Transcript Slide 1

Spintronics in metals and semiconductors
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. Tunneling anisotropic magnetoresistance in transition metals
2. Ferromagnetism in (Ga,Mn)As and related semiconductors
3. Spintronic transistors
Spintronics: Spin-orbit & exchange interactions
nucleus rest frame
I  Qv
E
electron rest frame
Q
4 0 r
 spin-orbit interaction
3
H SO
r
0 I  r
B
4 r 3
Thomas precession
1
 B   0 0 v  E  2 v  E
c
g B
e

SB 
S vE
2
2
2mc
DOS
Coulomb repulsion & Pauli exclusion principle  exchange interaction
 ferromagnetism
AMR
TMR
~ 1% MR effect
Exchange int.:
Spin-orbit int.:
~ 100% MR effect

M


 ~ vg (M vs.
I
)
 
magnetic anisotropy
TAMR

TDOS(M )
M (B)
Au
Exchange int.:
TDOS()  TDOS()
AFM-FM exchange bias
TAMR in CoPt structures
ab intio theory
experiment
Shick, et al, PRB '06, Park, et al, PRL '08
Park, et al, PRL '08
TAMR in TM structures
Consider uncommon TM combinations
Mn/W  ~100% TAMR Shick, et al,
unpublished
spontaneous moment
Consider both Mn-TM FMs & AFMs
exchange-spring rotation of the AFM
Scholl et al. PRL ‘04
Proposal for AFM-TAMR: first microelectronic device with active AFM component
Shick, et al,
unpublished
Outline
1. Tunneling anisotropic magnetoresistance in transition metals
2. Ferromagnetism in (Ga,Mn)As and related semiconductors
3. Spintronic transistors
TM-based  semiconducting multiferroic spintronics
sensors & memories  transistors & logic
Magnetic materials
spintronic magneto-sensors,
memories
Ferroelectrics/piezoelectrics
electro-mechanical transducors,
large & persistent el. fields
Semiconductors
transistors, logic,
sensitive to doping and
electrical gating
Ferromagnetic semiconductors
Need true FSs not FM inclusions in SCs
Ga
Mn
As
GaAs - standard III-V semiconductor
Group-II Mn - dilute magnetic moments
& holes
(Ga,Mn)As - ferromagnetic
semiconductor
Mn
GaAs:Mn – extrinsic p-type semiconductor
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 synthesis
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
Polyscrystalline
20% shorter bonds
Interstitial Mn out-diffusion limited by surface-oxide
O
GaMnAs-oxide
x-ray photoemission
GaMnAs
MnI++
Olejnik et al, ‘08
10x shorther annealing with etch
Optimizing annealing time & temperature
(removing int. Mn & keeping MnGa in place)
is essential
Rushforth et al, unpublished
Tc limit in (Ga,Mn)As remains open
180
160
140
120
Yu et al. ‘03
100
TC(K)
Indiana & California (‘03): “ .. Ohno’s ‘98
Tc=110 K is the fundamental upper limit ..”
80
Nottingham & Prague
(’08): Tc up to 188 K
so far
60
40
California (‘08): “…Tc =150-165 K
independent of xMn>10% contradicting
Zener kinetic exchange ...”
Mack et al. ‘08
“Combinatorial” approach to growth
with fixed growth and annealing cond.
20
0
0
1
2
3
4
5
6
7
Mntotal(%)
?
8
9
10
Other (III,Mn)V’s DMSs
Kudrnovsky et al. PRB 07
Weak hybrid.
Mean-field but
low TcMF
InSb
Strong hybrid.
Delocalized holes
long-range coupl.
d5
Impurity-band holes
short-range coupl.
Large TcMF but
low stiffness
GaP
(Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host
Other DMS candidates
III = I + II  Ga = Li + Zn
GaAs and LiZnAs are twin SC
(Ga,Mn)As and Li(Zn,Mn)As
should be twin ferromagnetic SC
But Mn isovalent in Li(Zn,Mn)As
Masek et al. PRL 07
 no Mn concentration limit and self-compensation
 possibly both p-type and n-type ferromagnetic SC
(Li / Zn stoichiometry)
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)???

Critical contribution at Tc to d/dT like TM FMs
(Ga,Mn)As
(Prague Nottingham)
Fe
Fisher & Langer ’68
Ni
Novak et al., ‘08
d/dT ~ cv
F ~ d-
 



2
 (T ) ~ ( Ri , T ) ~ J pd [ Si  S0    Si    S0 ]
uncor  small
Tc
uncor
2
~ ( S ) 
k 0
kd  1
kd ~ 1
(k ~ kF ~ 1/ d )
EuCdSe
Ni
d / dT ~ d / dT ~ cv
MF
(k ~ kF ~ 0) ~ 
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
p
Beff
Strong SO due to the As p-shell (L=1) character of the top of the valence band
Beff
Bex + Beff
TAMR discovered
in (Ga,Mn)As Gold et al. PRL’04
SO couped carries scattering coherently off
Coulomb & polarized-magnetic potential of Mn
AMR in DMSs
MnGa
~
magnetic. only
>
>
max AMR
sign and magnitude (numerical)
consistent with experiment
Remark: Extraordinary MRs & quantum coherent transport phenomena
dirty metal
UCF
Outline
1. Tunneling anisotropic magnetoresistance in transition metals
2. Ferromagnetism in (Ga,Mn)As and related semiconductors
3. Spintronic transistors
Gating of the highly doped (Ga,Mn)As: p-n junction FET
p-n junction depletion estimates
-3
carrier density [ 10 cm ]
10
8
19
0V
3V
5V
10V
6
4
2
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
GaMnAs layer thickness [nm]
~25% depletion feasible at low voltages
Olejnik et al., ‘08
Increasing  and decreasing AMR, Tc, coercivity with depletion
Vg = 0V
Vg = 3V
100
24.5
0
19.0
23.5
18.8
23.0
-6
24.0
19.2
d/dT [10 
-3
 [10 cm]
19.4
0
-100
-100
-200
-300
-200
18.6
20 22 24 26 28 30 32 34
22.5
20 22 24 26 28 30 32 34
T [K]
T [K]
Vg [V]
-1.0
0.0
1.0
2.0
2.5
3.0
3.5
1.00
0.98
0.96
-400 -200
0
B [Oe]
200
400
300
coercive field [Oe]
R(B)/R(B=0)
AMR
1.02
250
200
150
-1
0
1
Vg[V]
2
3
4
Persistent variations of magnetic properties with ferroelectric gates
Stolichnov et al.,
Nat. Mat.‘08
62K
65K
depletion
accumulation
dR/dT
200
100
30
40
50
60
T (K)
70
80
90
100
Electro-mechanical gating with piezo-stressors
exy = 0.1%
exy = 0%
Strain & SO 
Rushforth et al., ‘08
Electrically controlled magnetic anisotropies
(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
Device design
Physics of SO & exchange
Chemical potential
 CBAMR
Materials
TM FMs,
MnAs, MnSb
SET
(III,Mn)V, I(II,Mn)V
DMSs
Tunneling DOS
 TAMR
Tunneling device
Resistor
Mn-based TM FMs&AFMs
Group velocity & lifetime
 AMR
TM FMs