Spintronic transistors: magnetic anisotropy and direct charge depletion concepts

Tomas Jungwirth

University of Nottingham Bryan Gallagher, Tom Foxon,

Richard Campion, Kevin Edmonds, Andrew Rushforth, 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 TransistorLow-V 3-terminal

devices

STT MRAMspin-polarized charge current

1) indirect via magnetic anisotropy

2) direct charge depletion effects

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

parallel state

antiparallel state

ab intio theoryTAMR is generic to SO-coupled 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

Consider uncommon TM combinationse.g. Mn/W voltage-dependent upto ~100% TAMR

spontaneous momentmag

netic su

sceptib

ility

spin

-orb

it cou

plin

g

Optimizing TAMR in transition-metal structures

Shick, et al PRB ‘08

GM

MGG

C

C

e

MV

MVVCQC

QQU

)(&

)]([&2

)(0

20

electric && magneticmagnetic

control of CB oscillations

Source Drain

GateVG

VDQ

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

SO-coupling (M)

[010] M[110]

[100]

[110][010]

~ mV in GaMnAs~ 10mV in FePt

Wunderlich et al, PRL '06

(Ga,Mn)As nano-constriction SET CB oscillations shifted by changing M(CBAMR)

Electric-gate controlled magnitude and sign of magnetoresistance spintronic transistor

&

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

Mn-d-like localmoments

As-p-like holes

Mn

Ga

AsMn

EF

DO

S

Energy

spin

spin

Ferromagnetic semiconductor GaAs:Mn

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

- random dilute moment FM difficult to achieve high Tc

- intrinsically very disordered system

- heavily-doped SC difficult to grow and gate

Exchange-split, SO-coupled, & itinerant holes

FM & transport in the disordered GaMnAs DMS

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)

~)( dF

Eu0.95Cd0.05S

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

Fisher&Langer, PRL‘68

singular

UdF ~)~(

vcdTdUdTd /~/

Tc

Ni, Fe

singular

Scattering off correlated spin-fluctuations

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

Annealing sequence

In GaMnAs F~d- sharp singularity at Tc in d/dT

Novak et al., PRL ‚08

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

Complete spintronic FET characteristics

TcTc

Magnetization switching by short low-Vg pulses

Due to voltage-controlled Kc and Ku anisotropies

semiquantitative microscpic theory understanding

depletion/accumulation & high-frequency studies of DMS materials and spintronics

-1V +3 V

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) Optimized GaMnAs is excellent itinerant FM; low-voltage Optimized GaMnAs is excellent itinerant FM; low-voltage charge depletion effects on electric&magnetic properties charge depletion effects on electric&magnetic properties

demonstrated in all-semiconductor p-n junction transistordemonstrated in all-semiconductor p-n junction transistor - d- d/dT singularity at T/dT singularity at Tcc

- GaMnAs junction FET - GaMnAs junction FET

Tc

(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

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

Novak, et al. PRL‘08

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