Ferromagnetic semiconductor materials and spintronic transistors

Tomas Jungwirth

University of Nottingham Bryan Gallagher, Tom Foxon,

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

Hitachi Cambridge, Univ. 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.

Electric field controlled spintronics

HDD, MRAMcontrolled by Magnetic field

Spintronic Transistorcontrol by

electric gates

Low-voltage controlled magnetization and magnetotransport

STT MRAMspin-polarized charge current

From storage to logic

Magnetic race track memory

OutlineOutline

1) 1) Sensitivity to electric fields via magnetic anisotropiesSensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit couplinggeneric to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device- Tunneling AMR device - Coulomb blockade AMR spintronic SET- Coulomb blockade AMR spintronic SET

2) 2) Direct charge depletion effects on electric&magnetic proprtiesDirect charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here ferromagnetic semiconductors are the favorable systems here

- GaMnAs and related dilute-moment ferromagnetic semiconductors- GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET - GaMnAs-based p-n junction spintronic FET

AMRAMR TMRTMR

TAMRTAMR

) vs.( ~ IMvg

M

FM exchange int.:

Spin-orbit int.:

FM exchange int.:

)()( TDOSTDOS

)(MTDOS

Au

Discovered in GaMnAs Gould et al. PRL’04

ab intio theoryTAMR is generic to SO-coupled systems including room-Tc FMs

experiment

Bias-dependent magnitude and sign of TAMR

Shick et al PRB ’06, Parkin et al PRL ‘07, Park et al PRL '08

Park et al PRL '08

GMMGG0

20

C

C

e

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

C2

)QQ(U

electric && magneticmagnetic

control of Coulomb blockade oscillations

Q

0

'D

'

e

)M(Q)Q(VdQU

Source Drain

GateVG

VDQ

Devices utilizing M-dependent electro-chemical potentials: FM SET

SO-coupling (M)

[010] M[110]

[100]

[110][010]

(Ga,Mn)As nano-constriction SET

CB oscillations shifted by changing M

(CBAMR)

Electric-gate controlled magnitude and sign of magnetoresistance spintronic transistor

or

Magnetization controlled transistor characteristic (p or n-type) programmable logic

Wunderlich et al, PRL '06

OutlineOutline

1) 1) Sensitivity to electric fields via magnetic anisotropiesSensitivity to electric fields via magnetic anisotropies generic to both metals and semiconductors with spin-orbit couplinggeneric to both metals and semiconductors with spin-orbit coupling - Tunneling AMR device- Tunneling AMR device - Coulomb blockade AMR spintronic SET- Coulomb blockade AMR spintronic SET

2) 2) Direct charge depletion effects on electric&magnetic proprtiesDirect charge depletion effects on electric&magnetic proprties ferromagnetic semiconductors are the favorable systems here ferromagnetic semiconductors are the favorable systems here

- GaMnAs and related dilute-moment ferromagnetic semiconductors- GaMnAs and related dilute-moment ferromagnetic semiconductors - GaMnAs-based p-n junction spintronic FET - GaMnAs-based p-n junction spintronic FET

Mn-d-like localmoments

As-p-like holes

Mn

Ga

AsMn

EF

DO

S

Energy

spin

spin

Ferromagnetic semiconductor GaAs:Mn

FM due to p-d hybridization

(Zener 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: - heavily-doped SC difficult to grow and gate- dilute moment FM difficult to achieve high Tc

Jungwirth et al, RMP '06

(Ga,Mn)As growth

Low-T MBE to avoid precipitation & high enough T to maintain 2D growth need to optimize T & stoichiometry for each Mn-doping

high-T growth optimal-T growth

Annealing also needs to be optimized for each Mn-doping

Detrimental interstitial AF-coupled Mn-donors need to anneal out (Tc can increase by more than 100K)

Tc up to 187 K at 12% Mn doping

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

140

160

180

TC(K

)

Mntotal

(%)

No indication for reaching technological or physical Tc limit in (Ga,Mn)As yet

Novak et al. PRL ‘08

1998

2005Growth & post-growth optimized GaMnAs films

Weak hybrid.Delocalized holeslong-range coupl.

Strong hybrid.Impurity-band holesshort-range coupl.

InSb

GaP

d5

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

Magnetism in systems with coupled dilute moments and delocalized band electrons

cou

pli

ng

str

eng

th /

Fer

mi

ener

gy

band-electron density / local-moment density

Jungwirth et al, RMP '06

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

Ordered magnetic semiconductors

Eu - chalcogenides

Disordered DMSs

Sharp critical contribution to resistivity at Tc ~ magnetic susceptibility

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

Udkk F ~)/1~~(

2~ Suncor

~)0~/1~( dkk F

smalluncor

vcdTdUdTd /~/

Tc

Tc

Eu0.95Cd0.05S Ni, Fe

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

0k1kd

1~kd

Sharp d/dT singularity in GaMnAs at Tc – consistent with F~d-

Novak, et al. PRL‘08

Optimized GaMnAs materials with x~4-12% and Tc~80-185K: well behaved FMs

Annealing sequence

t=(Tc-T)/Tc

4.03.0~ tM

As-p-like holes

Strong spin-orbit coupling favorable for spintronics

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

Mn

Ga

AsMn

Low-voltage gating of the highly doped (Ga,Mn)As

p-n junction depletion simulations

~25-50% depletion feasible at low voltages

2x 1019 cm-3

Owen, et al. arXiv:0807.0906

10’s-100’s Volts in conventional MOS FETs Ohno et al. Nature ’00, APL ‘06

p-n junction FET

Basic charcteristics of the device

can deplete charge at low Vg

can “deplete” magnetization at low Vg

low Vg dependent competition of uniaxial and cubic anisotropies

30% AMR tuneable by low Vg

Magnetization switching by 10ms low-Vg pulses

ConclusionConclusion

1) 1) Studies in GaMnAs suggest new generic approaches to Studies in GaMnAs suggest new generic approaches to electric field controlled spintronics via magnetic anisotropieselectric field controlled spintronics via magnetic anisotropies - TAMR- TAMR - CBAMR - CBAMR

2) 2) Direct charge depletion effects on electric&magnetic properties Direct charge depletion effects on electric&magnetic properties of GaMnAs demonstrated at low gate voltagesof GaMnAs demonstrated at low gate voltages - GaMnAs junction FET - GaMnAs junction FET

Mn

GaAs

Mn