Spintronic Devices and Spin Physics in Bulk Semiconductors Marta Luengo-Kovac June 10, 2015

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

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

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, (2014) 4

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

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

This leads to dephasing 7 Total spin polarization Projection of S on horizontal axis Spin Polarization Tim e

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

Metal-oxide-semiconductor field-effect transistors (MOSFETs) 9 Source Drain Gate (off) n-doped p-doped V No current - 0

Metal-oxide-semiconductor field-effect transistors (MOSFETs) 10 Source Drain Gate (on) n-doped p-doped V Current flows

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

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

Magnetic Tunnel Junctions 13 V Insulator Current flows - 1 Pinned Magnetic Layer Free Magnetic Layer electrons tunnel

Magnetic Tunnel Junctions 14 V No current - 0 Insulator Pinned Magnetic Layer Free Magnetic Layer

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

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

Creating a Spin Polarization: Optical Selection Rules 17 Valence Band Conduction Band -1/21/2 -3/2 -1/2 1/23/ m =

Measuring a Spin Polarization: Faraday Rotation 18 Conduction Band -1/21/2 m = Valence Band -3/2 3/2m =

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

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

Cold Finger Pump-Probe Setup 21 Laser Pump Probe Wollaston Prism Linear Polarizer Half Wave Plate PEM Chopper

Time-Resolved Faraday Rotation 22 Faraday Rotation (a.u.)

Magnetic Field Scans (Resonant Spin Amplification) Faraday Rotation (a.u.) J. M. Kikkawa and D.D. Awschalom, PRL 80, 4313 (1997) 23

Spatial measurements map out the spin packet 24 0 V + 2 V

Fitting the spin-orbit fields 25

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

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

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

Measuring current-induced spin polarization 29 Measurement Projection Axis Current-induced: P ~ 0.1% Optical injection: P ~ 50%

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, (2014)

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, (2014) V V B spin-orbit

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