Transcript Spintronics in ferromagnetic semiconductor (Ga,Mn)As

Spintronics in ferromagnetic semiconductor (Ga,Mn)As
Tomas Jungwirth
Institute of Physics ASCR
Alexander Shick, Karel Výborný, Jan Zemen,
Jan Masek, Vít Novák, Kamil Olejník, et al.
Hitachi Cambridge
Jorg Wunderlich, Andrew Irvine, David Williams,
Elisa de Ranieri, Byonguk Park, Sam Owen, et al.
University of Nottingham
Bryan Gallagher, Tom Foxon,
Richard Campion, Kevin Edmonds,
Andrew Rushforth, Chris King et al.
Texas A&M
Jairo Sinova, et al.
University of Texas
Allan MaDonald, et al.
Outline
1. Curie temperature and critical transport anomaly
2. Low-voltage control of ferromagnetism in a p-n junction
3. Coulomb-blockade AMR single electron transistor
Electric field controlled spintronics
From storage to logic
HDD, MRAM
STT MRAM,
controlled by
Magnetic field
spin-polarized
charge current
Magnetic Transistor
control by
Electric field
Low-voltage controlled
magnetization and MR effects
FS spintronic transitor
J. Wunderlich, et al. 06
Ferromagnetic semiconductor GaAs:Mn
DOS
spin 
EF
<< 1% Mn
~1% Mn
>2% Mn
Energy
spin 
onset of ferromagnetism near MIT
As-p-like holes localized on Mn acceptors
valence band As-p-like holes
Ga
As-p-like holes
FM due to p-d hybridization
(Zener local-itinerant kinetic-exchange)
Mn
Mn-d-like local
moments
Mn
As
(Ga,Mn)As growth
high-T growth
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-donors
compensating holes and moments
 need to anneal out
optimal-T growth
Optimizing
annealing time &
temperature
(removing int. Mn &
keeping MnGa in
place)
again for each Mndoping is essential
Olejnik et al., ‘08
Tc limit in (Ga,Mn)As remains open
180
160
140
120
TC(K)
100
80
Nottingham & Prague
(’08): Tc up to 188 K
so far
60
40
Mack et al. ’08 : “…Tc =150-165 K
20
independent of xMn>10% contradicting
Zener kinetic exchange ...”
Mack et al. ‘08
“Combinatorial” approach to growth
with fixed growth and annealing conditions
0
0
1
2
3
4
5
6
7
Mntotal(%)
?
8
9
10
Towards spintronics in (Ga,Mn)As: FM & transport
Dense-moment MS
F<< d-
Dilute-moment MS
F~ d-
Eu - chalcogenides
Critical contribution to resistivity at Tc
~ magnetic susceptibility
Broad peak near Tc disappeares with
annealing (higher uniformity)???

 



2
 (T ) ~ ( Ri , T ) ~ J pd [ Si  S0    Si    S0  ]
uncor  small
Tc
uncor
 2
~ (S ) 
k 0
kd  1
kd ~ 1
(k ~ k F ~ 1 / d )
EuCdSe
Ni
d / dT ~ d / dT ~ cv
MF
(k ~ kF ~ 0) ~ 
Tc
d/dT singularity at Tc – consistent with kF~d-
Novak, et al.‘08
Annealing sequence
Optimized materials with x=4-12.5%
and Tc=80-185K
Remarkably
universal both
below and above Tc
Ferromagnetism & strong spin-orbit coupling
Ga
Mn
As
Mn
As-p-like holes
H SO


 
 eS   p  1 dV (r ) 
 r
    Beff  
   S  L

 mc   mc  er dr 
V
s
Beff
Strong SO due to the As p-shell (L=1) character of the top of the valence band
Beff
Bex + Beff
p
Electric field control of ferromagnetism
k.p kinetic exchange model predicst sensitivity to strains ~10-4
Strain & SO 
Rushforth et al., ‘08
slow and requires ~100V
and hole-density variations of ~1019-1020 cm-3
Gating of the highly doped (Ga,Mn)As: p-n junction
p-n junction depletion estimates
2x 1019 cm-3
~25% depletion feasible at low voltages
Olejnik et al., ‘08
Basic charcteristics of the device
can “deplete”
magnetization
at low Vg
can deplete
charge at low Vg
0
1.02
AMR(Vg) = R()/Rav
30% AMR
tuneable by
low Vg
-1V
3V
315
45
225
low Vg
dependent
90
competition
of uniaxial
and cubic
anisotropies
135
1.00
0.98
0.96 270
0.98
1.00
1.02
180
Magnetization switching
by 10ms low-Vg pulses
Due to the Vg-dependent
Stoner-Wolfarth “diamond”
(tuneable uniaxial and cubic
anisotropy terms)
dRc/dH
normalized dRc/dH
[0-10]
-6
-1V
-6.000
-5.836
-5.671
-5.507
-5.342
-5.178
-5.014
-4.849
-4.685
-4.521
-4.356
-4.192
-4.027
-3.863
-3.699
-3.534
-3.370
-3.205
-3.041
-2.877
-2.712
-2.548
-2.384
-2.219
-2.055
-1.890
-1.726
-1.562
-1.397
-1.233
-1.068
-0.9041
-0.7397
-0.5753
-0.4110
-0.2466
-0.08219
0.08219
0.2466
0.4110
0.5753
0.7397
0.9041
1.068
1.233
1.397
1.562
1.726
1.890
2.055
2.219
2.384
2.548
2.712
2.877
3.041
3.205
3.370
3.534
3.699
3.863
4.027
4.192
4.356
4.521
4.685
4.849
5.014
5.178
5.342
5.507
5.671
5.836
6.000
40
30
20
6
[100]
10
[100]
0
10
20
30
-1V
3V
[0-10]
3V
(Ga,Mn)As spintronic single-electron transistor
Wunderlich et al. PRL ‘06
Huge, gatable, and hysteretic MR
Single-electron transistor
Two "gates": electric and magnetic
AMR nature of the effect
normal AMR
Coulomb blockade AMR
Single-electron charging energy controlled by Vg and M
Source
QQind0 = (n+1/2)e
Q VD
Drain
QQ0ind = ne
Gate
VG
eE2/2C
C 
n-1

Q( M )
'
'
U   dQ VD ( Q ) 
e
0
n
n+1
n+2
Q


( Q  Q0 )
( M ) C
U
& Q0  CG [ VG  VM ( M )] & VM 
2C
e
CG
[110]
F
2
[100]
[110]
electric
& magnetic
control of Coulomb blockade oscillations
[010] M
[010]
SO-coupling 
(M)
Theory confirms chemical potential anisotropies in (Ga,Mn)As
& predicts CBAMR in SO-coupled room-Tc metal FMs
• CBAMR if change of |(M)| ~ e2/2C
• In our (Ga,Mn)As ~ meV (~ 10 Kelvin)
• In room-T ferromagnet change of
|(M)|~100K
• Room-T conventional SET
(e2/2C >300K) possible
Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device
1
0
V DD
ON
OFF
ON
VB
ON
OFF
VB
ON
OFF
10 Vout
10
ON
OFF
1
0
VA
1
0
1
0
0
1
VA
ON
OFF
OFF
ON
OFF
“OR”
A
0
1
0
1
B
0
0
1
1
Vout
0
1
1
1
Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device
1
0
V DD
ON
OFF
1
0
VA
VB
ON
OFF
Vout
VB
VA
“OR”
A
0
1
0
1
B
0
0
1
1
Vout
0
1
1
1