Tomas Jungwirth, Jan Mašek, Alexander Shick Karel Výborný, Jan Zemen, Vít Novák, et al. Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew.

Tomas Jungwirth, Jan Mašek, Alexander Shick Karel Výborný, Jan Zemen, Vít Novák, et al.

Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, et al.

Joerg Wunderlich, Andrew Irvine, Elisa Ranieri, et al.

Cambridge

Nottingham

Prague

Jairo Sinova (TAMU),

Allan MacDonald (UT), et al (except

Nov. 26th)

Making semiconductors magnetic: A physics tango approach to

engineering quantum materialsFuture Directions

WorkshopOctober 11th 2007,

Austin TX

Technologically motivated and scientifically fueled

Incorporate magnetic properties with semiconductor

tunability (MRAM, etc)

Understanding complex phenomena:•Spherical cow of ferromagnetic systems (still very complicated)•Engineered control of collective phenomena•Benchmark for our understanding of strongly correlated systems

Generates new physics:•Tunneling AMR•Coulomb blockade AMR•Nanostructure magnetic anisotropy engineering

ENGINEERING OF QUANUTM MATERIALS

More knobs than usual in semiconductors: density, strain, chemistry/pressure, SO coupling engineering. Same as in oxides but better control of its consequences.

Mn

Ga

AsMn

FeFerromagnetic semiconductorsrromagnetic 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

More tricky than just hammering an iron nail in a silicon wafer

Mn

Ga

AsMn

Mn–hole spin-spin interaction

hybridization

Hybridization like-spin level repulsion Jpd SMn shole interaction

Mn-d

As-p

Heff

= Jpd

<shole> || -x

MnAs

Ga

heff

= Jpd

<SMn> || x

Hole Fermi surfaces

Ferromagnetic Mn-Mn coupling mediated by holes

Magnetism in systems with coupled dilute moments and delocalized band electrons

(Ga,Mn)As

cou

plin

g s

tren

gth

/ F

erm

i en

erg

y

band-electron density / local-moment density

Dilute moment nature of ferromagnetic semiconductorsDilute moment nature of ferromagnetic semiconductors

GaAs Mn

Mn

10-100x smaller Ms

One

Current induced switchingreplacing external field Tsoi et al. PRL 98, Mayers Sci 99

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

Sinova et al., PRB 04, Yamanouchi et al. Nature 04

Integrated read-out, storage, and transistor

Low current driven magnetization reversal

Parkin, US Patent (2004)

Magnetic race track memory

Yamanouchi et al., Nature (2004)

2 orders of magnitude lower criticalcurrents in dilute moment (Ga,Mn)As than in conventional metal FMsSinova, Jungwirth et al., PRB (2004)

Wunderlich, et al., PRL (2006)

Ideal to study spintronics fundamentals

Family of extraordinary MR effects, R(M),in ohmic, tunneling, Coulomb-blockade regimes

Low Ms (1-10% Mn moment doping): 100-10 x weaker mag. dipolar interactions can allow for100-10 x denser integration without unintentional dipolar cross-links

Strong SO-coupling:magnetocrystalline anisotropy ~ 10mT & doping and strain dependent can replacedemagnetizing shape anisotropy fields & local control of magnetic configurations

... and more yet to be fully exploited

GaMnAsAuAlOx Au

Tunneling anisotropic Tunneling anisotropic magnetoresistance (TAMR)magnetoresistance (TAMR)

Bistable memory device with a single magnetic layer only

Gould, Ruster, Jungwirth, et al., PRL '04

Giant magneto-resistance

(Tanaka and Higo, PRL '01)

[100]

[010]

[100]

[010]

[100]

[010]

Recently discovered in metals as well!

