Post on 21-Jan-2016
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Jairo Sinova (TAMU)
NRI e-WorkshopMaking semiconductors magnetic:
A new approach to engineering quantum materials
Tomas Jungwirth (TAMU, Institute of Physics, Czech Republic, U. of
Nottingham)
NERCSWAN
OUTLINE
• Motivation• Ferromagnetic semiconductor materials:
– (Ga,Mn)As - general picture– Growth and physical limits on Tc
– Related FS materials• Ferromagnetic semiconductors & spintronics
– Tunneling anisotropic magnetoresistive device– Transistors
Ferromagnetic semiconductor research for spintronics:
Motivations and strategies
1.Find new effects in this new material and utilize in conventional metal-based spintronics
2. Develop a three-terminal gatable spintronic device to progress from sensors & memories to transistors & logic
In the 2nd part of the talk we show examples of 1. & 2. and a combination of both principles
Ferromagnetic 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
Need true FSs not FM inclusions in SCs
Mn
Ga
AsMn
Mn
Ga
AsMn
What happens when a Mn is placed in Ga sites:Mn–hole spin-spin interaction
hybridization
Hybridization like-spin level repulsion Jpd SMn shole interaction
Mn-d
As-p
In addition to the Kinetic-exchange coupling, for a single Mn ion, the coulomb interaction gives a trapped hole (polaron) which resides just above the valence band
5 d-electrons with L=0 S=5/2 local moment
intermediate acceptor (110 meV) hole
Mn
Ga
AsMn
EF
DO
S
Energy
spin
spin
Transition to a ferromagnet when Mn concentration increasesGaAs:Mn – extrinsic p-type semiconductor
FM due to p-d hybridization (Zener local-itinerant kinetic-exchange)
valence band As-p-like holes
As-p-like holes localized on Mn acceptors
<< 1% Mn ~1% Mn >2% Mn
onset of ferromagnetism near MIT
Mn
Ga
As
Mn
Ga
AsMn
(Ga,Mn)As synthesis
•Low-T MBE to avoid precipitation
•High enough T to maintain 2D growth
need to optimize T & stoichiometry for each Mn-doping
•Inevitable formation of interstitial Mn-double-donors compensating holes and moments need to anneal out but without loosing MnGa
high-T growth
optimal-T growth
Interstitial Mn out-diffusion limited by surface-oxide
GaMnAs
GaMnAs-oxide
Polyscrystalline20% shorter bonds
MnI++
O
Optimizing annealing-T another key factorRushforth et al, ‘08
x-ray photoemission
Olejnik et al, ‘08
10x shorther annealing with etch
OUTLINE
• Motivation• Ferromagnetic semiconductor materials:
– (Ga,Mn)As - general picture– Growth and physical limits on Tc
– Related FS materials• Ferromagnetic semiconductors & spintronics
– Tunneling anisotropic magnetoresistive device– Transistors
0 1 2 3 4 5 6 7 8 9 100
20
40
60
80
100
120
140
160
180
TC(K
)
Mntotal
(%)
“... Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..” Yu et al. ‘03
“…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...” Mack et al. ‘08
“Combinatorial” approach to growthwith fixed growth and annealing T’s
Tc limit in (Ga,Mn)As remains open
2008Olejnik et al
185K!!
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 IDENTIFICATION OF AN INTRINSIC LIMIT NO EXTRINSIC LIMIT
(lines – theory, Masek et al 05)
Relative Mn concentrations obtained through hole density measurements and saturation moment densities measurements.
Qualitative consistent picture within LDA, TB, and k.p
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
Weak hybrid.Delocalized holeslong-range coupl.
Strong hybrid.Impurity-band holesshort-range coupl.
InSb
GaP
d5
(Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host
Other (III,Mn)V’s DMSs
Mean-field butlow Tc
MF
Large TcMF but
low stiffness
Kudrnovsky et al. PRB 07
III = I + II Ga = Li + Zn
GaAs and LiZnAs are twin SC
Masek, et al. PRB (2006)
LDA+U says that Mn-doped are also twin DMSs
n and p type doping through Li/Zn stoichiometry
No solubility limit for group-II Mn
substituting for group-II Zn !!!!
