Transcript Document 7443945

Challenges and Chemical Trends in Achieving a Room
Temperature Dilute Magnetic Semiconductor:
A Spintronics Tango
JAIRO SINOVA
2007: Frontiers in Chemical Physics Workshop
February 22th 2007, Knoxville TN
Research fueled by:
NERC
Mario Borunda
Texas A&M U.
Tomas Jungwirth
Inst. of Phys. ASCR
U. of Nottingham
Xin Liu
Alexey Kovalev
Texas A&M U. Texas A&M U.
Allan MacDonald
U of Texas
Joerg Wunderlich
Cambridge-Hitachi
Ewelina Hankiewicz
U. of Missouri
Texas A&M U.
Bryan Gallagher
U. Of Nottingham
Other collaborators: Jan Masek, Karel Vyborny, Bernd
Kästner, Carten Timm, Laurens Molencam, Charles
Gould, Tomas Dietl, Tom Fox, Richard Campton
OUTLINE
Intro: Spintronics and Semiconductor
Spintronics
 Intro to Diluted Magnetic Semiconductors
 Can we have a high Tc DMS?
 What is the data telling us in GaMnAs: thumbs
up or down?
 Strategies to achieve high Tc DMSs

ELECTRONICS
UP TO NOW: electronics mostly based on the
manipulation of the charge of the electron
CHARGE
Mr. Electron
Two parts to his
personality !
SPIN
Spintronics = Controlling spin and spin flow
with
electric fields
magnetic fields
…
Courtesy of The Archives, California Institute of Technology
There is plenty of room
at the bottom - Feynman,
1959
"... Up to now we have been
content to dig in the ground to
find minerals.
... We should consider,
whether - in the great future we can arrange atoms the way
we want, the very atoms, all the
way down!
... We can use not just circuits,
but some system involving the
quantized energy levels, or
interactions of
quantized spins."
Why semiconductors are better
Bloch Wilson
allowed energy
band in crystal
isolated atomic
levels
allowed energy
band in crystal
Pauli 1931: “One shouldn’t work with semiconductors,
that is a filthy mess; who knows whether they really exist”
New a la Feynman revolution in spintronics:
diluted magnetic semiconductors
Charge: used for computation
+


Spin: used for storage



Goal: use the storing capacity of
ferromagnetism and the processing
capacity of semiconductors
Does nature give us the material already?
No
Can we make it? Yes
Can we understand it? Getting there
Can we improve it: that is the tango
Dilute Magnetic Semiconductors: the simple picture
5 d-electrons with L=0
 S=5/2 local moment
- Mn local moments too dilute
(near-neghbors cople AF)
- Holes do not polarize
in pure GaAs
- Hole mediated Mn-Mn
FM coupling
moderately shallow
acceptor (110 meV)
 hole
FERROMAGNETISM MEDIATED
BY THE CARRIERS!!!
Jungwirth, Sinova, Mašek, Kučera,
MacDonald, Rev. Mod. Phys. (2006),
http://unix12.fzu.cz/ms
Ga1-xMnxAs
BUT THINGS ARE NOT THAT SIMPLE
As anti-site deffect: Q=+2e
Antisite
Mn
Substitutioanl Mn:
acceptor +Local 5/2
moment
As
Interstitial
Ga
Ferromagnetic: x=1-8%
courtesy of D. Basov
Interstitial Mn:
double donor
Low Temperature
- MBE
(III,V) DMS: man made ferromagnets a la Feynman (1992-2000)
•Ohno, et al PRL (1992) (InMnAs) 7.5 K
•Ohno, et al APL (1996) (GaMnAs) 55 K
•Ohno, Science (1998) 110 K
Tc~ 100 K, metallic
hard magnets
anomalous Hall effect
normal: RHall ~ B
anomalous: R ~ M

Curie temperature limited to ~110K.

Only metallic for ~3% to 6% Mn

High degree of compensation

Unusual magnetization (temperature dep.)

