1. POTENTIAL APPLICATIONS OF SPINTRONICS Dept. of ECECS, Univ.of Cincinnati, Cincinnati, Ohio 45221 http://www.ececs.uc.edu/~mcahay M.Cahay February 4, 2005

2. Outline A Little quBit of History Success Story: Giant Magnetoresistance Spin Valve Requirements for spintronics Zeeman, Spin-Orbit Effects Injection, Manipulation, Detection Magnetoresistive biosensors Conclusions

3. Brief History Spintronics-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.

4. Brief History Spintronics-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

5. Brief History Spintronics-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

6. SIA ROADMAP - Moore’s Law

7. 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

8. Preliminaries: Stoner Model Exchange Energy E(k) k Ferromagnetic Contact

11. GMR Read Head

12. RAM with GMR Elements

13. Preliminiaries: Zeeman Effect

14. Preliminaries:Spin-Orbit Interaction

15. Ferromagnet/Sm Interface

16. 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)

17. Ferromagnetic contact/semiconductor interfaces How 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 Predictions  Classical diffusion eq. predicts very small spin injection efficiency across Fe/Sm interface (G. Schmidt et al. PRB 62,R4790 (2000). Main reason: Large conductivity mismatch between the two materials.  Not so fast! E.I.Rashba (Phys.Rev.B 62 R16267 (2000)). “If you can adjust interface resistance by using a tunneling barrier, the situation can improved drastically!”.

18. Ferromagnetic contact/semiconductor interfaces How good are they? Rashba's prediction was confirmed experimentally using (a) Schottky barriers H.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).

20. Spin Relaxation Mechanisms The Elliot-Yafet Scattering Mechanism As a result of the SO-contribution to the crystal Hamiltonian, conduction-band states of some semiconductors are not spin eigenstates. This leads to the possibility for spin-flip scattering even for spin independent impurity scattering (due to Coulombic scattering for instance). For the same reason, spin-independent electron-electron scattering can also cause spin-flip transitions

21. DYAKONOV-PEREL SPIN RELAXATION IN A QUANTUM WIRE DRESSELHAUS HAMILTONIAN RASHBA HAMILTONIAN y z x x

23. RAMSAUER (FABRY-PERROT) RESONANCES

25. ONE REPULSIVE IMP. 300 angs from left contact INFLUENCE OF SCATTERING STRENGTH

26. Magnetoresistive-Based Biosensors D.L.Graham et al, Trends in biotechnology vol.22, 455 (2004)

27. 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.