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SPINTRONICSSPINTRONICS
Tomáš Jungwirth
Fyzikální ústav AVČR University of Nottingham
1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
2.2. Physical principles of operation of current spintronic devices Physical principles of operation of current spintronic devices
3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs
4. Summary4. Summary
Current spintronics applications Current spintronics applications
First hard discFirst hard disc (1956) (1956) - - classical electronics for read-outclassical electronics for read-out
From PC hard drives ('90)From PC hard drives ('90)to mto miicro-discscro-discs - spintroni - spintronic read-headsc read-heads
MByteMByte
GByteGByte
1 bit: 1mm x 1mm1 bit: 1mm x 1mm
1 bit: 101 bit: 10-3-3mm x 10mm x 10-3-3mmmm
HARD DISKSHARD DISKS
HARD DISK DRIVE READ HEADSHARD DISK DRIVE READ HEADS
horse-shoe read/write heads
spintronic read heads
Anisotropic magnetoresistance (AMR) read headAnisotropic magnetoresistance (AMR) read head
1992 - dawn of spintronics1992 - dawn of spintronics
Appreciable sensitivity, simple design, scalable, cheap
Giant magnetoresistance (GMR) read headGiant magnetoresistance (GMR) read head
19971997
High sensitivity
MEMORY CHIPSMEMORY CHIPS
.DRAMDRAM (capacitor) - high density, cheephigh density, cheep x slow,
high power, volatile
.SRAMSRAM (transistors) - low power, fastlow power, fast x low density,
expensive, volatile
.Flash (floating gate) - non-volatilenon-volatile x slow, limited life,
expensive
Operation through electron chargecharge manipulation
MRAM – universal memoryMRAM – universal memory fast, small, non-volatile
RAM chip that won't forget
↓
instant on-and-off computers
Tunneling magneto-resistance effect (TMR)
First commercial 4Mb MRAM
MRAM – universal memoryMRAM – universal memory fast, small, non-volatile
RAM chip that won't forget
↓
instant on-and-off computers
Tunneling magneto-resistance effect (TMR)
First commercial 4Mb MRAM
1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
2.2. Physical principles of current spintronic devices operationPhysical principles of current spintronic devices operation
3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs
4. Summary4. 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 Dirac eq.
E=p2/2mE ih d/dtp -ih d/dr. . .
E2/c2=p2+m2c2
(E=mc2 for p=0)
high-energy physics solid-state physicsand microelectronics
ResistorResistor
classicalclassical
spinspintronic tronic
ee--
external manipulation ofexternal manipulation ofcharge & spincharge & spin
internal communication between internal communication between charge & spincharge & spin
Pauli exclusion principle & Coulomb repulsionPauli exclusion principle & Coulomb repulsion FerromagnetismFerromagnetism
total wf antisymmetric = orbital wf antisymmetric * spin wf symmetric (aligned)
FEROFERO MAGMAG NETNET
ee--
• RobustRobust (can be as strong as bonding in solids)(can be as strong as bonding in solids)
• Strong coupling to magnetic fieldStrong coupling to magnetic field (weak fields = anisotropy fields needed (weak fields = anisotropy fields needed only to reorient macroscopic moment)only to reorient macroscopic moment)
Non-relativistic (except for the spin) many-body
Ingredients: - potential V(r)
- motion of an electron
Producesan electric field
In the rest frame of an electronthe electric field generates and effective magnetic field
- gives an effective interaction with the electron’s magnetic moment
E
)(1
rVe
E
ee--
Relativistic "single-particle"
effSO BsH
p)V(cm2
1B
22eff
V
BBeffeff
pss
Spin-orbit couplingSpin-orbit coupling (Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit)
• Current sensitive to magnetizationCurrent sensitive to magnetization directiondirection
Spin-orbit couplingSpin-orbit coupling Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit
ee--
effSO BsH
p)V(cm2
1B
22eff
V
BBeffeff
pss
SpintronicsSpintronics
FerromagnetismFerromagnetism Coulomb repulsion & Pauli exclusion principle
~(k . s)2
ky
kx~Mx . sx
Fermi surfaces
FM without SO-coupling SO-coupling without FM FM & SO-coupling
~(k . s)2
+ Mx . sx
FM without SO-coupling SO-coupling without FM FM & SO-coupling
~(k . s)2 ~(k . s)2
+ Mx . sx
ky
kx
kx
kx
k y
k y
M
M
scattering
~Mx . sx
Fermi surfaces
AMR Ferromagnetism: sensitivity to magnetic field
SO-coupling: anisotropies in Ohmic transportcharacteristics; ~1-10% MR sensor
hot spots for scattering of states moving M R(M I)> R(M || I)
DiodDiodee
classicalclassical
spinspin-valve-valve
TMRBased on ferromagnetism only; ~100% MR sensor or memory
no (few) spin-up DOS available at EF large spin-up DOS available at EF
1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
2.2. Physical principles of current spintronic devices operationPhysical principles of current spintronic devices operation
3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs
4. Summary4. Summary
Removing external magnetic fields (down-scaling problem)Removing external magnetic fields (down-scaling problem)
EXTERNAL MAGNETIC FIELDEXTERNAL MAGNETIC FIELD
