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Spintronic Devices and Spin Physics in Bulk Semiconductors Marta Luengo-Kovac June 10, 2015
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Outline Motivation Basic spin dynamics Precession Dephasing Spin-based devices Datta-Das Spin Modulator Magnetic Tunnel Junctions MRAM My own research Measurement techniques Current-induced spin polarization 2
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Computers - The Past Moore’s law has held for the past 50 years But a limit is being reached Photolithography limit Features smaller than the wavelength of light Quantum limit Tunneling causes gate leakage Huge power dissipation Overheating and low energy efficiency 3
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Spintronics - The Future? Why spins? Exploit quantum features Additional degree of freedom Spin current doesn’t need electrical current – less power dissipation Non-volatile – “normally off” computers Ando et al., J.A.P. 115, 172607 (2014) 4
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Intrinsic angular momentum of an electron Treat semi-classically ( / ) Has magnetic moment μ B Magnetic field applies torque on magnetic moments Can use magnetic fields to control orientation of spins What is spin? 5 B
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But it’s not that simple - spin orbit effects Due to spin-orbit effects – an electron moving through an electric field sees an effective magnetic field Electrons are moving at different speeds in different directions Every spin sees a slightly different magnetic field 6 GaAs crystal structure B total = B external + B spin-orbit
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This leads to dephasing 7 Total spin polarization Projection of S on horizontal axis Spin Polarization Tim e
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Devices and their Spintronic Counterparts Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) Datta-Das Spin Modulator Dynamic Random Access Memory (DRAM) Magnetic tunnel junctions (MTJs) Magnetoresistive Random Access Memory (MRAM) 8
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Metal-oxide-semiconductor field-effect transistors (MOSFETs) 9 Source Drain Gate (off) n-doped p-doped V No current - 0
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Metal-oxide-semiconductor field-effect transistors (MOSFETs) 10 Source Drain Gate (on) n-doped p-doped V Current flows - 1 ++++++++ - - - -
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Yes measured current No measured current Datta-Das Spin Modulator Proposed: S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990). Demonstrated in InGaAs: Chuang, et al., Nature Nanotech. 10, 35–39 (2015). NOT a transistor! Doesn’t amplify spin signal 11 SourceDrain Gate V
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Dynamic Random Access Memory (DRAM) Main type of RAM used in computers nowadays Uses a capacitor to store a bit Charged – 1 Discharged – 0 Due to capacitor discharging, must be periodically refreshed Every 64 ms 12
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Magnetic Tunnel Junctions 13 V Insulator Current flows - 1 Pinned Magnetic Layer Free Magnetic Layer electrons tunnel
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Magnetic Tunnel Junctions 14 V No current - 0 Insulator Pinned Magnetic Layer Free Magnetic Layer
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Magnetoresistive Random Access Memory (MRAM) 15 Albert Fert, Nobel Lecture; Sbiaa et al., PSS RRL 5, 413 (2011) Writing (flipping the top layer): Run current through one Bit and one Word line Induced magnetic field only exerts enough torque to flip the magnetization where the Bit and Word lines overlap
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My Research Optical measurements of spins Creating a spin polarization Measuring a spin polarization (Faraday rotation) Measuring spin-orbit fields Current-induced spin polarization 16
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Creating a Spin Polarization: Optical Selection Rules 17 Valence Band Conduction Band -1/21/2 -3/2 -1/2 1/23/2 31 1 3 m =
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Measuring a Spin Polarization: Faraday Rotation 18 Conduction Band -1/21/2 m = Valence Band -3/2 3/2m =
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Measuring a Spin Polarization: Faraday Rotation σ + and σ - absorbed at slightly different energies Different absorption Different index of refraction ( n) Different n for σ + and σ - (“circular birefringence”) 19 Kramers-Kronig Relations Angle of rotation (“Faraday angle”) Spin Polarization
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Pump-Probe Setup Pump laser pulse Circularly polarized Optically injects a spin polarization Probe laser pulse Linearly polarized Measure Faraday rotation after transmission Faraday rotation proportional to spin polarization 20
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Cold Finger Pump-Probe Setup 21 Laser Pump Probe Wollaston Prism Linear Polarizer Half Wave Plate PEM Chopper
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Time-Resolved Faraday Rotation 22 Faraday Rotation (a.u.)
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Magnetic Field Scans (Resonant Spin Amplification) Faraday Rotation (a.u.) J. M. Kikkawa and D.D. Awschalom, PRL 80, 4313 (1997) 23
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Spatial measurements map out the spin packet 24 0 V + 2 V
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Fitting the spin-orbit fields 25
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All-Electrical Manipulation of Spin Polarizations Why all-electrical? More compatible with current computation technology Electric fields can be applied more locally than magnetic fields Easier to make high magnitude and high frequency electric fields than magnetic fields Spin-orbit fields create an internal magnetic field for spin manipulation using only an applied voltage 26
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All-Electrical Creation of Spin Polarizations Why all-electrical? Alternatives: Laser light – complicates device design Injection from a ferromagnet – complicates sample design Large external magnetic field – difficult and expensive All-electrical more compatible with current technology Current-induced spin polarization 27
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Measuring current-induced spin polarization “CISP” Block the pump (no optical injection of spins) Apply an electric field Measured spin polarization is due to the electric field 28
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Measuring current-induced spin polarization 29 Measurement Projection Axis Current-induced: P ~ 0.1% Optical injection: P ~ 50%
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Understanding current- induced spin polarization To maximize CISP, we must understand CISP “Common sense” explanation CISP is due to the spin-orbit effect – coupling of an electron’s motion to its spin Therefore, larger spin-orbit field should mean larger CISP – right? Measurement doesn’t match theory! 30 B. M. Norman, et al. PRL 112, 056601 (2014)
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CISP device concept 1.Apply voltage to create spin polarization 2.Apply voltage to create spin-orbit field – this manipulates the spins 3.Measure voltage through “inverse CISP” 31 I. Stepanov, et al. APL 104, 062406 (2014) V V B spin-orbit
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Conclusion Spintronic devices offer several advantages, e.g. Information density Power consumption Current-induced spin polarization could be used for all-electrical, all-semiconductor spintronic devices However, we need to understand it first (no theory yet) 32
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