Transcript talk-czech tech. univ.-07

SPINTRONICS
Tomáš Jungwirth
Fyzikální ústav AVČR
University of Nottingham
1. Current spintronics in HDD read-heads and memory chips
2. Physical principles of operation of current spintronic devices
3. Research at the frontiers of spintronics
4. Summary
Current spintronics applications
First hard disc (1956) - classical electronics for read-out
1 bit: 1mm x 1mm
MByte
From PC hard drives ('90)
to micro-discs - spintronic read-heads
GByte
1 bit: 10-3mm x 10-3mm
HARD DISKS
HARD DISK DRIVE READ HEADS
spintronic read heads
horse-shoe read/write heads
Anisotropic magnetoresistance (AMR) read head
1992 - dawn of spintronics
Appreciable sensitivity, simple design,
scalable, cheap
Giant magnetoresistance (GMR) read head
1997
High sensitivity
MEMORY CHIPS
.
DRAM (capacitor) - high density, cheep x slow,
high power, volatile
.
SRAM (transistors) - low power, fast x low density,
expensive, volatile
.
Flash (floating gate) - non-volatile x slow, limited life,
expensive
Operation through electron charge manipulation
MRAM – universal memory
fast, small, non-volatile
First commercial 4Mb MRAM
Tunneling magneto-resistance effect (TMR)
RAM chip that won't forget
↓
instant on-and-off computers
MRAM – universal memory
fast, small, non-volatile
First commercial 4Mb MRAM
Tunneling magneto-resistance effect (TMR)
RAM chip that won't forget
↓
instant on-and-off computers
1. Current spintronics in HDD read-heads and memory chips
2. Physical principles of current spintronic devices operation
3. Research at the frontiers of spintronics
4. Summary
Electron has a charge (electronics) and
spin (spintronics)
Electrons do not actually “spin”,
they produce a magnetic moment that is
equivalent to an electron spinning clockwise
or anti-clockwise
quantum mechanics & special relativity  particles/antiparticles & spin
E=p2/2m
E ih d/dt
p -ih d/dr
...
E2/c2=p2+m2c2
(E=mc2 for p=0)
high-energy physics
Dirac eq.
solid-state physics
and microelectronics
Resistor
classical
spintronic
external manipulation of
charge
&
spin
internal communication between
charge & spin
e-
Non-relativistic (except for the spin) many-body
e-
Pauli exclusion principle & Coulomb repulsion  Ferromagnetism
total wf antisymmetric
FERO
= orbital wf antisymmetric * spin wf symmetric
(aligned)
MAG
• Robust (can be as strong as bonding in solids)
• Strong coupling to magnetic field
(weak fields = anisotropy fields needed
only to reorient macroscopic moment)
NET
Relativistic "single-particle"
Spin-orbit coupling
(Dirac eq. in external field V(r) & 2nd-order
in v /c around non-relativistic limit)
Ingredients: - potential V(r)
E


- motion of an electron
e-
Produces
an electric field

1
E   V (r )
e
In the rest frame of an electron
the electric field generates and
effective magnetic field

- gives an effective interaction with the electron’s Beff 
magnetic moment

1
(

V
)

p
2m 2 c 2
• Current sensitive to magnetization
direction
V
Beff
s
p
 
H SO  s  Beff
e-
Spintronics
Ferromagnetism
Coulomb repulsion & Pauli exclusion principle
Spin-orbit coupling
Dirac eq. in external field V(r) & 2nd-order
in v /c around non-relativistic limit
V
s
p
Beff
 
H SO  s  Beff

Beff 

1
(

V
)

