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POTENTIAL APPLICATIONS OF SPINTRONICS
Dept. of ECECS, Univ.of Cincinnati,
Cincinnati, Ohio 45221
http://www.ececs.uc.edu/~mcahay
M.CahayM.Cahay
February 4, 2005February 4, 2005
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Outline
A Little quBit of HistorySuccess Story: Giant MagnetoresistanceSpin ValveRequirements for spintronicsZeeman, Spin-Orbit EffectsInjection, Manipulation, DetectionMagnetoresistive biosensorsConclusions
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Brief HistorySpintronics-Magnetoelectronics
• Stern-Gerlach Experiment (Early 1920s)• spin concept – 1920s• Pauli-Dirac Equation (Late 1920s)• 1980s… : Study of Mesoscopic systems – Landauer-
Buttiker Formalism • Breakdown of Moore’s Law?• 1988: Giant Magnetoresistance in Magnetic multilayers,
magnetic read heads, magnetic sensors, spin valves.
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Brief HistorySpintronics-Magnetoelectronics
• 1990: SPINFET proposal by Datta and Das
• 1990s: Lots of work on Ferromagnet/ (metal, semiconductor, superconductor) interfaces.
• DMS – diluted magnetic semiconductors (ZnMnSe, GaMnAs,GaMnN,…)
• 1985: David Deutsch:
Quantum Mechanical Turing Machine
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Brief HistorySpintronics-Magnetoelectronics
• 1995: P.Schor’s algorithm for fast factorization of large integers (cryptography)
• 1997: L.K.Grover’s search algorithm for efficient search of large database
• 1990s: Lots of proposal for implementation of qubits and quantum computers (NMR, Ion trap, quantum dot)
• Search in Spintronics and Quantum Computing will continue to feed on each other
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SIA ROADMAP - Moore’s Law
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Requirements for Spintronic Integrated Circuits
• Simple device structure for high degree
of integration and high process yield.
• Large magnetocurrent for high speed operation
• High transconductance for high speed operation
• High amplification capability (V, I, and/or power)
• Small power delay product and small off-current
for low power dissipation
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Preliminaries: Stoner Model
Exchange Energy
E(k)
k
Ferromagnetic Contact
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GMR Read Head
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RAM with GMR Elements
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Preliminiaries: Zeeman Preliminiaries: Zeeman EffectEffect
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Preliminaries:Spin-Orbit Preliminaries:Spin-Orbit InteractionInteraction
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Ferromagnet/Sm Interface
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Contact Selection
• Ferromagnetic contact (Fe, Ni, Co)• HMF candidates: Heusler Materials• Dilute Magnetic Semiconductors (GaMnAs,
ZnMnSe, ZnMnTe,GaMnN…)• More recently, wide band gap ferromagnetic
semiconductors and oxides S.J. Pearson et al., “Wide band gap ferromagnetic
Semiconductors and oxides”, Journal of Applied Physics, Vol.93, pp.1-13 (2003)
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Ferromagnetic contact/semiconductor interfacesHow good are they?
Why Ferromagnetic Contacts (Fe, Ni, Co)? Because Curie Temperature Is Above Room Temperature!
Hence, devices could work at 300k. FM are good source of spin polarized electron sources
(Stoner model)
Theoretical PredictionsTheoretical Predictions Classical diffusion eq. predicts very small spin Classical diffusion eq. predicts very small spin injection efficiency across Fe/Sm interface (G. Schmidt et injection efficiency across Fe/Sm interface (G. Schmidt et al. PRB 62,R4790 (2000). Main reason: Large conductivity al. PRB 62,R4790 (2000). Main reason: Large conductivity mismatch between the two materials.mismatch between the two materials. Not so fast! E.I.Rashba (Phys.Rev.B 62 R16267 (2000)).Not so fast! E.I.Rashba (Phys.Rev.B 62 R16267 (2000)).““If you can adjust interface resistance by using a If you can adjust interface resistance by using a tunneling barrier, the situation can improved tunneling barrier, the situation can improved drastically!”.drastically!”.
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Ferromagnetic contact/semiconductor interfacesHow good are they?
Rashba's prediction was confirmed
experimentally using
(a) Schottky barriersH.J.Zhu et al., PRL 87, 016601 (2001) (Fe/GaAs), 2% efficiency
A.T.Hanbicki et al, APL 80, 1240 (2002)
A.T.Hanbicki et al, APL 82, 4092 (2003), (Fe/AlGaAs), 33% efficiency
(b) Thin Metal Oxides V.F. Motsynyi et al, APL 81, 265 (2002)
T. Manago and H. Akinaga, APL 81, 694 (2002)
(c) AlAs barriers S.H.Chun et al, PRB 66, R100408 (2002).
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Spin Relaxation Mechanisms
The Elliot-Yafet Scattering Mechanism
As a result of the SO-contribution to the crystalHamiltonian, conduction-band states of somesemiconductors are not spin eigenstates. This leads tothe possibility for spin-flip scattering even for spinindependent impurity scattering (due to Coulombicscattering for instance).
For the same reason, spin-independent electron-electronscattering can also cause spin-flip transitions
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DYAKONOV-PERELSPIN RELAXATION IN A QUANTUM WIRE
• DRESSELHAUS HAMILTONIAN
• RASHBA HAMILTONIAN
y
z
x
x
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RAMSAUER (FABRY-PERROT) RESONANCES
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ONE REPULSIVE IMP. 300 angs from left contact
INFLUENCE OF SCATTERING STRENGTH
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Magnetoresistive-Based BiosensorsD.L.Graham et al, Trends in biotechnology vol.22, 455 (2004)
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Conclusions
Spintronics has already some success stories!
(giant magnetoresistance/spin valve)
Quantum Computing: Too early to tell!
Other potential: Spintronics & organics,
Spintronics & Biosensors, Magnetic Sensors.
Want to know more about it? Buy the book:
“Introduction to Spintronics”….in 2006.