Spintronics and Magnetic

SemiconductorsJoaquín Fernández-Rossier,

Department of Applied Physics, University of Alicante (SPAIN)

Alicante, June 18 2003

•Spintronics•Magnetic Semiconductors•Ferromagnetism in GaAsMn

Spintronics

Semiconductors

Source GateDrain

Semiconductor

Semiconductors:•Low carrier density (p, n)•Heterostructures•Electrical Control of p,n•Volatile information

Dimensional Reduction,

L<BMesoscopic Behavior,

L<ls

New exciting physics:Quantum

Hall Effects,Conductanc

e Quantizatio

n

Single electron transistor

Size reduction

Metallic Ferromagnets

Ferromagnetic metals:•Collective Coordinate •Permanent information•High carrier density•Heterostructures•Magnetic Control of current

‘Single atom’ magnet

New PhysicsGMR

L<lsr

Size reductio

n

Superparamagnetism

‘There is plenty of room at the bottom’

R. P. Feynman

‘There is plenty of room at the bottom’

R. P. Feynman

Alternative Approach: Spintronics

BUT:•Spin injection problem•Spin scattering problem

Source Gate Drain

InAs

Das and Datta spin transistor,APL 1990

•Rashba spin-orbit controls spin orientation•Spin dependent transmission controls resistance

High ResistanceLow Resistance

SPINTRONICS: Merger of semiconductor based

and ferromagnet based information technologies

Spintronics (from the table of contents of

“Semiconductor Spintronics and Quantum Computation”

• Device concepts• Interface Physics (spin injection)• Spin dynamics, spin decoherence• Optical Manipulation• Materials

Magnetic Semiconductors(materials for spintronics)

“History” of magnetic semiconductorsMaterial Year Tc Transport

EuO, EuS 1960-70 <65 K Insulating

(II,Mn)VI 1970-80 0 Semic.

PbSnMnTe

CdTeMn:N

ZnMnTe:X

1980-1990 <1.5 K P-type semiconduc.

InMnAs 1992 7 K Ins.

(Ga,Mn)As 1996 110 K Semic, bad metal

(Ga,Sb)Mn, GaP:Mn, GaN:Mn, ZnCrTe

1996-2003(DARPA time)

900 K??

Ferromagnetism vs Paramagnetism

B C

Al Si

N O

P S

Ga Ge

In Sn

As Se

Sb

II

Zn

Cd

Hg

IV VIII VI

Te

EGEF

II-VIZn-SeZn-S Cd-Te

EGEF

Paramagnetic DMS

II

B C

Al Si

N O

P S

Ga Ge

In Sn

As Se

Sb

IV VIII VI

Te

Zn

Cd

Hg

Mn

(II,Mn)-VI(Zn,Mn)-Se(Zn,Mn)-S (Cd,Mn)-Te

B C

Al Si

N O

P S

Ga Ge

In Sn

As Se

Sb

II

Zn

Cd

Hg

IV VIII VI

Te

EGEF

III-VGa-AsIn-As Ga-Sb

EG

(diluted Ferromagnetic semiconductor)

B C

Al Si

N O

P S

Ga Ge

In Sn

As Se

Sb

Mn

II

Zn

Cd

Hg

IV VIII VI

Te

EF

(III,Mn)-V(Ga, Mn)-As(In, Mn)-As (Ga, Mn)-Sb

“Chemistry” of II-VI:Mn and III-V:Mn

Electronic configuration of Mn: 4s2 3d5 4p0

Electronic configuration of Ga (III): 4s2 3d10 4p1

Electronic configuration of Cd (II): 4s2 3d10 4p0

•Mn in III-V: gives magnetic moment and holes •Mn in II-VI: gives magnetic moment

Why DMS?

Y. OHNO et al., Nature 402, 790 (1999)

Magnetic Light emitting diode

•Spin injection•Compatible with GaAs

‘All semiconductor’ Magnetic Tunnel Junction•Large TMR (at 4 Kelvin)

Tanaka, Higo, PRL 2001

‘Electric Control of Ferromagnetism’

•First electrically tunable ferromagnet•Reversible change of Tc

H. Ohno, Nature (2000)

•Spin Injection•Heterostructures III-V + (III,Mn)-V•Magnetic control of transport•Electric control of Magnetism

BUT:• Working at low temperature•Small effects•Curie Temperature < 150 Kelvin

Magnetic Semiconductors Summary

• 2 types (ferro and para)• Compatible with semiconductor

technology

• Issue: Increase Tc

Microscopic mechanism of ferromagnetism in

(III,Mn)-V

The mechanism. Experimental evidence

• II-VI+Mn = PARAMAGNETIC• II-VI+Mn+electrons, PARAMAGNETIC• II-VI+Mn+ holes: FERROMAGNETIC• III-V+Mn= FERROMAGNETIC

Material: Ga(1-x) AsMnx

• Ferromagnetic below 160 kelvin• Homogeneous alloy for x<0.10

• Transport: p-doped semiconductor (p<cMn)

FERRO

PARA

x0.080.050.03

Giant Zeeman Splitting in (II,Mn)-VI

<M>=0

jsdcMn<M>

jpdcMn<M>

B

McJ

RrMJH

Mnsd

IIIsdexch

McJ

RrMJH

Mnpd

IIIpdexch

4 Exchange Interactions• Coulomb

Exchange: ferromagnetic. (Reduction of Coulomb repulsion )

• Kinetic Exchange: Antiferromagnetic

d5

d6

As Mnd5

d6

As MnMSJH hEXCH

p,s

p,s d

d

d

d d

d

dddF

pdKE UUVJ

112

The “model Hamiltonian”

I

IIimp MrSJVHH

)(0

GaAs Hamiltonian

Non MagneticScattering

Exchange

Sum overImpurities

SO

2

3

)1(pdcB Jx

SSTk

3/1)(mp

p

EF

Carrier mediated ferromagnetism

MTSkMETMG Bcarrier

,

Entropic Penalty

Paramagneticgain

MSJMEMEcarrier

0

Functional of carrier density matrix

2

3

)1(pdcB Jx

SSTk

cB

pd Tk

SSJx

3

)1(2

Model Hamiltonian “chemistry”

EG

EF

dp p

s

d

Holes in Valence

BandNo

fluctuationin d levels

dp p

s

dHoles in d

levels.

Hund Exchange

Half metal?

“Ab initio” “chemistry”

dp p

s

d

Holes both in d

levels and valence band.

Both Hund and

Kinetic exchange

What about this “chemistry”?

Microscopic Mechanism: Summary and questions

• Model Hamiltonian: no d-charge fluctuations, holes in valence band, kinetic exchange. Weak coupling

• “Double Exchange”: d charge fluctuations. Hund exchange. Strong coupling.

• Who is right? (from ab-initio)• Interpolation from DE to KE

C. PiermarocchiMichigan State

P.C. ChenUC Berkeley

L. J. ShamUCSD

JFR , in collaboration with

A. H. MacDonaldUT Austin

M. Abolfath,UT Austin

A. S. Núñez,UT Austin

Conclusions

• Spintronics: “make it happen” ideology

• Magnetic Semiconductors: bricks to build spintronics

• FM Mechanism: maybe an open problem.