p
2m 2 c 2
Fermi surfaces
ky
~Mx . sx
FM without SO-coupling
kx
~(k . s)2
SO-coupling without FM
~(k . s)2
+ Mx . sx
FM & SO-coupling
Fermi surfaces
ky
~Mx . sx
kx
FM without SO-coupling
~(k .
~(k . s)2
+ Mx . s x
s)2
SO-coupling without FM
FM & SO-coupling
AMR
M
Ferromagnetism: sensitivity to magnetic field
scattering
SO-coupling: anisotropies in Ohmic transport
characteristics; ~1-10% MR sensor
M
hot spots for scattering of states moving  M
 R(M  I)> R(M || I)
Diode
classical
spin-valve
TMR
Based on ferromagnetism only; ~100% MR sensor or memory
no (few) spin-up DOS available at EF
large spin-up DOS available at EF
1. Current spintronics in HDD read-heads and memory chips
2. Physical principles of current spintronic devices operation
3. Research at the frontiers of spintronics
4. Summary
Removing external magnetic fields (down-scaling problem)
EXTERNAL MAGNETIC FIELD
problems with integration - extra wires, addressing neighboring bits
Current (instead of magnetic field) induced switching
Angular momentum conservation  spin-torque
magnetic field
current
Myers et al., Science '99; PRL '02
local, reliable, but fairly
large currents needed
Likely the future of MRAMs
Spintronics in the footsteps of classical electronics
from resistors and diodes to transistors
AMR based diode
- TAMR sensor/memory elemets
TAMR
TMR
no need for exchange biasing
or spin
Au coherent tunneling
FM
AFM
Simpler design without exchange-biasing
the fixed magnet contact
Spintronic transistor based on AMR type of effect
Huge, gatable, and hysteretic MR
Single-electron transistor
Two "gates": electric and magnetic
Spintronic transistor based on CBAMR
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)
CBAMR SET
• Generic effect in FMs with SO-coupling
• Combines electrical transistor action
with magnetic storage
• Switching between p-type and n-type transistor
by M  programmable logic
In principle feasible but difficult
to realize at room temperature
Spintronics in the footsteps of classical electronics
from metals to semiconductors
Spin FET – spin injection from ferromagnet & SO coupling in semiconductor
V
Beff
Difficulties with injecting spin polarized currents from
metal ferromagnets to semiconductors, with spincoherence, etc.  not yet realized
s
p
Ferromagnetic semiconductors – all semiconductor spintronics
More tricky than just hammering an iron nail in a silicon wafer
Ga
Mn
As
GaAs - standard semiconductor
Mn
Mn - dilute magnetic element
(Ga,Mn)As - ferromagnetic
semiconductor
Ga
(Ga,Mn)As (and other III-Mn-V)
ferromagnetic semiconductor
• compatible with conventional III-V semiconductors (GaAs)
• dilute moment system  e.g., low currents needed for writing
• Mn-Mn coupling mediated by spin-polarized delocalized
holes  spintronics
• tunability of magnetic properties as in the more conventional
semiconductor electronic properties.
• strong spin-orbit coupling  magnetic and magnetotransport
anisotropies
• Mn-doping (group II for III substitution) limited to ~10%
• p-type doping only
• maximum Curie temperature below 200 K
Mn
As
Mn
(Ga,Mn)As material
Ga
Mn
As
Mn
- Mn local moments too dilute
(near-neghbors cople AF)
- Holes do not polarize
in pure GaAs
- Hole mediated Mn-Mn
FM coupling
5 d-electrons with L=0
 S=5/2 local moment
moderately shallow
acceptor (110 meV)
 hole
Mn–hole spin-spin interaction
Ga
Mn
As
Mn
As-p
Mn-d
hybridization
Hybridization  like-spin level repulsion  Jpd SMn  shole interaction
Ferromagnetic Mn-Mn coupling mediated by holes
heff = Jpd <SMn> || x
Hole Fermi surfaces
Mn
As
Ga
Heff = Jpd <shole> || -x
No apparent physical barriers for achieving room Tc in III-Mn-V
or related functional dilute moment ferromagnetic semiconductors
Need to combine detailed understanding of physics and technology
Weak hybrid.
Delocalized holes
long-range coupl.
InSb, InAs, GaAs
d5
Strong hybrid.
GaP
Impurity-band holes
short-range coupl.
And look into related semiconductor host families like e.g. I-II-V’s
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
 no Mn concentration limit
 possibly both p-type and n-type ferromagnetic SC
Spintronics in non-magnetic semiconductors
way around the problem of Tc in ferromagnetic semiconductors &
back to exploring spintronics fundamentals
Spintronics relies on extraordinary magnetoresistance
Ordinary magnetoresistance:
response in normal metals to external
magnetic field via classical Lorentz force
B
Extraordinary magnetoresistance:
response to internal spin polarization in ferromagnets
often via quantum-relativistic spin-orbit coupling
anisotropic
magnetoresistance
_ _ _ _ _ _ _ _ _ _
_
FL
+++++++++++++
V
I
M
__
FSO
I
e.g. ordinary (quantum)
Hall effect
V
and anomalous
Hall effect
Known for more than 100 years
but still controversial
Anomalous Hall effect in ferromagnetic conductors:
spin-dependent deflection & more spin-ups  transverse voltage
skew scattering
intrinsic
side jump
majority
_
__
FSO
FSO
I
_
minority
FSO
__
FSO
non-magnetic
V
I
V=0
Spin Hall effect in non-magnetic conductors:
spin-dependent deflection  transverse edge spin polarization
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
supravodivý magnet, 100 A
SHE edge spin accumulation can be
extracted and moved further into the circuit
1. Current spintronics in HDD read-heads and memory chips
2. Physical principles of current spintronic devices operation
3. Research at the frontiers of spintronics
4. Summary
Downscaling approach about to expire
currently ~ 30 nm feature size
interatomic distance in ~20 years
Spintronics: from straighforward downscaling to
more "intelligent" device concepts:
• simpler more efficient realization for a given functionality (AMR sensor)
• multifunctional (integrated reading, writing, and processing)
• new materials (ferromagnetic semiconductors)
• fundamental understanding of quantum-relativistic electron transport (extraordinary MR)
Anisotropic magneto-resistance
sensor
Electromagnet
• Information reading

Magnetization

Current
• Information reading & storage
Tunneling magneto-resistance sensor and memory bit
• Information reading & storage & writing
Current induced magnetization rotation
• Information reading & storage & writing & processing
Spintronic single-electron transistor:
magnetoresistance controlled by gate voltage
• New materials
Ga
As
Dilute moment ferromagnetic semiconductors
Mn
• Spintronics fundamentals
AMR, anomalous and spin Hall effects
Mn