One

Dipolar-field-free current induced switching nanostructures

Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy)

~100 nm

(b)

Domain wall

Strain controlled magnetocrystalline (SO-induced) anisotropy

Can be moved by ~100x smaller currents than in metals

Humpfner et al. 06,Wunderlich et al. 06

Huge & tunable magnetoresistance in (Ga,Mn)As side-gated nano-Huge & tunable magnetoresistance in (Ga,Mn)As side-gated nano-constriction FETconstriction FET

Wunderlich, Jungwirth, Kaestner, Shick, et al., preprint

Single-electron Single-electron transistortransistor

Coulomb blockade anisotropic magnetoresistanceCoulomb blockade anisotropic magnetoresistance

Band structure (group velocities, scattering rates, chemical potentialchemical potential) depend on M

Spin-orbit couplingSpin-orbit coupling

If lead and dot differentIf lead and dot different (different carrier concentrations in our (Ga,Mn)As SET)

Q

0

DL'

D' )M()M()M(&

e

)M(Q)Q(VdQU

GMMGG0

20

C

C

e

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

C2

)QQ(U

electric && magneticmagneticcontrol of Coulomb blockade oscillations

Mn-d-like localmoments

As-p-like holes

Mn

Ga

AsMn

EF

DO

S

Energy

spin

spin

GaAs:Mn – extrinsic p-type semiconductor

with 5 d-electron local momenton the Mn impurity

valence band As-p-like holes

As-p-like holes localized on Mn acceptors

<< 1% Mn

onset of ferromagnetism near MIT

Jungwirth et al. RMP ‘06

~1% Mn >2% Mn

STILL LARGELY UNEXPLORED SYSTEMATICALLY: MIT

Impurity band to disordered-valence-band cross over in high-doped GaAs:Mn: red-shift of the IR peak in GaMnAs

At low doping near the MI transitionNon-momentum conserved transitions to localized states at the valence edge take away spectral weight from the low frequency

As metallicity/doping increases the localized states near the band edge narrow and the peak red-shifts as the inter-band part adds weight to the low-frequency part

Curie temperature limited to ~110K.

Only metallic for ~3% to 6% Mn

High degree of compensation

Unusual magnetization (temperature dep.)

Significant magnetization deficit

But are these intrinsic properties of GaMnAs ??

“110K could be a fundamental limit on TC”

As

GaMn

Mn Mn

Problems for GaMnAs (late 2002)

Can a dilute moment ferromagnet have a high Curie temperature ?

The questions that we need to answer are:

1. Is there an intrinsic limit in the theory models (from the physics of the phase diagram) ?

2. Is there an extrinsic limit from the ability to create the material and its growth (prevents one to reach the optimal spot in the phase diagram)?

EXAMPLE OF THE PHYSICS TANGO

As

GaMn

Mn Mn

Tc linear in MnGa local moment concentration; falls rapidly with decreasing hole density in more than 50% compensated samples; nearly independent of hole density for compensation < 50%.

Jungwirth, Wang, et al. Phys. Rev. B 72, 165204 (2005)

3/1pxT MnMF

c

Intrinsic properties of (Ga,Mn)As

Extrinsic effects: Interstitial Mn - a magnetism killer

Yu et al., PRB ’02:

~10-20% of total Mn concentration is incorporated as interstitials

Increased TC on annealing corresponds to removal of these defects.

Mn

As

Interstitial Mn is detrimental to magnetic order:

compensating double-donor – reduces carrier density

couples antiferromagnetically to substitutional Mn even in

low compensation samples Blinowski PRB ‘03, Mašek, Máca PRB '03

Theoretical linear dependence of Mnsub on total Mn confirmed experimentally

Mnsub

MnIntObtain Mnsub

& MnInt assuming change in

hole density due to Mn out

diffusion

Jungwirth, Wang, et al.Phys. Rev. B 72, 165204 (2005)

SIMS: measures total Mn concentration. Interstitials only compensation assumed

Experimental partial concentrations of MnGa and MnI in as grown samples

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 INTRINSIC LIMIT NO EXTRINSIC LIMIT

