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Transcript of Tomas Jungwirth, Jan Mašek, Alexander Shick Karel Výborný, Jan Zemen, Vít Novák, et al. Bryan...
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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
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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.
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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
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Mn
Ga
AsMn
Mn–hole spin-spin interaction
hybridization
Hybridization like-spin level repulsion Jpd SMn shole interaction
Mn-d
As-p
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Heff
= Jpd
<shole> || -x
MnAs
Ga
heff
= Jpd
<SMn> || x
Hole Fermi surfaces
Ferromagnetic Mn-Mn coupling mediated by holes
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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
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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
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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
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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
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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!
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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- 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
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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
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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
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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
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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)?
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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 !!!!
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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
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● 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?
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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
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EXTRA
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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:
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Potashnik et al 2001Lopez-Sanchez and Bery 2003Hwang and Das Sarma 2005
Resistivity temperature dependence of metallic GaMnAs
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theory theory
experiment
Ferromagnetic resonance: Gilbert damping
Aa,k()
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I
M
Anisotropic Magnetoresistance
exp.
T. Jungwirth, M. Abolfath, J. Sinova, J. Kucera, A.H. MacDonald, Appl. Phys. Lett. 2002
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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
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ANOMALOUS HALL EFFECT IN GaMnAs
Experiments
Clean limit theory
Minimal disorder theory
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-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
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(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
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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
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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