Making semiconductors magnetic:

new materials properties, devices, and future

NRISWAN

JAIRO SINOVATexas A&M University

Institute of Physics ASCR

Hitachi CambridgeJorg Wunderlich, A. Irvine, et al

Institute of Physics ASCRTomas Jungwirth, Vít Novák, et al

Texas A&M L. Zarbo

University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al.

215th ECS Meeting - San Francisco, CA May 27th 2009

OUTLINE

• Motivation• Ferromagnetic semiconductor materials:

– (Ga,Mn)As - general picture– Growth, physical limits on Tc – Related FS materials (searching for room temperature)– Understanding critical behavior in transport

• Ferromagnetic semiconductors & spintronics– Tunneling anisotropic magnetoresistive device– Transistors (4 types)

1.Create a material that marriages the tunability of semiconductors and the collective behavior of ferromagnets; once created search for room temperature systems

2.Study new effects in this new material and utilize in metal-based spintronics

3.Develop a three-terminal gated spintronic device to progress from sensors & memories to transistors & logic

Ferromagnetic semiconductor research :

Motivations and strategies

(Ga,Mn)As GENERAL PICTURE

Ferromagnetic semiconductors

GaAs - standard III-V semiconductor

Group-II Mn - dilute magnetic moments & holes

(Ga,Mn)As - ferromagnetic semiconductor

Need true FSs not FM inclusions in SCs

Mn

Ga

AsMn

+

Mn

Ga

As

What happens when a Mn is placed in Ga sites:Mn–hole spin-spin interaction

hybridization

Hybridization like-spin level repulsion Jpd SMn shole interaction

Mn-d

As-p

In addition to the Kinetic-exchange coupling, for a single Mn ion, the coulomb interaction gives a trapped hole (polaron) which resides just above the valence band

5 d-electrons with L=0 S=5/2 local moment

intermediate acceptor (110 meV) hole

Mn

Ga

AsMn

EF

DO

S

Energy

spin

spin

Transition to a ferromagnet when Mn concentration increasesGaAs: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

Mn

Ga

As

Mn

Ga

AsMn

•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-double-donors compensating holes and moments need to anneal out but without loosing MnGa

high-T growth

optimal-T growth

(Ga,Mn)As 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

Tc LIMITS AND STRATEGIES

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

140

160

180

TC(K

)

Mntotal

(%)

“... Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..” Yu et al. ‘03

“…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...” Mack et al. ‘08

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

Tc limit in (Ga,Mn)As remains open

2008Olejnik et al

188K!!

Can we have high Tc in Diluted Magnetic Semicondcutors?

Tc linear in MnGa local (uncompensated) moment concentration; falls rapidly with decreasing hole density in heavily compensated samples.

Define Mneff = Mnsub-MnInt

NO IDENTIFICATION OF AN INTRINSIC LIMITNO EXTRINSIC LIMIT

(lines – theory, Masek et al 05)

Relative Mn concentrations obtained through hole density measurements and saturation moment densities measurements.

Qualitative consistent picture within LDA, TB, and k.p

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

140

160

180

TC(K

)

Mntotal

(%)

8% Mn

Open symbols as grown. Closed symbols annealed

0 1 2 3 4 5 6 70

20

40

60

80

100

120

140

160

180

TC(K

)

Mneff

(%)

Tc as grown and annealed samples

● Concentration of uncompensated MnGa moments has to reach ~10%. Only 6.2% in the current record Tc=173K sample

● Charge compensation not so important unless > 40%

● No indication from theory or experiment that the problem is other than technological - better control of growth-T, stoichiometry

How well do we understand (Ga,Mn)As?

In the metallic optimally doped regime GaMnAs is well described by a disordered-valence band picture: both dc-data and ac-data are consistent with this scenario.

