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.
University of Nottingham
Bryan Gallagher, Tom Foxon,
Richard Campion, Kevin Edmonds,
Andrew Rushforth, Chris King et al.
Hitachi Cambridge
Jorg Wunderlich, Andrew Irvine, David Williams,
Elisa de Ranieri, Sam Owen, 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unpublished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unpublished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
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
Note: TAMR discovered
in (Ga,Mn)As Gold et al. PRL’04
(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-T another key factor
Rushforth et al, ‘08
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 185K
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 T’s
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 ]
2
0 ~ uncor ~ (S ) 
(q ~ kF ~ 0) ~ 
(q ~ kF ~ 1/ d ) ~ cv
Fisher&Langer ‘68
d/dT
Tc
Scattering off short range
correlated spin-fluctuation
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 and Tc with depletion
Vg = 0V
Vg = 3V
24.5
-3
 [10 cm]
19.4
24.0
19.2
19.0
23.5
18.8
23.0
18.6
20
22
24
26
28
30
32
34
22.5
T [K]
-6
d/dT [10 
AMR
100
0
0
-100
-100
-200
-300
-200
20
22
24
26
28
T [K]
30
32
34
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
END
Dawn of spintronics
Magnetoresistive read element
Inductive read/write element
Anisotropic magnetoresistance (AMR) – 1850’s  1990’s
Giant magnetoresistance (GMR) – 1988  1997
MRAM – universal memory
fast, small, low-power, durable, and non-volatile
2006- First commercial 4Mb MRAM
Based on Tunneling Magneto-Resistance (similar to GMR but insulating spacer)
RAM chip that actually won't forget  instant on-and-off computers
DOS
Giant Magneto-Resistance
>
P AP
  
~ 10% MR effect
Tunneling Magneto-Resistance
DOS  DOS
~ 100% MR effect
Spin Transfer Torque writing
Dilute moment nature of ferromagnetic semiconductors
Key problems with increasing MRAM capacity (bit density):
- Unintentional dipolar cross-links
- External field addressing neighboring bits
One
10-100x weaker dipolar fields
10-100x smaller Ms
Ga
As
Mn
10-100x smaller currents for switching
Mn
coupling strength / Fermi energy
Magnetism in systems with coupled dilute moments
and delocalized band electrons
band-electron density / local-moment density
(Ga,Mn)As
Hole transport and ferromagnetism at relatively large dopings
conducting p-type GaAs:
- shallow acc. (C, Be) ~ 1018 cm-3
- Mn ~1020 cm-3
Non-equilibrium growth - technological difficulties
Photogenerated
ferromagnetism
Electric-field controlled
ferromagnetism in FET or piezo/FM hybrid
Vgate
ħw
Ferro SC
Ferro SC
Magnetization
Magnetization
GaSb
B (mT)
Variable controlled strain using a Piezo stressor
A.W. Rushforth, J. Zemen, K. Vyborny, et al. arXiv:0801.0886
Strain induced by piezo voltage +/- 150V:
~ 2 10-4 (at 50K)
M. Overby, et al., arXiv:0801.4191
Easy axis rotation by
50 deg for
Vpiezo = -150V  +150V
Fast Precessional switching via gatevoltage
(I)
(II)
Beff
M=(M,0,0)
(III)
M=(0,M,0)
M
Beff
Beff
0
VG = V0, t < 0
VG
0
VG = VC, t = 0
x
0
x
VG = V0, t > Δt90°
VC
Δt90°
(a)
time
V0
M=(0,0,M)
(I)
z
(II)
z
Beff
VG
VG = V0, t < 0
(III)
Beff M=(0,0,-M)
Beff
VG = VC, t = 0
VG = V0, t > Δt180°
VC
Δt180°
(b)
V0
time
Spintronics with spin-currents only
Magnetic domain “race-track” memory
Spin Hall effect detected optically
in GaAs-based structures
Same magnetization achieved
by external field generated by
a superconducting magnet
with 106 x larger dimensions &
106 x larger currents
p
n
n
SHE mikročip, 100A
SHE detected elecrically in metals
Cu
supercondicting magnet, 100 A
SHE edge spin accumulation can be
extracted and moved further into the circuit
Spintronics in nominally non-magnetic materials
Datta-Das transistor
Spin Hall effect
spin-dependent deflection  transverse edge spin polarization
_
__
FSO
FSO
I
intrinsic
skew scattering
Magnetization
Spintronics explores new avenues for:
• Information reading


Current
• Information reading & storage
Tunneling magneto-resistance sensor and memory bit
• Information reading & storage & writing
Current induced magnetization switching
• Information reading & storage & writing & processing
Spintronic transistor:
magnetoresistance controlled by gate voltage
Ga
As
• New materials
Ferromagnetic semiconductors, Multiferroics
Non-magnetic SO-coupled systems
Mn
Mn