Spintronic Devices and Spin
Physics in Bulk Semiconductors
Marta Luengo-Kovac
June 10, 2015
2
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
4
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)
5
What is spin?
• 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
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
Btotal = Bexternal + Bspin-orbit
GaAs crystal structure
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This leads to dephasing
Spin Polarization
Tim
e
Total spin
polarization
Projection of S on horizontal axis
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)
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Metal-oxide-semiconductor field-effect
transistors (MOSFETs)
No current - 0
V
Source
n-doped
p-doped
Gate (off)
Drain
n-doped
10
Metal-oxide-semiconductor field-effect
transistors (MOSFETs)
Current flows - 1
V
Source
Gate (on)
Drain
++++++++
-------n-doped
p-doped
n-doped
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Datta-Das Spin Modulator
Gate
Source
V
Drain
Yes
measured current
current
No measured
• 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
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
Current flows - 1
Pinned Magnetic Layer
V
electrons tunnel
Insulator
Free Magnetic Layer
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Magnetic Tunnel Junctions
No current - 0
Pinned Magnetic Layer
V
Insulator
Free Magnetic Layer
Magnetoresistive Random
Access Memory (MRAM)
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
Albert Fert, Nobel Lecture; Sbiaa et al., PSS RRL 5, 413 (2011)
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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
Creating a Spin Polarization:
Optical Selection Rules
-1/2
m=
3
m=
-3/2
1/2
1
1
-1/2
1/2
Conduction
Band
3
3/2
Valence
Band
17
Measuring a Spin Polarization:
Faraday Rotation
m=
-1/2
1/2
Conduction
Band
m=
-3/2
3/2
Valence
Band
18
Measuring a Spin Polarization:
Faraday Rotation
• σ+ and σ - absorbed at slightly different energies
• Different absorption
Kramers-Kronig
Relations
Different index of refraction (n)
• Different n for σ+ and σ - (“circular birefringence”)
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
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Pump-Probe Setup
Linear
Polarizer
Laser
Half
Wave
Plate
Probe
Wollaston
Prism
Pump
Chopper
PEM
Cold Finger
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Faraday Rotation (a.u.)
Time-Resolved Faraday Rotation
Faraday Rotation (a.u.)
Magnetic Field Scans
(Resonant Spin Amplification)
J. M. Kikkawa and D.D. Awschalom, PRL 80, 4313 (1997)
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Spatial measurements map out
the spin packet
0V
+2V
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Fitting the spin-orbit fields
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
28
Measuring current-induced
spin polarization
Measurement
Projection Axis
• Current-induced: P ~ 0.1%
• Optical injection: P ~ 50%
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Understanding currentinduced 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!
B. M. Norman, et al. PRL 112, 056601 (2014)
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CISP device concept
Bspin-orbit
1. Apply voltage to create
spin polarization
V
2. Apply voltage to create
spin-orbit field – this
manipulates the spins
3. Measure voltage
through “inverse
CISP”
V
I. Stepanov, et al. APL 104, 062406 (2014)
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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)