Significant magnetization deficit
Curie temperature (K)
Problems for GaMnAs (late 2002)
120
[3] [4]
[2]
80
[1]
40
0
1996
1998
2000
Time
Mn Mn
As
“110K could be a fundamental limit on TC”
Mn
Ga
But are these intrinsic properties of GaMnAs ??
2002
OUTLINE
Intro: Spintronics and Semiconductor
Spintronics
 DMS’s Introduction
 Can we have a high Tc DMS?
 What is the data telling us in GaMnAs: thumbs
up or down?
 Strategies to achieve high Tc DMSs

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)?
Origin of coupling between localized
moments and dilute carriers
Microscopic: atomic orbitals & Coulomb correlation of d-electrons & hopping
Jpd = + 0.6 meV nm3
Jpd SMn.shole
Effective magnetic coupling:
Coulomb correlation of d-electrons & hopping
AF kinetic-exchange coupling
coupling strength / Fermi energy
Magnetism in systems with coupled dilute moments
and delocalized band electrons
band-electron density / local-moment density
(Ga,Mn)As
Theoretical Approaches to DMSs
• First Principles Local Spin Density Approximation (LSDA)
PROS: No initial assumptions, effective Heisenberg model can be
extracted, good for determining chemical trends
CONS: Size limitation, difficulty dealing with long range interactions, lack of
quantitative predictability, neglects SO coupling (usually)
• Microscopic Tight Binding models
PROS: “Unbiased” microscopic approach, correct capture of band
structure and hybridization, treats disorder microscopically (combined with
CPA), good agreement with LDA+U calculations
CONS: neglects coulomb interaction effects, difficult to capture nontabulated chemical trends, hard to reach large system sizes
•Phenomenological k.p  Local Moment
PROS: simplicity of description, lots of computational ability, SO coupling
can be incorporated,
CONS: applicable only for metallic weakly hybridized systems (e.g.
optimally doped GaMnAs), over simplicity (e.g. constant Jpd), no good for
deep impurity levels (e.g. GaMnN)
Which theory is right?
Impurity bandit vs Valence Joe
KP Eastwood
Fast principles Jack
Intrinsic properties of (Ga,Mn)As
TcMF  xMn p1/ 3
Mn Mn
Jungwirth, Wang, et al. Phys.
Rev. B 72, 165204 (2005)
As
Mn
Ga
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: Interstitial Mn - a magnetism killer
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
Mn
As
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.
MnGa and MnI partial concentrations
As grown
Materials
calculation
Jungwirth, Wang, et al.
Phys. Rev. B 72, 165204 (2005)
Microscopic defect formation energy calculations:
No signs of saturation in the dependence of MnGa concentration
on total Mn doping
Experimental hole densities: measured by ordinary Hall effect
Open symbols & half closed as grown. Closed symbols annealed
18
p/Mnsub
1.5
20
-3
p (x 10 cm )
15
12
1.0
Low
Compensation
0.5
0.0
0
2
9
4
6
Mntotal(%)
8
Obtain Mnsub
assuming change
in hole density
due to Mn out
diffusion
10
6
3
0
0
2
4
6
Mntotal(%)
8
10
High
compensation
Jungwirth, Wang, et al.
Phys. Rev. B 72, 165204 (2005)
Annealing can very significantly increases hole densities.
Experimental partial concentrations of MnGa and MnI in as grown samples
Theoretical linear dependence of Mnsub on total Mn confirmed
experimentally
Mnsub
Obtain Mnsub &
MnInt assuming
change in hole
density due to
Mn out
diffusion
Jungwirth, Wang, et al.
Phys. Rev. B 72, 165204 (2005)
MnInt
SIMS: measures total Mn concentration.
Interstitials only compensation assumed
Can we have high Tc in Diluted Magnetic Semicondcutors?
NO INTRINSIC LIMIT
NO EXTRINSIC LIMIT
Tc linear in MnGa local (uncompensated)
moment concentration; falls rapidly with
decreasing hole density in heavily
compensated samples.
There is no observable limit to
the amount of substitutional
Mn we can put in
Define Mneff = Mnsub-MnInt
OUTLINE
Intro: Spintronics and Semiconductor
Spintronics
 DMS’s Introduction
 Can we have a high Tc DMS?
 What is the data telling us in GaMnAs: thumbs
up or down?
 Strategies to achieve high Tc DMSs

