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Spintronics

Spintronics. The Search for Effective Spin Polarized Current Injection Into S emiconductors. Presented by Alan Gabel Boston University Introduction to Solid State. Spintronics: Spin-based-electronics. Using Spin as well as charge to control electrons and holes. This may make possible:

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Spintronics

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  1. Spintronics The Search for Effective Spin Polarized Current Injection Into Semiconductors Presented by Alan Gabel Boston University Introduction to Solid State

  2. Spintronics: Spin-based-electronics Using Spin as well as charge to control electrons and holes • This may make possible: • Decreased volatility • Increased Processing Speeds • Decreased Power Consumption • Increased integrated circuit density

  3. What is a Transistor? • Basic logic component of an integrated circuit • Device where a small applied voltage can control a large current C. WoodfordTransistors. Explain That Stuff. [Online] 9/10/2008. [Cited: April 25, 2009.] http://www.explainthatstuff.com/howtransistorswork.html.

  4. Field Effect Transistor (FET) Source Electrode Drain Electrode Gate n-type P-type n-type Φ(x) E E x

  5. Field Effect Transistor (FET) Source Electrode Drain Electrode Gate + + + + + + + + + + n-type P-type n-type Φ(x) E E x

  6. Properties of Ferromagnets • Ferromagnets have asymmetric density of states with respect to electron spin • Electrons see an effective magnetic field from magnetization of ferromagnet • Leads to a ‘Zeeman Splitting’ effect M

  7. Properties of Ferromagnets • Conduction electrons form a polarized current M

  8. A Spin Based Transistor • If current is polarized in same direction as Drain electrode: low resistance • If current is polarized opposite to drain electrode: high resistance Source Electrode Drain Electrode Gate Ferromagnet 2-D Semiconductor Ferromagnet Substrate Electronic Analog of Electro-optic Modulator. S. Datta, B. Das.7 56Applied Physics Letters (1990)

  9. A Spin Based Transistor • Voltage on gate creates an electric field, which induces an effective magnetic field – Rashba Effect • Magnetic field causes the spins to precessso polarization is anti-parallel to drain electrode Source Electrode Drain Electrode Gate + + + + + + + + + + Ferromagnet 2-D Semiconductor Ferromagnet Substrate Electronic Analog of Electro-optic Modulator. S. Datta, B. Das.7 56Applied Physics Letters (1990)

  10. Key Ingredients • Injection of Spin polarized current into semiconductor from source electrode • Propagation through the semiconductor • Induced spin precession • Spin-selective collection of current by drain electrode Spintronics: A Spin-Based Electronics. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molna. Science, 294 (2001), p. 1488.

  11. Key Ingredients • Injection of Spin polarized current into semiconductor from source electrode • Propagation through the semiconductor • Induced spin precession • Spin-selective collection of current by drain electrode Spintronics: A Spin-Based Electronics. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molna. Science, 294 (2001), p. 1488.

  12. What’s the problem anyway? • Direct current from ferromagnet to semiconductor produces very low polarization, <1% • ‘Conductivity Mismatch’ P0 =polarization far inside the ferromagnet σF , σSC = conductivity of the ferromagnet , semiconductor λF, λSC = mean distance travelled by spin carriers before a spin flipping scattering occurs. Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor. G. Schmidt, D. Ferrand, L. W. Molenkamp.Physical Review B. 8 62 (2000)

  13. What if Ferromagnet was a Semiconductor? • Use a ferromagneticaly doped semiconductor • It Works! P measured between 90-100% • BUT… Electrical spin injectin in a ferromagnetic semiconductor heterostructure. Y. Ohno, D. Young, B. Beschoten, F. Matsukura, H. Ohno, D. Awschalom.Nature 402 790 (1999) Injection and detection of a spin-polarized current in a light-emitting diode. R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L. Molenkamp. Nature 402 787 (1999)

  14. What if Ferromagnet was a Semiconductor? • Use a ferromagneticaly doped semiconductor • It Works! P measured between 90-100% • BUT… • Need high magnetic fields (~1.5T) • Need super-low temperatures (<40K) • Not viable for commercial application Electrical spin injectin in a ferromagnetic semiconductor heterostructure. Y. Ohno, D. Young, B. Beschoten, F. Matsukura, H. Ohno, D. Awschalom.Nature 402 790 (1999) Injection and detection of a spin-polarized current in a light-emitting diode. R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L. Molenkamp. Nature 402 787 (1999)

  15. Tunneling Junction • Tunneling current remains polarized • Measured P=2% at room temperature • Trade-off to insulating layer: • Increases injection efficiency • Decreases overall current Ferromagnet Current Flow Insulator Semiconductor

  16. Conclusions • Spintronics promises great, if vague, improvements – but is yet to be realized • Obstacles to a working spin transistor are substantial • Device was proposed 20 years ago, and no working model has ever been made • Will take hard work and possibly a major breakthrough to succeed.

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