries scattering coherently off Coulomb & polarized-magnetic potential of Mn

>

magnetic. only

max AMR

>

MnGa

~

AMR in DMSs

sign and magnitude (numerical) consistent with experiment

Remark: Extraordinary MRs & quantum coherent transport phenomena

dirty metal UCF

OutlineOutline

11.. Tunneling anisotropic magnetoresistance in transition metals Tunneling anisotropic magnetoresistance in transition metals

2. Ferromagnetism in (Ga,Mn)As and related semiconductors2. Ferromagnetism in (Ga,Mn)As and related semiconductors

3. Spintronic transistors3. Spintronic transistors

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

2

4

6

8

10

0V 3V 5V 10V

carr

ier

dens

ity

[ 10

19 c

m-3

]

GaMnAs layer thickness [nm]

Gating of the highly doped (Ga,Mn)As: p-n junction FET

p-n junction depletion estimates

Olejnik et al., ‘08

~25% depletion feasible at low voltages

20 22 24 26 28 30 32 34

18.6

18.8

19.0

19.2

19.4

[10

-3c

m]

T [K]

Vg = 0V

22.5

23.0

23.5

24.0

24.5 Vg = 3V

20 22 24 26 28 30 32 34

-200

-100

0

100

d/d

T [1

0-6

T [K]

-300

-200

-100

0

AM

RIncreasing and decreasing AMR, Tc, coercivity with depletion

-400 -200 0 200 400

0.96

0.98

1.00

1.02 Vg [V] -1.0 0.0 1.0 2.0 2.5 3.0 3.5

R(

B)/R

(B=0

)

B [Oe]-1 0 1 2 3 4

150

200

250

300

coer

cive f

ield [

Oe]

Vg[V]

30 40 50 60 70 80 90 100

100

200

65K62K

dR/d

T

T (K)

depletion accumulation

Persistent variations of magnetic properties with ferroelectric gates

Stolichnov et al., Nat. Mat.‘08

exy = 0.1%

exy = 0%

Electro-mechanical gating with piezo-stressors

Rushforth et al., ‘08

Strain & SO

Electrically controlled magnetic anisotropies

Single-electron transistor

Two "gates": electric and magnetic

(Ga,Mn)As spintronic single-electron transistor

Huge, gatable, and hysteretic MR

Wunderlich et al. PRL ‘06

AMR nature of the effect

normal AMR Coulomb blockade AMR

GMMGG0

20

C

C

e

)M(V&)]M(VV[CQ&

C2

)QQ(U

electric && magneticmagnetic

control of Coulomb blockade oscillations

n-1 n n+1 n+2n-1 n n+1 n+2

EC

QQindind = = nnee

QQindind = (= (n+1/2)n+1/2)eeQ0

Q0

e2/2C

Q

0

'D

'

e

)M(Q)Q(VdQU

[010]

M[110]

[100]

[110][010]

SO-coupling (M)

Source Drain

GateVG

VDQ

Single-electron charging energy controlled by Vg and M

• CBAMR if change of |CBAMR if change of |((MM)| ~ )| ~ ee22//22CC

• In our (Ga,Mn)As ~ meV (~ 10 Kelvin)In our (Ga,Mn)As ~ meV (~ 10 Kelvin)

• In room-T ferromagnet change of |In room-T ferromagnet change of |((MM)|~100K )|~100K

• Room-T conventional SET (e2/2C >300K) possible

Theory confirms chemical potential anisotropies in (Ga,Mn)As& predicts CBAMR in SO-coupled room-Tc metal FMs

Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device

0

ONONOFFOFF

1

0

ONON OFFOFF

1

VDD

VA VB

VA

VB

Vout

0

0

0

OFFOFFONON

ONON

OFFOFF

0

0

1

1

ONONOFFOFF

A B Vout0 0 01 0 10 1 11 1 1

0

01

ONON

OFFOFF

0

0

OFFOFF

1

ONON

1

1

1

1

OFFOFF

ONON

1

1

ONON

OFFOFF

1

“OR”

Nonvolatile programmable logic

VDD

VA VB

VA

VB

Vout

Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device

0

ONONOFFOFF

1

0

ONON OFFOFF

1

A B Vout0 0 01 0 10 1 11 1 1

“OR”

Nonvolatile programmable logicNonvolatile programmable logic

Physics of SO & exchange

SET

Resistor

Tunneling device

Chemical potential CBAMR

Tunneling DOS TAMR

Group velocity & lifetime AMR

Device design Materials

TM FMs

(III,Mn)V, I(II,Mn)VDMSs

Mn-based TM FMs&AFMs

TM FMs,MnAs, MnSb