problems with integration - extra wires, addressing neighboring bits
Current (instead of magnetic field) induced switching
Angular momentum conservation spin-torque
current
magnetic field
local, reliable, but fairlylarge currents needed
Myers et al., Science '99; PRL '02
Likely the future of MRAMsLikely the future of MRAMs
SpintronSpintronics in the footsteps of classical electronicsics in the footsteps of classical electronicsfrom resistors and diodes to transistors
TAMR
Au
TMR
- TAMR sensor/memory elemets
no need for exchange biasing or spin coherent tunneling
AMR based diode
FM
AFM
Simpler design without exchange-biasingthe fixed magnet contact
Single-electron transistor
Two "gates": electric and magnetic
Spintronic transistor based on AMR type of effect
Huge, gatable, and hysteretic MR
GMMGG0
20
C
C
e
)M(V&)]M(VV[CQ&
C2
)QQ(U
electric && magneticmagnetic
control of Coulomb blockade oscillations
n-1 n n+1 n+2n-1 n n+1 n+2
EC
QQindind = = nnee
QQindind = (= (n+1/2)n+1/2)eeQ0
Q0
e2/2C
Q
0
'D
'
e
)M(Q)Q(VdQU
[010]
M[110]
[100]
[110][010]
SO-coupling (M)
Spintronic transistor based on CBAMR
Source Drain
GateVG
VDQ
• 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
CBAMR SET
In principle feasible but difficultto realize at room temperature
SpintronSpintronics in the footsteps of classical electronicsics in the footsteps of classical electronicsfrom metals to semiconductors
Spin FET – spin injection from ferromagnet & SO coupling in semiconductor
V
BBeffeff
pss
Difficulties with injecting spin polarized currents from metal ferromagnets to semiconductors, with spin-coherence, etc. not yet realized
FeFerromagnetic semiconductors – all semiconductor spintronicsrromagnetic semiconductors – all semiconductor spintronics
GaAs - GaAs - standard semiconductorstandard semiconductor
Mn - Mn - dilute dilute magneticmagnetic element element
(Ga,Mn)As - fe(Ga,Mn)As - ferrromagneticromagnetic semiconductorsemiconductor
Mn
Ga
AsMn
More tricky than just hammering an iron nail in a silicon wafer
(Ga,Mn)As (and other III-Mn-V)ferromagnetic semiconductor
Mn
Ga
AsMn
• 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 material
5 d-electrons with L=0 S=5/2 local moment
moderately shallow acceptor (110 meV) hole
- Mn local moments too dilute (near-neghbors cople AF)
- Holes do not polarize in pure GaAs
- Hole mediated Mn-Mn FM coupling
Mn
Ga
AsMn
Mn
Ga
AsMn
Mn–hole spin-spin interaction
hybridization
Hybridization like-spin level repulsion Jpd SMn shole interaction
Mn-d
As-p
Heff
= Jpd
<shole> || -x
MnAs
Ga
heff
= Jpd
<SMn> || x
Hole Fermi surfaces
Ferromagnetic Mn-Mn coupling mediated by holes
No apparent physical barriers for achieving room Tc in III-Mn-Vor related functional dilute moment ferromagnetic semiconductors
Need to combine detailed understanding of physics and technology
Weak hybrid.Delocalized holeslong-range coupl.
InSb, InAs, GaAsd5
Strong hybrid.Impurity-band holesshort-range coupl.
GaP
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
SpintronSpintronics in non-magnetic semiconductorsics in non-magnetic semiconductorsway around the problem of Tc in ferromagnetic semiconductors & back to exploring spintronics fundamentals
Spintronics relies on extraordinary magnetoresistance
B
V
I
_
+ + + + + + + + + + + + +
_ _ _ _ _ _ _ _ _ _ FL
Ordinary magnetoresistance:response in normal metals to external magnetic field via classical Lorentz force
Extraordinary magnetoresistance:response to internal spin polarization in ferromagnets often via quantum-relativistic spin-orbit coupling
e.g. ordinary (quantum) Hall effect
I
_ FSO__
Vand anomalous Hall effect
anisotropic magnetoresistance
M
Known for more than 100 years but still controversial
intrinsic skew scattering side jump
I
_ FSO
FSO
_ __majority
minority
V
Anomalous Hall effect in ferromagnetic conductors:spin-dependent deflection & more spin-ups transverse voltage
I
_ FSO
FSO
_ __
V=0
non-magnetic
Spin Hall effect in non-magnetic conductors:spin-dependent deflection transverse edge spin polarization
n
n
p
SHE mikročip, 100A supravodivý magnet, 100 A
Spin Hall effect detected optically in GaAs-based structures
Same magnetization achievedby external field generated bya superconducting magnet with 106 x larger dimensions & 106 x larger currents
Cu
SHE detected elecrically in metals SHE edge spin accumulation can beextracted and moved further into the circuit
1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
2.2. Physical principles of current spintronic devices operationPhysical principles of current spintronic devices operation
3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs
4. Summary4. Summary
Downscaling approach about to expire
currently ~ 30 nm feature sizeinteratomic 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)
• Information reading
Electromagnet Anisotropic magneto-resistance sensor
Ferro 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 materialsDilute moment ferromagnetic semiconductors
Mn
Ga
AsMn
• Spintronics fundamentals
AMR, anomalous and spin Hall effects