Transcript talk-czech tech. univ.-08

Spintronics
Tomas Jungwirth ([email protected])
Institute of Physics ASCR, Prague
University of Nottingham
1. Current spintronics in HDD read-heads and MRAMs
2. Basic physical principles of spintronics
3. Spintronics research – overview
4. Ferromagnetic semiconductors
5. Single-electron spintronic transistor
6. Summary
Hard disk drive
First hard disc (1956) - classical electromagnet for read-out
1 bit: 1mm x 1mm
MB’s
From PC hard drives ('90)
to micro-discs - spintronic read-heads
1 bit: 10-3mm x 10-3mm
10’s-100’s GB’s
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
1. Current spintronics in HDD read-heads and MRAMs
2. Basic physical principles of spintronics
3. Spintronics research – overview
4. Ferromagnetic semiconductors
5. Single-electron spintronic transistor
6. Summary
Spin-orbit coupling from classical E&M and postulated electron spin
nucleus rest frame
electron rest frame
I  Qv
1
B   0 0 v  E  2 v  E
c
H SO
E
Q
40 r
3
r
0 I  r
B
4 r 3
g B
e

SB 
S vE
2
2
2mc
Lorentz transformation  Thomas precession
e… it’s all about spin and charge
of electron communicating
SO coupling from relativistic QM
quantum mechanics & special relativity  Dirac equation
E=p2/2m
E2/c2=p2+m2c2
E ih d/dt
Spin
(E=mc2 for p=0)
p -ih d/dr
Anisotropic Magneto-Resistance
& HSO (2nd order in v/c around
the non-relativistic limit)
~ 1% MR effect
Current sensitive to magnetization direction
Ferromagnetism = Pauli exclusion principle & Coulomb repulsion
etotal wf antisymmetric
e-
= orbital wf antisymmetric * spin wf symmetric
(aligned)
DOS
e-
DOS
• Robust (can be as strong as bonding in solids)
• Strong coupling to magnetic field
(weak fields = anisotropy fields needed
only to reorient macroscopic moment)
DOS
Giant Magneto-Resistance
>
P AP
  
~ 10% MR effect
Tunneling Magneto-Resistance
DOS  DOS
~ 100% MR effect
1. Current spintronics in HDD read-heads and MRAMs
2. Basic physical principles of spintronics
3. Spintronics research – overview
4. Ferromagnetic semiconductors
5. Single-electron spintronic transistor
6. Summary
GMR
~ 1% MR effect
~ 10% MR effect
<
AMR
FM & SO-coupling
(M )
FM only (  )
+ larger MR
+ linear sensing, low-noise
- low MR, low-resistance
-
TAMR
AlOx
Au
TDOS
low-resistance, non-linear, spin-coherence,
exchange biasing or interlayer coupling,
higher noise
TMR
Au
~ 100% MR effect
TDOS  TDOS
(M )
Combining “+” and eliminating “-” of
AMR and TMR(GMR)
+ very large MR, high resistance,
bistable  memory
-
non-linear, spin-coherence, exchange
biasing, higher noise
Spin Transfer Torque writing
Semiconducting multiferroic structures
Ferromagnetic/magnetostrictive
magneto-sensors, transducors,
memory, storage
piezo/FM
hybrids
FM semiconductors
Semicondicting/gatable
Ferroelectric/piezoelectric
electro-sensors, transducors,
memory
FeFET
transistors, processors
Systems integrating all three basic elements of current microelectronics
Photogenerated
ferromagnetism
Electric-field controlled
ferromagnetism in FET or piezo/FM hybrid
Vgate
ħw
Ferro SC
Ferro SC
Magnetization
Magnetization
GaSb
B (mT)
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
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
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
“NAND”
A
0
1
0
1
B
0
0
1
1
Vout
1
1
1
0
Spintronics with spin-currents only
Magnetic domain “race-track” memory
Spintronics in nominally non-magnetic materials
Datta-Das transistor
Spin Hall effect
spin-dependent deflection  transverse edge spin polarization
skew scattering
intrinsic
_
__
side jump
FSO
FSO
I
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
1. Current spintronics in HDD read-heads and MRAMs
2. Basic physical principles of spintronics
3. Spintronics research – overview
4. Ferromagnetic semiconductors
5. Single-electron spintronic transistor
6. Summary
Dilute moment ferromagnetic semiconductors
More tricky than just hammering an iron nail in a silicon wafer
Ga
Mn
As
Mn
GaAs - standard III-V semiconductor
Group-II Mn - dilute magnetic moments
& holes
(Ga,Mn)As - ferromagnetic
semiconductor
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 coupling between Mn local
moments mediated by SC
valence band holes
Mn
Mn-d-like local
moments
Mn
As
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
Strong spin-orbit coupling
Ga
As-p-like holes
Mn
As
Mn
Mn-d-like local
moments
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
Ga
As
• (Ga,Mn)As ferromagnetic semiconductor
• dilute moment system  e.g., low currents needed for writing
• Mn-Mn coupling mediated by spin-polarized & spin-orbit
coupled delocalized holes  spintronics
• tunability of magneto-electronics properties by same means
as in conventional semiconductors – doping, gating (normal,
piezo).
• but maximum Curie temperature so far below 200 K
Mn
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
1. Current spintronics in HDD read-heads and MRAMs
2. Basic physical principles of spintronics
3. Spintronics research – overview
4. Ferromagnetic semiconductors
5. Single-electron spintronic transistor
6. Summary
(Ga,Mn)As spintronic single-electron transistor
Spintronic transistor - magnetoresistance controlled by gate voltage
Bptp
B90
Huge hysteretic
low-field MR
Sign & magnitude
tunable by small
gate valtages
I
B0
Strong dependence
on field angle
hints to AMR origin
Anisotropic magnetoresistive effect
AMR in the resistor
AMR in the transistor
Single electron transistor
Narrow channel SET
dots due to disorder potential fluctuations
(similar to non-magnetic narrow-channel GaAs or Si SETs)
Coulom blockade
oscillations
low Vsd  blocked
due to SE charging
CB oscillation shifts by magnetication rotations
magnetization angle 
At fixed Vg peak  valley
or valley  peak

MR comparable to CB
negative or positive MR(Vg)
Single Electron Transistor
Source
Q VD
Drain
• Vg = 0
Q
Q2
U   dQ VD ( Q ) &VD  Q / C  U 
2C
0
'
Gate
VG
e2
 k BT
2C
'
 Coulomb blockade
• Vg  0
( Q  Q0 )2
U
& Q0  CGVG
2C
QQind0 = (n+1/2)e
Q=ne - discrete
Q0=CgVg - continuous
QQ0ind = ne
eE2/2C
C 
n-1
n
n+1
n+2
Q0=-ne  blocked
Q0=-(n+1/2)e  open
Coulomb blockade AMR
QQind0 = (n+1/2)e
QQ0ind = ne
eE2/2C
C 
n-1
n
n+1
n+2
[110]
F
[100]
[110]

Q( M )
'
'
U   dQ VD ( Q ) 
e
0
Q


( Q  Q0 )
( M ) C
U
& Q0  CG [ VG  VM ( M )] & VM 
2C
e
CG
2
electric
& magnetic
control of Coulomb blockade oscillations
[010] M
[010]
SO-coupling 
(M)
1. Current spintronics in HDD read-heads and MRAMs
2. Basic physical principles of spintronics
3. Spintronics research – overview
4. Ferromagnetic semiconductors
5. Single-electron spintronic transistor
6. Summary
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