Transcript Slide 1

Ferromagnetic ordering in (Ga,Mn)As related
zincblende semiconductors
Tomáš Jungwirth
Institute of Physics ASCR
University of Nottingham
František Máca, Jan Mašek, Jan Kučera
Josef Kudrnovský, Alexander Shick
Karel Výborný, Jan Zemen,
Vít Novák, Miroslav Cukr, Kamil Olejník, et al.
Bryan Gallagher, Tom Foxon,
Richard Campion, Kevin Edmonds,
Andrew Rushforth, Devin Giddings et al.
in collaboration with
Hitachi Cambridge
Polish Acad. of Sci.
UT & Texas A&M
Jorg Wunderlich
David Williams, et al.
Tomasz Dietl
Mike Sawicki
Allan MacDonald
Jairo Sinova
Univ. Wuerzburg
Laurenc Molenkamp
Charles Gould, et al.
Spintronics in (Ga,Mn)As dilute moment ferromagnetic semiconductor
Spintronic transistor
Huge hysteretic
low-field MR
Sign & magnitude
tunable by small
gate valtages
Wunderlich, et al., PRL (2006)
Current driven magnetization reversal
Magnetic race track memory
2 orders of magnitude lower critical
currents in dilute moment (Ga,Mn)As
than in conventional metal FMs
Sinova, Jungwirth et al., PRB (2004)
Parkin, US
Patent (2004)
Yamanouchi et al., Nature (2004)
OUTLINE
1. Mn-doped GaAs material - only a factor of 2 short room-T
otherwise outstanding properties
2. 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 - still very closely related to (Ga,Mn)As and might
heal its key problems
(Ga,Mn)As material
Mn
As
Ga
- Mn local moments too dilute
(near-neghbors cople AF)
- Holes do not polarize
in pure GaAs
- Hole mediated Mn-Mn
FM coupling
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
Universal scaling of (Tc / Mn-moment) vs. (hole / Mn-moment)
theory
expectations
Robust mean-field-like
ferromagnet
experiment
hole density / Mn-moment density
Jungwirth, Wang, et al. PRB (2005)
coupling strength / Fermi energy
Magnetism in systems with coupled dilute moments
and delocalized band electrons
band-electron density / local-moment density
(Ga,Mn)As
More Mn - interstitial incorporation
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
As
MnInt
Mnsub
Can be annealed out
MnInt
Tc 95K in as-grown (9% Mn)
theory & exp.
to 173 in annealed (6% Mnsub)
Ga
but MnGa < nominal Mn
Jungwirth, Wang, et al. PRB (2005)
More Mn - problem with solubility
- 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
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)
lattice constant (A)
(Al,Ga)As & Ga(As,P) hosts
5.7
(Al,Ga)As
Mn
As
5.4
0
Ga(As,P)
Ga
1
conc. of wide gap component
local moment - hole spin-spin coupling Jpd S . s
Mn d - As(P) p overlap
GaAs & (Al,Ga)As
Mn d level - valence band splitting
d5
Ga(As,P)
GaAs
d5
(Al,Ga)As & Ga(As,P)
(Al,Ga)As
p-d coupling and Tc in mixed
(Al,Ga)As and Ga(As,P)
theory
10% Mn
Ga(As,P)
Smaller lattice const. more important
for enhancing p-d coupling than larger gap

Mixing P in GaAs more favorable
for increasing mean-field Tc than Al
10% Mn
Ga(As,P)
Up to a factor of ~1.5 Tc enhancement
5% Mn
theory
Mašek, et al. preprint (2006)
Microscopic TBA/CPA or LDA+U/CPA
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)
No reduction of Mnint in (Al,Ga)As
Mixing P in GaAs more favorable for
suppressing Mnint formation
theory
Limits to carrier-mediated
Weak hybrid.
Delocalized holes
long-range coupl.
ferromagnetism in (Mn,III)V
InSb, InAs, GaAsTc: 7  173 K
d5
Similar hole localization tendencies
in (Al,Ga)As and Ga(As,P)
Strong hybrid.
GaP Tc: 65 K
d5d4
Impurity-band holes
short-range coupl.
Scarpulla, et al. PRL (2005)
no holes
d
(GaN ?)
d4
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
LDA+U sais that Mn-doped are also twin DMSs
Masek, et al. preprint (2006)
No solubility limit for group-II Mn
substituting for group-II Zn
theory
Additional interstitial Li in
Ga tetrahedral position - donors
n-type Li(Zn,Mn)As
Electron mediated Mn-Mn coupling n-type Li(Zn,Mn)As similar to hole mediated coupling in p-type (Ga,Mn)As
EF
Ga s-orb.L
As p-orb.
As p-orb.
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
Conclusions
1. Relatevily small technological investment in Mn-doped (Al,Ga)As useful info about trends in III-V's and for Ga(As,P)
2. More tech. difficult Mn-doped Ga(As,P) - could lead to higher Tc
3. Adventureous Mn-doped LiZnAs - might heal key problems in (Ga,Mn)As
& n-type FS
super-exchange
(anti-ferro)
kinetic exchange
(RKKY)
Intrinsic properties of Ga1-xMnxAs
Mn Mn
As
Mn
Ga
room-Tc
for x=10%
hole density (nm-3)
hole density / Mn density
Effective kinetic-exchange Hamiltonian,
microscopic TBA or LDA+U
• 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%.
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
+
MnI
Ga
Tc in as-grown and annealed samples
Open symbols as-grown. Closed symbols annealed
180
140
0.1
M[110](T) / MSat(5K)
160
8% (Ga,Mn)As
T = 172 K
0.0
120
-0.1
-1
Magnetic
0
Field
1
[ Oe ]
180
180
160
80
40
1.7% Mn
2.2%
1.7% Mn
3.4%Mn
Mn
2.2%
4.5%Mn
Mn
3.4%
5.6%Mn
Mn
4.5%
6.7%Mn
Mn
5.6%
9%Mn
Mn
6.7%
Mn
8%
9% Mn
C
20
160
140
140
180
120
1.7% Mn
120
160
2.2%100
Mn
100
3.4%
Mn
140
80
4.5%80
Mn
120
5.6% 60
Mn
66.7%60
7
Mn
1005
40
9% Mn
40
80 doping (%)
Mn
20
total
20
60
TC(K)
T (K)
60
0
0
1
2
3
4
Mn
Total
TC(K)
TC(K)
100
Tc=173K
(%)
8
9
10
A
A
A
A
Linear increase of Tc with effective Mn moment doping
Mneff = MnGa-MnI
180
160
MnGa
-
140
As
MnI
Ga
120
TC(K)
+
Closed symbols are
annealed samples
100
80
60
High
compensation
40
20
0
0
1
2
3
4
5
(%)
EffectiveMn
Mn
eff doping (%)
Tc increases with Mneff when compensation is less than ~40%.
No saturation of Tc at high Mn concentrations
6
7
(III,Mn)V materials: Microscopic picture of Mn-hole coupling in (Ga,Mn)As
Mn
GaAs
d5 
Mn
As
Ga
Ga s-orb.
L
1 Mn
0.1eV acceptor
As p-orb.
many Mn


As 4p
- Mn 3d
hybridization
d5 
Mixed (Al,Ga)As and Ga(As,P) hosts
Mn d  level
Ed
Ed
Mn d  level
|Vpd|2 ~ alc-7
1/|Ed| + 1/|Ed| ~ const.
Hole - local moment Kondo coupling:
Mean-field Curie temperature:
=
50% in GaP
4% in GaP and AlAs