There is no observable limit to the amount of substitutional Mn we can put in

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

- Effective concentration of uncompensated MnGa moments has to increase beyond 6% of the current record Tc=173K sample. A factor of 2 needed 12% Mn would still be a DMS

- Low solubility of group-II Mn in III-V-host GaAs makes growth difficult

Low-temperature MBEStrategy A: stick to (Ga,Mn)As

- alternative growth modes (i.e. with proper

substrate/interface material) allowing for larger

and still uniform incorporation of Mn in zincblende GaAs

More Mn - problem with solubility

Getting to higher Tc: Strategy A

Find DMS system as closely related to (Ga,Mn)As as possible with

• larger hole-Mn spin-spin interaction

• lower tendency to self-compensation by interstitial Mn

• larger Mn solubility

• independent control of local-moment and carrier doping (p- & n-type)

Getting to higher Tc: Strategy B

conc. of wide gap component0 1

latti

ce c

onst

ant (

A)

5.4

5.7

(Al,Ga)As

Ga(As,P)

(Al,Ga)As & Ga(As,P) hosts

d5 d5

local moment - hole spin-spin coupling Jpd S . s

Mn d - As(P) p overlap Mn d level - valence band splitting

GaAs & (Al,Ga)As

(Al,Ga)As & Ga(As,P)GaAs

Ga(As,P)

MnAs

Ga

Smaller lattice const. more importantfor enhancing p-d coupling than larger gap

Mixing P in GaAs more favorable

for increasing mean-field Tc than Al

Factor of ~1.5 Tc enhancement

p-d coupling and Tc in mixed

(Al,Ga)As and Ga(As,P)

Mašek, et al. PRB (2006)Microscopic TBA/CPA or

LDA+U/CPA

(Al,Ga)As

Ga(As,P)

Ga(As,P)

10% Mn

10% Mn

5% Mn

theory

theory

Using DEEP mathematics to find a new material

3=1+2

Steps so far in strategy B:

• larger hole-Mn spin-spin interaction : DONE BUT DANGER IN PHASE DIAGRAM

• lower tendency to self-compensation by interstitial Mn: DONE

• larger Mn solubility ?

• independent control of local-moment and carrier doping (p- & n-type)?

III = I + II Ga = Li + Zn

GaAs and LiZnAs are twin SC

Wei, Zunger '86;Bacewicz, Ciszek '88;Kuriyama, et al. '87,'94;Wood, Strohmayer '05

Masek, et al. PRB (2006)

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

No solubility limit for group-II Mn

substituting for group-II Zn !!!!

Electron mediated Mn-Mn coupling n-type Li(Zn,Mn)As -

similar to hole mediated coupling in p-type (Ga,Mn)As

L

As p-orb.

Ga s-orb.As p-orb.

EF

Comparable Tc's at comparable Mn and carrier doping and

Li(Mn,Zn)As lifts all the limitations of Mn solubility, correlated local-moment and carrier densities, and p-type only in (Ga,Mn)As

Li(Mn,Zn)As just one candidate of the whole I(Mn,II)V family

● Apply same strategic approach to Oxides, other strongly correlated materials (new different DMSs, etc)

● Exploit further new properties and physics: are the same physics present in DMSs in Oxides and other strongly correlated materials?

● Fill in the phase diagram

SO WHAT NEXT?

BEFORE 2000

CONCLUSION:directors cut

BUT it takes MANY to do the physics tango!!

Texas A&M U., U. Texas, Nottingham, U. Wuerzburg,

Cambirdge Hitachi, ….

It IS true that it takes two to tango

2000-2004

2006NEW DMSs ,More heterostructures

NEXT

EXTRA

MAGNETIC ANISOTROPY

M. Abolfath, T. Jungwirth, J. Brum, A.H. MacDonald, Phys. Rev. B 63, 035305 (2001)

Condensation energy dependson magnetization orientation

<111>

<110>

<100>

compressive strain tensile strain

experiment:

Potashnik et al 2001Lopez-Sanchez and Bery 2003Hwang and Das Sarma 2005

Resistivity temperature dependence of metallic GaMnAs

theory theory

experiment

Ferromagnetic resonance: Gilbert damping

Aa,k()

I

M

Anisotropic Magnetoresistance

exp.