Closed symbols annealed
140
TC(K)
TTC(K)
(K)
120
C
TC(K)
100
120
100
100
100
80
80
80
60
40
20
0
0
1
2
3
4
5
6
Mntotal(%)
7
8
80
60
60
1.7%Mn
Mn Linear increase of Tc
1.7%
2.2%
Mn Mn = Mn -Mn
1.7%Mn
2.2%
eff
sub
Int
3.4%
Mn
2.2%
Mn
3.4% Mn
4.5%Mn
Mn 180
4.5%
3.4%
Mn
5.6%Mn
Mn 160
5.6%
4.5%
Mn
6.7%Mn
Mn 140
6.7%
5.6%
Mn
9%
Mn
9%Mn
Mn
6.7%
Mn 120
8%
9% Mn 100
TC(K)
180
180
Tc as grown and annealed samples
180
160
160
160
140
180
140
140
160 Open symbols as grown.
120
120
60
40
40
40
20
20
80
60
40
20
20
0 10
90
0
1
1
0 0
0
1
22
33
2
3
0
04 4
15 5 26
●
●
As-Grown
As-Grown
Annealed
As-Grown
Annealed
High
Annealed
compensation
7
6 37 7 4 8 85 9 9610 10
Mn
(%)
Mneff(%)
(%)
total
4Mn
5
6
7
8
9
10
total
Mn
●
with
(%)
total
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
OUTLINE
Intro: Spintronics and Semiconductor
Spintronics
 DMS’s Introduction
 Can we have a high Tc DMS?
 What is the data telling us in GaMnAs: thumbs
up or down?
 Strategies to achieve high Tc DMSs

Getting to higher Tc: Strategy A
- 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 MBE
Strategy 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 B
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)
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)
10% Mn
theory
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)
Factor of ~1.5 Tc enhancement
5% Mn
theory
Mašek, et al. PRB (2006)
Microscopic TBA/CPA or
LDA+U/CPA
Mni 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 MnI in (Al,Ga)As
Mixing P in GaAs more favorable for
suppressing Mnint formation
theory
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)?
Using DEEP mathematics to find a new material
3=1+2
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 says that Mn-doped are also twin DMSs
Masek, et al. PRB (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
As p-orb.
Ga s-orb.
L
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
Li(Mn,Zn)As just one candidate of the whole I(Mn,II)V family
SUMMARY





No intrinsic limit to Tc: need to think in terms of
effective Mn concentration
Should use knowledge gained in (Ga,Mn)As to
create new a la carte materials
Using multiple theoretical approaches and knowing
what each is best and when IS the theory silver
bullet
Alloying P can help Tc
3=2+1
CONCLUSION:
directors cut
It IS true that it takes two to tango
BUT it takes MANY to do the spintronics tango!!
Texas A&M U., U. Texas, U.
Buffalo, Nottingham, U.
Wuerzburg, U. Notre Dame, U. of
Tennessee, U. of Maryland,
UCSB, Penn State, ….
NEW
DMSs ,
2006
BEFORE 2000
Heterostructures
NEXT
2000-2004
EXTRA
MAGNETIC ANISOTROPY
<111>
experiment:
<110>
<100>
Condensation energy depends
on magnetization orientation
M. Abolfath, T. Jungwirth, J. Brum,
A.H. MacDonald, Phys. Rev. B 63, 035305 (2001)
compressive strain

 tensile strain
Resistivity temperature dependence of metallic GaMnAs
Potashnik et al 2001
Lopez-Sanchez and Bery 2003
Hwang and Das Sarma 2005
Ferromagnetic resonance: Gilbert damping

