Spintronics and magnetic semiconductors Spintronics and magnetic semiconductors Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al. Hitachi Cambridge Jorg Wunderlich, David Williams, et al. Institute of Physics ASCR, Prague sha Shick, Jan Mašek, Vít Novák, et al. University of Texas Texas A&M Univ. Allan MacDonald, Qian Niu et al. Jairo Sinova, et al. NERC SWAN
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Spintronics and magnetic semiconductors Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al. Hitachi Cambridge.
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Spintronics and magnetic semiconductorsSpintronics and magnetic semiconductors
Tomas Jungwirth
University of Nottingham
Bryan Gallagher, Tom Foxon, Richard Campion, et al.
Hitachi Cambridge
Jorg Wunderlich, David Williams, et al.
Institute of Physics ASCR, Prague
Sasha Shick, Jan Mašek, Vít Novák, et al.
University of Texas Texas A&M Univ.
Allan MacDonald, Qian Niu et al. Jairo Sinova, et al.
NERCSWAN
1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
2.2. Basic Basic physical principles of the operation of spintronic devices physical principles of the operation of spintronic devices
3.3. Semiconductor Semiconductor sspintronipintronics researchcs research
4. Summary4. Summary
Current spintronics applications Current spintronics applications
First hard discFirst hard disc (1956) (1956) - - classical electromagnet for read-outclassical electromagnet for read-out
From PC hard drives ('90)From PC hard drives ('90)to mto miicro-discscro-discs - - spintronispintronic read-headsc read-heads
MBMB’s’s
10’s-100’s 10’s-100’s GBGB’s’s
1 bit: 1mm x 1mm1 bit: 1mm x 1mm
1 bit: 101 bit: 10-3-3mm x 10mm x 10-3-3mmmm
Anisotropic magnetoresistance (AMR) read headAnisotropic magnetoresistance (AMR) read head1992 - dawn of spintronics1992 - dawn of spintronics
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)
many-body
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
Conventional ferromagnetic metals
itinerant 4s:no exch.-split
no SO
localized 3d:exch. split
SO coupled
ss sd
sdss
Mott’s model of transportAb initio Kubo (CPA) formula forAMR and AHE in FeNi alloys
More tricky than just hammering an iron nail in a silicon wafer
Mn-d-like localmoments
As-p-like holes
Mn
Ga
AsMn
- carriers with both strong SO carriers with both strong SO coupling coupling and exchange splitting, yet simpleand exchange splitting, yet simple semiconductor-like bandssemiconductor-like bands
- Mn 3d5 (S=5/2, L=0): no SO coupling just help to stabilize ferromagnetism
Favorable systems for exploring physical origins of old spintronics effects and for finding new ones
FM without SO-couplingSO-coupling without FM
FM & SO-coupling
~(k . s)2
~(k . s)2 + Mx . sx
ky
kx
kx
k y
M
kx
k y
M
Enhanced interbandscattering near degeneracy
~Mx . sx
Hot spots for scattering of states moving M R(M I)> R(M || I)
AMR: a reflection of Fermi surface spin textures in transportAMR: a reflection of Fermi surface spin textures in transport
Family of new AMR effects: TAMR – anisotropic TDOSFamily of new AMR effects: TAMR – anisotropic TDOS
TAMR – discovered in GaMnAs
AuGaMnAs
AuAlOx Au
predicted and observed in metals
[100]
[010]
[100]
[010]
[100]
[010]
Gould, et al., PRL'04, Brey et al. APL’04,Ruster et al.PRL’05, Giraud et al. APL’05, Saito et al. PRB’05,
[010]
M[110]
[100]
[110][010]
Shick et al.PRB'06, Bolotin et al. PRL'06, Viret et al. EJP’06, Moser et al. 06, Grigorenko et al. ‘06
Res
ista
nce
TAMR spintronic dTAMR spintronic diodiodee
classicalclassical
sspipintronic TMRntronic TMR
Au
No need for exchange biased fixed magnetor spin coherent tunneling
sspipintronic TAMRntronic TAMR
Au
TMR
electric && magneticmagnetic
control of CB oscillations
Coulomb blockade AMR spintronic transistorCoulomb blockade AMR spintronic transistor
Wunderlich et al. PRL 06
Source Drain
GateVG
VDQ
[010]
M[110]
[100]
[110][010]
Anisotropic chemical potential
• Generic effect in FMs with SO-coupling (predicted higher-T CBAMR for metals)
• Combines electrical transistor action with magnetic storage
• Switching between p-type and n-type transistor by M programmable logic
CBAMR SET
Dilute moment nature of ferromagnetic semiconductorsDilute moment nature of ferromagnetic semiconductors
GaAs Mn
Mn
10-100x smaller Ms
One
Current induced switchingreplacing external field Tsoi et al. PRL 98, Mayers Sci 99
Key problems with increasing MRAM capacity (bit density):
- Unintentional dipolar cross-links- External field addressing neighboring bits
10-100x weaker dipolar fields
10-100x smaller currents for switching
Sinova et al., PRB 04, Yamanouchi et al. Nature 04
One
Dipolar-field-free current induced switching nanostructuresDipolar-field-free current induced switching nanostructures
Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy)
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