T. Jungwirth, M. Abolfath, J. Sinova, J. Kucera, A.H. MacDonald, Appl. Phys. Lett. 2002

ANOMALOUS HALL EFFECT

T. Jungwirth, Q. Niu, A.H. MacDonald, Phys. Rev. Lett. 88, 207208 (2002)

anomalous velocity:

M0M=0

JpdNpd<S> (meV)

Berry curvature:

AHE without disorder

ANOMALOUS HALL EFFECT IN GaMnAs

Experiments

Clean limit theory

Minimal disorder theory

-0.08 0.00 0.080

20

RC [

M

]

-BC1 -BC2BC2

BC1

B [ T ]

POWER“OFF”

Electrical operation mode

“READ”:measure RC

at VG = VG1

“1” (M1)

“0” (M0)

“WRITE”permanently

POWER “ON”

POWER“OFF”

Electrical operation mode

Electrical operation mode

“READ”:measure RC

at VG = VG1

“1” (M1)

“0” (M0)

“WRITE”permanently

POWER “ON”

M1 -M1

M0 M0

VG = VG1 = 1.04V

1.00 1.01 1.02 1.03 1.046

8

10

12

14

16

18

20

VG0

VG1

RC [

M

]

VG [ V ]

electric modeelectric mode

magneticmagneticnonnon--volatilevolatile

modemode

0.6 0.8 1.00

25

50

RC [

M

]

VG [ V ]

““00””

““11””

MM00(a)

(b)

(c)

MM11

Magnetic non-volatile mode

VG = VG1 : M0 (“0”) M1 (“1”)

[[InverseInverse:: VG = VG0 : M0 (“1”) M1 (“0”)]

M0 : B B0 0 BC1 < B0 < BC2

M1 : B B1 0 B1 < -BC2

Electric modeM = M1 : VG0 (“0”) VG1 (“1”)

[[InverseInverse:: M = M0 : VG0 (“1”) VG1 (“0”)]

(d)

Magnetic non-volatile mode

VG = VG1 : M0 (“0”) M1 (“1”)

[[InverseInverse:: VG = VG0 : M0 (“1”) M1 (“0”)]

M0 : B B0 0 BC1 < B0 < BC2

M1 : B B1 0 B1 < -BC2

Electric modeM = M1 : VG0 (“0”) VG1 (“1”)

[[InverseInverse:: M = M0 : VG0 (“1”) VG1 (“0”)]

(d)

CBAMR CBAMR new device concepts new device concepts

(Ga,Mn)As material(Ga,Mn)As material

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

intermediate acceptor (110 meV) hole

- Mn local moments too dilute (near-neghbors cople AF)

- Holes do not polarize in pure GaAs

- Hole mediated Mn-Mn FM coupling

Mn

Ga

AsMn

Ohno, Dietl et al. (1998,2000);Jungwirth, Sinova, Mašek, Kučera, MacDonald, Rev. Mod. Phys. (2006), http://unix12.fzu.cz/ms

Additional interstitial Li in

Ga tetrahedral position - donors

n-type Li(Zn,Mn)As

No solubility limit for group-II Mn

substituting for group-II Zn

theory

d4

d

Weak hybrid.Delocalized holeslong-range coupl.

Strong hybrid.Impurity-band holesshort-range coupl.

d 5 d 4 no holes

InSb, InAs, GaAs Tc: 7 173 K

(GaN ?)

GaP Tc: 65 K

Similar hole localization tendencies

in (Al,Ga)As and Ga(As,P)

Scarpulla, et al. PRL (2005)

d5

Limits to carrier-mediated

ferromagnetism in (Mn,III)V