Ferromagnetic ordering in (Ga,Mn)As relatedFerromagnetic ordering in (Ga,Mn)As relatedzincblende semiconductorszincblende semiconductors

Tomáš Jungwirth

Institute of Physics ASCR

František Máca, Jan Mašek, Jan Kučera Josef Kudrnovský, Alexander Shick

Karel Výborný, Jan Zemen, Vít Novák, Miroslav Cukr, Kamil Olejník, et al.

University of Nottingham

Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds,

Andrew Rushforth, Devin Giddings et al.

Hitachi Cambridge Polish Acad. of Sci. UT & Texas A&M Univ. Wuerzburg

Jorg Wunderlich Tomasz Dietl Allan MacDonald Laurenc MolenkampDavid Williams, et al. Mike Sawicki Jairo Sinova Charles Gould, et al.

in collaboration with

Huge hysteretic low-field MR

Sign & magnitudetunable by smallgate valtages

Spintronic transistor

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)

Spintronics in (Ga,Mn)As dilute moment ferromagnetic semiconductor

Wunderlich, et al., PRL (2006)

OUTLINEOUTLINE

11.. Mn-doped GaAs material Mn-doped GaAs material - only a factor of 2 short room-T otherwise outstanding properties

22.. Mn-doped (Al,Ga)As and Ga(As,P) Mn-doped (Al,Ga)As and Ga(As,P) - useful materials to compare with and Ga(As,P) could lead to higher Tc

3. Mn-doped LiZnAs 3. Mn-doped LiZnAs - still very closely related to (Ga,Mn)As and might heal its key problems

(Ga,Mn)As material

Mn

As

Ga

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

moderately shallow acceptor (110 meV) hole

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

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

- Holes do not polarize in pure GaAs

- Hole mediated Mn-Mn FM coupling

Universal scaling of (Tc / Mn-moment) vs. (hole / Mn-moment)

hole density / Mn-moment density

theory theory expectationsexpectations

experimentexperiment

Robust mean-field-likeferromagnet

Jungwirth, Wang, et al. PRB (2005)

Magnetism in systems with coupled dilute moments and delocalized band electrons

(Ga,Mn)As

coup

ling

stre

ngth

/ Fe

rmi e

nerg

y

band-electron density / local-moment density

Jungwirth, Wang, et al. PRB (2005)

Covalent SCs do not like doping self-compensation by interstitial Mn

Interstitial MnInt is detrimental to magnetic order

charge and moment compensation defect

Yu et al., PRB ’02; Blinowski PRB ‘03; Mašek, Máca PRB '03

Mnsub

MnInt

Mnsub

As

Ga

MnInt+-

Can be annealed out

Tc 95K in as-grown (9% Mn)

to 173 in annealed (6% Mnsub)

but MnGa < nominal Mn

More Mn - interstitial incorporation

theory & exp.

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

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

Low-temperature MBE

More Mn - problem with solubility

A startegy:

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

• larger hole-Mn spin-spin interaction

• lower tendency to self-compensation by Mnint

• larger Mn solubility

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

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

Up to a factor of ~1.5 Tc enhancement

p-d coupling and Tc in mixed

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

Mašek, et al. preprint (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

No reduction of Mnint in (Al,Ga)As

Mixing P in GaAs more favorable forsuppressing Mnint formation

Mnint formation in mixed (Al,Ga)As and Ga(As,P)

higher in (Al,Ga)As

and Ga(As,P)

than in GaAs

smaller interstitial space

only in Ga(As,P) theory

d4

d

Weak hybrid. Delocalized holeslong-range coupl.

Strong hybrid.Impurity-band holesshort-range coupl.

d 5 d 4 no holes

InSb, InAs, GaAsTc: 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

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. preprint (2006)

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

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

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

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

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

ConclusionsConclusions

11.. Relatevily small technological investment in Mn-doped (Al,Ga)As Relatevily small technological investment in Mn-doped (Al,Ga)As -

useful info about trends in III-V's and for Ga(As,P)

22.. More tech. difficult Mn-doped More tech. difficult Mn-doped Ga(As,P) Ga(As,P) - could lead to higher Tc

3. Adventureous Mn-doped LiZnAs 3. Adventureous Mn-doped LiZnAs - might heal key problems in (Ga,Mn)As & n-type FS

• 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%.

Intrinsic properties of Ga1-xMnxAs

As

GaMn

Mn Mn

super-exchange (anti-ferro)kinetic exchange

(RKKY)

hole density (nm-3) hole density / Mn density

room-Tcfor x=10%

Effective kinetic-exchange Hamiltonian, microscopic TBA or LDA+U

Extrinsic effects - covalent SC do not like doping self-compensation by interstitial Mn

Interstitial MnI is detrimental to magnetic order:

compensating double-donor – reduces carrier density

attracted to substitutional MnGa acceptor and couples

antiferromagnetically to MnGa even at low compensation

Yu et al., PRB ’02; Blinowski PRB ‘03; Mašek, Máca PRB '03

MnGa

As

Ga

MnI

+

-

Tc in as-grown and annealed samples

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

140

160

180

T C(K

)

Mntotal(%)

-1 0 1-0.1

0.0

0.1T = 172 K8% (Ga,Mn)As

M [1

10](T

) / M Sa

t(5K

)

Magnetic Field [ Oe ]

Tc=173K

8% Mn

Open symbols as-grown. Closed symbols annealed

Total Mn doping (%)

MnGa

As

Ga

MnI

+

-

Linear increase of Tc with effective Mn moment doping

Tc increases with Mneff when compensation is less than ~40%.

No saturation of Tc at high Mn concentrations

Closed symbols are annealed samples

0 1 2 3 4 5 6 70

20

40

60

80

100

120

140

160

180

T C(K)

Mneff(%)

High compensatio

n

Mneff = MnGa-MnI

Effective Mn doping (%)

(III,Mn)V materials: Microscopic picture of Mn-hole coupling in (Ga,Mn)As

L

Mn

Ga

As

Ga s-orb.

As p-orb.

1 Mn0.1eV acceptor many Mn

d5

d5

As 4p - Mn 3d

hybridization

GaAs Mn

Mn d level

Mn d level

Ed

Ed

|Vpd|2 ~ alc-7

Hole - local moment Kondo coupling:

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

1/|Ed| + 1/|Ed| ~ const.

=Mean-field Curie temperature:

4% in GaP and AlAs50% in GaP