SPINTRONICS
Tom Jungwirth
Fyzikln stav AVR 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
MByte 1 bit: 1mm x 1mm
From PC hard drives ('90)
to micro-discs - spintronic read-heads
1 bit: 10-3mm x 10-3mm
GByte
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 E2/c2=p2+m2c2 Dirac eq.
E ih d/dt (E=mc2 for p=0)
p -ih d/dr
... high-energy physics solid-state physics
and microelectronics
Resistor
classical
spintronic
external manipulation of internal communication between
charge & spin charge & spin
e-
Non-relativistic (except for the spin) many-body
e-
Pauli exclusion principle & Coulomb repulsion Ferromagnetism
total wf antisymmetric = orbital wf antisymmetric * spin wf symmetric
(aligned)
FERO MAG NET
Robust (can be as strong as bonding in solids)
Strong coupling to magnetic field
(weak fields = anisotropy fields needed
only to reorient macroscopic moment)
Relativistic "single-particle"
Spin-orbit coupling e-
(Dirac eq. in external field V(r) & 2nd-order
in v /c around non-relativistic limit)
1
Produces E V (r )
Ingredients: - potential V(r)
an electric field e
E
In the rest frame of an electron
- motion of an electron the electric field generates and
effective magnetic field
1
Beff
- gives an effective interaction with the electrons ( V ) p
magnetic moment 2m 2 c 2
Current sensitive to magnetization
direction p
s
V
H SO s Beff
Beff
Spintronics e-
Ferromagnetism
Coulomb repulsion & Pauli exclusion principle
p
s H SO s Beff
Spin-orbit coupling V 1
Dirac eq. in external field V(r) & 2nd-order Beff ( V ) p
in v /c around non-relativistic limit 2m 2c 2
Beff
Fermi surfaces
ky
~(k . s)2
~Mx . sx kx
~(k . s)2 + Mx . s x
FM without SO-coupling SO-coupling without FM FM & SO-coupling
Fermi surfaces
ky
~(k . s)2
~Mx . sx ~(k . s) 2
kx + Mx . s x
FM without SO-coupling SO-coupling without FM FM & SO-coupling
AMR
Ferromagnetism: sensitivity to magnetic field M
scattering
SO-coupling: anisotropies in Ohmic transport kx
y
k
characteristics; ~1-10% MR sensor
M
ky
kx
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
Q VD Q0
Source Drain
Q0
Gate
VG e2/2
C
Q
Q ( M )
U dQ VD ( Q )
' '
[010] M
0
e [110]
( Q Q0 )2 ( M ) C [100]
U & Q0 CG [ VG VM ( M )] & VM
2C e CG
[110]
[010]
electric & magnetic SO-coupling
control of Coulomb blockade oscillations (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
p
s
V
Beff
Difficulties with injecting spin polarized currents from
metal ferromagnets to semiconductors, with spin-
coherence, etc. not yet realized
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
Mn
(Ga,Mn)As (and other III-Mn-V) As
ferromagnetic semiconductor
Mn
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
(Ga,Mn)As material
Ga
Mn
As
Mn
5 d-electrons with L=0
- Mn local moments too dilute S=5/2 local moment
(near-neghbors cople AF)
moderately shallow
- Holes do not polarize acceptor (110 meV)
in pure GaAs hole
- Hole mediated Mn-Mn
FM coupling
Mnhole 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
Mn
Hole Fermi surfaces
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 Impurity-band holes
Strong hybrid.
long-range coupl. short-range coupl.
InSb, InAs, GaAs
d5 GaP
And look into related semiconductor host families like e.g. I-II-Vs
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: Extraordinary magnetoresistance:
response in normal metals to external response to internal spin polarization in ferromagnets
magnetic field via classical Lorentz force often via quantum-relativistic spin-orbit coupling
B anisotropic
magnetoresistance
_ _ _ _ _ _ _ _ _ _
_ FL
+++++++++++++
I
V M __ FSO
e.g. ordinary (quantum) I
Hall effect
and anomalous
V 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
intrinsic skew scattering 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
Spin Hall effect detected optically Same magnetization achieved
in GaAs-based structures by external field generated by
a superconducting magnet
with 106 x larger dimensions &
106 x larger currents
n
p
n
SHE mikroip, 100A supravodiv magnet, 100 A
SHE detected elecrically in metals SHE edge spin accumulation can be
extracted and moved further into the circuit
Cu
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)
Electromagnet Anisotropic magneto-resistance
sensor
Ferr
Information reading o
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
Ga
New materials Mn
As
Dilute moment ferromagnetic semiconductors
Mn
Spintronics fundamentals
AMR, anomalous and spin Hall effects