OUTLINE
• Motivation• Ferromagnetic semiconductor materials:
– (Ga,Mn)As - general picture– Growth and physical limits on Tc
– Related FS materials• Ferromagnetic semiconductors & spintronics
– Tunneling anisotropic magnetoresistive device– Transistors
AMRAMR~ 1% MR effect~ 1% MR effect
TMRTMR~ 100% MR effect~ 100% MR effect
TAMRTAMR
) vs.( ~ IMvgExchange split & SO-coupled bands:
Exchange split bands:
)()(~ TDOSTDOS
)(~ MTDOS
Au
discovered in (Ga,Mn)As Gold et al. PRL’04
As-p-like holes
Strong exchange splitting & SO coupling in (Ga,Mn)As
Standard MBE techniques for high-quality tunneling structures
MnGa
As
Mn
ab intio theory Shick, et al, PRB '06, Park, et al, PRL '08
TAMR in metal structures
experiment Park, et al, PRL '08
Also studied by Parkin et al., Weiss et al., etc.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0
2
4
6
8
10
0V 3V 5V 10V
carr
ier
dens
ity
[ 10
19 c
m-3
]
GaMnAs layer thickness [nm]
Gating of highly doped (Ga,Mn)As: p-n junction FET
p-n junction depletion estimates
Olejnik et al., ‘08
~25% depletion feasible at low voltages
(Ga,Mn)As/AlOx FET with large gate voltages, Chiba et al. ‘06
20 22 24 26 28 30 32 34
18.6
18.8
19.0
19.2
19.4
[10
-3c
m]
T [K]
Vg = 0V
22.5
23.0
23.5
24.0
24.5 Vg = 3V
20 22 24 26 28 30 32 34
-200
-100
0
100
d/d
T [1
0-6
T [K]
-300
-200
-100
0
AM
RIncreasing and decreasing AMR and Tc with depletion
Tc Tc
30 40 50 60 70 80 90 100
100
200
65K62K
dR/d
T
T (K)
depletion accumulation
Persistent variations of magnetic properties with ferroelectric
gates
Stolichnov et al., Nat. Mat.‘08
exy = 0.1%
exy = 0%
Electro-mechanical gating with piezo-stressors
Rushforth et al., ‘08
Strain & SO
Electrically controlled magnetic anisotropies via strain
Single-electron transistor
Two "gates": electric and magnetic
(Ga,Mn)As spintronic single-electron transistor
Huge, gatable, and hysteretic MR
Wunderlich et al. PRL ‘06
AMR nature of the effect
normal AMR Coulomb blockade AMR
GMMGG C
C
e
MVMVVCQ
C
QQU
)(&)]([&
2
)(0
20
electric && magneticmagneticcontrol of Coulomb blockade oscillations
n-1 n n+1 n+2n-1 n n+1 n+2
EC
QQindind = = nnee
QQindind = (= (n+1/2)n+1/2)eeQ0
Q0
e2/2C
Q
D e
MQQVdQU
0
'' )()(
[010]
M[110]
[100]
[110][010]
SO-coupling (M)
Source Drain
GateVG
VDQ
Single-electron charging energy controlled by Vg and M
•CBAMR if change of |CBAMR if change of |((MM)| ~ )| ~ ee22//22CC
•In our (Ga,Mn)As ~ meV (~ 10 In our (Ga,Mn)As ~ meV (~ 10 Kelvin)Kelvin)
•In room-T ferromagnet change of |In room-T ferromagnet change of |((MM)|~100K )|~100K
•Room-T conventional SET (e2/2C >300K) possible
Theory confirms chemical potential anisotropies in (Ga,Mn)As& predicts CBAMR in SO-coupled room-Tc metal FMs
Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device
0
ONONOFFOFF
1
0
ONON OFFOFF
1
VDD
VA VB
VA
VB
Vout
0
0
0
OFFOFFONON
ONON
OFFOFF
0
0
1
1
ONONOFFOFF
A B Vout0 0 01 0 10 1 11 1 1
0
01
ONON
OFFOFF
0
0
OFFOFF
1
ONON
1
1
1
1
OFFOFF
ONON
1
1
ONON
OFFOFF
1
“OR”
Nonvolatile programmable logic
VDD
VA VB
VA
VB
Vout
Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device
0
ONONOFFOFF
1
0
ONON OFFOFF
1
A B Vout0 0 01 0 10 1 11 1 1
“OR”
Nonvolatile programmable logicNonvolatile programmable logic
Physics of SO & exchange
SET
Resistor
Tunneling device
Chemical potential CBAMR
Tunneling DOS TAMR
Group velocity & lifetime AMR
Device design
Materials
metal FMs
FSs
FSs and metal FS with strong SO
Allan MacDonald U of Texas
Tomas JungwirthInst. of Phys. ASCRU. of Nottingham
Joerg WunderlichCambridge-Hitachi
Laurens MolenkampWuerzburg
Mario BorundaTexas A&M U.
Other collaborators: Bernd Kästner, Satofumi Souma, Liviu Zarbo, Dimitri Culcer , Qian Niu, S-Q Shen, Brian Gallagher, Tom Fox, Richard
Campton
Alexey KovalevTexas A&M U.
Liviu ZarboTexas A&M U.
Matching TAMU funds
Xin LiuTexas A&M U.
Bryan GallagherU. Of Nottingham
Sankar Das SarmaU. of Maryland
30
Conclusion (checks of theory)
In the metallic optimally doped regime GaMnAs is well described by a disordered-valence band picture: both dc-data and ac-data are consistent with this scenario.
The effective Hamiltonian (MF) and weak scattering theory (no free parameters) describe (III,Mn)V metallic DMSs very well in the optimally annealed regime:
• 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
TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity formation energies, lattice constant variations upon doping