Aa,k()
theory
theory
experiment
Anisotropic Magnetoresistance
exp.
T. Jungwirth, M. Abolfath, J. Sinova, J. Kucera,
A.H. MacDonald, Appl. Phys. Lett. 2002
Tunneling anisotropic
magnetoresistance (TAMR)
AlOx
Au
GaMnAs
Giant magneto-resistance
Au
Gould, Ruster, Jungwirth,
et al., PRL '04
[100]
[100]
[010]
[100]
[010]
[010]
Bistable memory
device with a single
magnetic layer only
(Tanaka and Higo, PRL '01)
ANOMALOUS HALL EFFECT
anomalous velocity:
AHE without disorder
Berry curvature:
M=0
M 0
JpdNpd<S> (meV)
T. Jungwirth, Q. Niu, A.H. MacDonald,
Phys. Rev. Lett. 88, 207208 (2002)
ANOMALOUS HALL EFFECT IN GaMnAs
Clean limit theory
Experiments
Minimal disorder theory
The valence band picture of IR absorption
F =  dRe[()] = e2p / 2mopt
hole density: p=0.2, 0.3, ....., 0.8 nm-3
x=5%
mopt independent of
(within 10%):
· density
· disorder
· magnetic state
exp.
GaAs
J. Sinova, et al. Phys. Rev. B
66, 041202 (2002).
mop  0.24
me
Exps: Singley et al Phys. Rev. Lett. 89, 097203 (2002)
Hirakawa, et al Phys. Rev. B 65, 193312 (2002)
infrared absorption accurate density measurement
FINITE SIZE EXACT DIAGONALIZATION STUDIES
6-band K-L model
GaAs
p=0.33 nm-3, x=4.5%,
compensation from anti-sites
parabolic band
model
6-band K-L model
GaAs
p=0.2 nm-3, x=4.0%,
compensation from anti-sites
f-sum rule accurate within 10 %
S.-R. E. Yang, J. Sinova, T. Jungwirth, Y.P. Shim,
and A.H. MacDonald, PRB 67, 045205 (03)
Possible issues regarding IR absorption
●
Energy dependence of Jpd
●
Localization effects
●
●
Contributions due to impurity states: Flatte’s approach of
starting from isolated impurities
Systematic p and xeff study (need more than 2 meff data
points)
Keeping Score
The effective Hamiltonian (MF) and weak scattering theory (no free
parameters) describe (III,Mn)V shallow acceptor metallic DMSs very
well in the regime that is valid:
• 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 
BUT it is only a peace of the theoretical mosaic with many
remaining challenges!!
TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends,
impurity formation energies, lattice constant variations upon doping
Which theory is right?
Impurity bandit vs Valence Joe
KP Eastwood
Fast principles Jack
OUTLINE


Intro: Spintronics and Semiconductor Spintronics
DMS’s Intro:




Can we have a high Tc DMS?







III-V Semiconductors: the basic picture
Perhaps Pauli was right: the messy reality
Discovery and cool down of expectations
Origin of the coupling between local moments and delocalized carriers
Basic phase diagram
Theoretical approaches: no silver bullet but a good arsenal
Is there an intrinsic limitation from theory
Extrinsic limitations
What is the data telling us in GaMnAs: thumbs up or down?
Strategies to achieve high Tc DMSs


Strategy A: focus on GaMnAs
Strategy B: use what we have learned from DMSs to search for other materials


Increasing coupling strength but still sallow levels
Mathematical strategy: decompose 3
Limits to carrier-mediated
Weak hybrid.
Delocalized holes
long-range coupl.
ferromagnetism in (Mn,III)V
InSb, InAs, GaAs Tc: 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