The effective Hamiltonian (MF) and weak scattering theory (no free parameters) describe (III,Mn)V metallic DMSs very well in the optimally annealed regime:• Ferromagnetic transition temperatures

Magneto-crystalline anisotropy and coercively Domain structure Anisotropic magneto-resistance Anomalous Hall effect MO in the visible range Non-Drude peak in longitudinal ac-conductivity • Ferromagnetic resonance • Domain wall resistance • TAMR •Transport critical behaviour

TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity formation energies, lattice constant variations upon doping

III = I + II Ga = Li + Zn

GaAs and LiZnAs are twin SC

Masek, et al. PRB (2006)

LDA+U says that Mn-doped are also twin DMSs

n and p type doping through Li/Zn stoichiometry

No solubility limit for group-II Mn

substituting for group-II Zn !!!!

UNDERSTANDING CRITICAL

BEHAVIOUR IN TRANSPORT

Solving a puzzle in (Ga,Mn)As: FM & transport

Dense-moment MSF<< d-

Eu - chalcogenides

Dilute-moment MSF~ d-

Critical contribution to resistivity at Tc

~ magnetic susceptibilityBroad peak near Tc disappeares with annealing (higher uniformity)???

~)0~~( Fkk

smalluncor Tc

EuCdSe

When density of carriers is smaller than density of local moments what matters is the long range behavior of Γ (which goes as susceptibility)

)/1~~( dkk F

vcdTddTd ~/~/

When density of carriers is similar to density of local moments what matters is the short range behavior of Γ (which goes as the energy)

Ni

Tc

Optimized materials with x=4-12.5% and Tc=80-185K

Remarkably universal both below and above Tc

Annealing sequence

d/dT singularity at Tc – consistent with kF~d-

V. Novak, et al “Singularity in temperature derivative of resistivity in (Ga,Mn)As at the Curie point”, Phys. Rev. Lett. 101, 077201 (2008).

OUTLINE

• Motivation• Ferromagnetic semiconductor materials:

– (Ga,Mn)As - general picture– Growth, physical limits on Tc – Related FS materials (searching for room temperature)– Understanding critical behavior in transport

• Ferromagnetic semiconductors & spintronics– Tunneling anisotropic magnetoresistive device– Transistors (4 types)

AMRAMR~ 1% MR effect~ 1% MR effect

TMRTMR~ 100% MR effect~ 100% MR effect

TAMRTAMR

) vs.( ~ IMvgExchange split & SO-coupled bands:

Exchange split bands:

)()(~ TDOSTDOS

)(~ MTDOS

Au

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

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

TAMR in metal structures

experiment Park, et al, PRL '08

Also studied by Parkin et al., Weiss et al., etc.

DMS DEVICES

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

p-n junction depletion estimates

Olejnik et al., ‘08

~25% depletion feasible at low voltages

(Ga,Mn)As/AlOx FET with large gate voltages, Chiba et al. ‘06

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

Tc Tc

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 via strain

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

GMMGG C

C

e

MVMVVCQ

C

QQU

)(&)]([&

2

)(0

20

electric && magneticmagneticcontrol 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

D e

MQQVdQU

0

'' )()(

[010]

M[110]

[100]

[110][010]

SO-coupling (M)

Source Drain

GateVG

VDQ

Single-electron charging energy controlled by Vg and M

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 logic

Physics of SO & exchange

SET

Resistor

Tunneling device

Chemical potential CBAMR

Tunneling DOS TAMR

Group velocity & lifetime AMR

Device design

Materials

metal FMs

FSs

FSs and metal FS with strong SO

Allan MacDonald U of Texas

Tomas JungwirthInst. of Phys. ASCRU. of Nottingham

Joerg WunderlichCambridge-Hitachi

Laurens MolenkampWuerzburg

Mario BorundaTexas A&M U.

Other collaborators: Bernd Kästner, Satofumi Souma, Liviu Zarbo, Dimitri Culcer , Qian Niu, S-Q Shen, Brian Gallagher, Tom Fox, Richard

Campton

Alexey KovalevTexas A&M U.

Liviu ZarboTexas A&M U.

Matching TAMU funds

Xin LiuTexas A&M U.

Bryan GallagherU. Of Nottingham

33

EXTRAS

AMR nature of the effect

normal AMR Coulomb blockade AMR

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

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

As-p-like holes

Strong exchange splitting & SO coupling in (Ga,Mn)As

Standard MBE techniques for high-quality tunneling structures

MnGa

As

Mn