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

Spintronic Devices and Spin Physics in Bulk Semiconductors

Marta Luengo-KovacJune 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

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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

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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)

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• Intrinsic angular momentum of an electron

• Treat semi-classically (/)

• Has magnetic moment μB

• 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 structureBtotal = Bexternal + Bspin-

orbit

This leads to dephasing

7

Total spin polarizatio

n

Projection of S on horizontal axis

Spin Polarization

Time

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 DrainGate (off)

n-doped n-doped

p-doped

V

No current - 0

Metal-oxide-semiconductor field-effect transistors (MOSFETs)

10

Source DrainGate (on)

n-doped n-doped

p-doped

V

Current flows - 1

++++++++

- - - - - - - -

Yes measured currentNo 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

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Source Drain

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

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Magnetic Tunnel Junctions

13

VInsulator

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)

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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

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Creating a Spin Polarization:Optical Selection Rules

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Valence Band

Conduction Band

-1/2 1/2

-3/2 -1/2 1/2 3/2

3 11 3

m =

m =

Measuring a Spin Polarization:

Faraday Rotation

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Conduction Band

-1/2 1/2m =

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”)

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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

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Cold Finger

Pump-Probe Setup21

Lase

r

Pump

ProbeWollasto

nPrism

Linear

Pola

riz

er

Half Wave Plate

PEM

Chopper

Time-Resolved Faraday Rotation

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Fara

day R

ota

tion

(a.u

.)

Magnetic Field Scans(Resonant Spin Amplification)

Fara

day R

ota

tion

(a.u

.)

J. M. Kikkawa and D.D. Awschalom, PRL 80, 4313 (1997)

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Spatial measurements map out the spin packet

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0 V + 2 V

Fitting the spin-orbit fields

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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

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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

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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

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Measuring current-induced spin polarization

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MeasurementProjection 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!

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B. M. Norman, et al. PRL 112, 056601 (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”

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I. Stepanov, et al. APL 104, 062406 (2014)

V

V

Bspin-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)

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