Spintronics in metals and semiconductors Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth,

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, Sam Owen, et al.

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Shick, et al,unpublished

Shick, et al,unpublished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

As-p-like holes

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 Note: TAMR discovered in (Ga,Mn)As Gold et al. PRL’04

Mn

Ga

AsMn

(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-T another key factorRushforth et al, ‘08

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 185Kso far

“Combinatorial” approach to growthwith fixed growth and annealing T’s

?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

vF cdkq ~)/1~~(

Tc

d/dT

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

2)(~~0 Suncor

~)0~~( Fkq

Scattering off short rangecorrelated spin-fluctuation

Fisher&Langer ‘68

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 and Tc with depletion

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

END

Dawn of spintronicsDawn of spintronics

Anisotropic magnetoresistance (AMR) – 1850’s Anisotropic magnetoresistance (AMR) – 1850’s 1990’s 1990’s

Giant magnetoresistance (GMR) – 1988 Giant magnetoresistance (GMR) – 1988 1997 1997

Inductive read/write element

Magnetoresistive read element

MRAM – universal memoryMRAM – universal memory fast, small, low-power, durable, and non-volatile

2006- First commercial 4Mb MRAM

RAM chip that actually won't forget instant on-and-off computers

Based on Tunneling Magneto-Resistance (similar to GMR but insulating spacer)

Giant Magneto-ResistanceGiant Magneto-Resistance

~ 10% MR effect~ 10% MR effect

DOS

AP

P

>

Tunneling Magneto-ResistanceTunneling Magneto-Resistance

~ 100% MR effect~ 100% MR effect

DOS DOS

Spin Transfer Torque writingSpin Transfer Torque writing

GaAs Mn

Mn

10-100x smaller Ms

One

Key problems with increasing MRAM capacity (bit density):

- Unintentional dipolar cross-links- External field addressing neighboring bits

10-100x weaker dipolar fields

10-100x smaller currents for switching

Dilute moment nature of ferromagnetic semiconductorsDilute moment nature of ferromagnetic semiconductors

Magnetism in systems with coupled dilute moments and delocalized band electrons

(Ga,Mn)As

cou

pli

ng

str

eng

th /

Fer

mi

ener

gy

band-electron density / local-moment density

Hole transport and ferromagnetism at relatively large dopings

conducting p-type GaAs:- shallow acc. (C, Be) ~ 1018 cm-3

- Mn ~1020 cm-3

Non-equilibrium growth - technological difficulties

Electric-field controlledferromagnetism in FET or piezo/FM hybrid

Vgate

Ferro SC

Photogenerated ferromagnetism

Ferro SC

GaSb

B (mT)

ħ

Mag

net

izat

ion

Mag

net

izat

ion

Variable controlled strain using a Piezo stressor A.W. Rushforth, J. Zemen, K. Vyborny, et al. arXiv:0801.0886

Strain induced by piezo voltage +/- 150V:

~ 2 10-4 (at 50K)

Easy axis rotation by

50 deg for

Vpiezo = -150V +150V

M. Overby, et al., arXiv:0801.4191

Beff

VG = V0, t < 0VG

timeV0

VC

Δt180°

Beff

VG = VC, t = 0

zM=(0,0,M)

Beff

VG = V0, t > Δt180°

M=(0,0,-M)z(I) (II) (III)

M

Beff

VG = V0, t < 0

(I)

VG

timeV0

VC

Δt90°

Beff

VG = VC, t = 0

(II)

0 x

M=(M,0,0)

Beff

VG = V0, t > Δt90°

(III)

0 x

M=(0,M,0)

0

(a)

(b)

Beff

VG = V0, t < 0VG

timeV0

VC

Δt180°

Beff

VG = VC, t = 0

zM=(0,0,M)

Beff

VG = V0, t > Δt180°

M=(0,0,-M)z(I) (II) (III)

M

Beff

VG = V0, t < 0

(I)

VG

timeV0

VC

Δt90°

Beff

VG = VC, t = 0

(II)

0 x

M=(M,0,0)

Beff

VG = V0, t > Δt90°

(III)

0 x

M=(0,M,0)

0

(a)

(b)

Beff

VG = V0, t < 0VG

timeV0

VC

Δt180°

Beff

VG = VC, t = 0

zM=(0,0,M)

Beff

VG = V0, t > Δt180°

M=(0,0,-M)z(I) (II) (III)

M

Beff

VG = V0, t < 0

(I)

VG

timeV0

VC

Δt90°

Beff

VG = VC, t = 0

(II)

0 x

M=(M,0,0)

Beff

VG = V0, t > Δt90°

(III)

0 x

M=(0,M,0)

0

(a)

(b)

Fast Precessional switching via gatevoltage

Spintronics with spin-currents onlySpintronics with spin-currents only

Magnetic domain “race-track” memory

n

n

p

SHE mikročip, 100A supercondicting magnet, 100 A

Spin Hall effect detected optically in GaAs-based structures

Same magnetization achievedby external field generated bya superconducting magnet with 106 x larger dimensions & 106 x larger currents

Cu

SHE detected elecrically in metals SHE edge spin accumulation can beextracted and moved further into the circuit

Datta-Das transistor

Spintronics in nominally non-magnetic materialsSpintronics in nominally non-magnetic materials

intrinsic skew scattering

I

_ FSO

FSO

_ __

Spin Hall effectspin-dependent deflection transverse edge spin polarization

• Information reading

Ferro

Magnetization

Current

• Information reading & storage

Tunneling magneto-resistance sensor and memory bit

• Information reading & storage & writing

Current induced magnetization switching

• Information reading & storage & writing & processing

Spintronic transistor::magnetoresistance controlled by gate voltage

• New materialsFerromagnetic semiconductors, MultiferroicsNon-magnetic SO-coupled systems

Mn

GaAs Mn

Spintronics explores new avenues for: