Spin injection and detection in Cu spin valve structures

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Spin injection and detection in Cu spin valve
structures
Samir Garzon Richard Webb
Center for Superconductivity Research Department
of Physics University of Maryland
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Motivation-Devices
Spintronic applications
Spin transport in Cu wires
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Motivation-Science
Spectroscopic tool to study

• symmetry properties of unconventional
  superconductors
• excitations in the quantum Hall regime
• spin-charge separation in non-Fermi liquids

1,2
3,4
5,6
Johnson and Silsbee, PRL 55, 1790 (1985)
pioneered field spin injection
1 Vasko et. al, PRL 78, 1134 (1997)
4 Chan et. al, PRL 83, 3258 (1999)
2 Ngai et. al, APL 84, 1907 (2004)
5 Si et. al, PRL 81, 3191 (1998)
3 MacDonald et. al, PRL 83, 3262 (1999)
6 Balents et. al, PRL 85, 3464 (2000)
Spin transport in Cu wires
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Our research

• Produce and detect spin-polarized currents

• Find spin relaxation length and understand the
  mechanisms that are responsible

• Study a nonlocal geometry different from that
  used for GMR and MTJ applications

• Gain understanding of interfacial spin transport

• Understand the high temperature behavior of spin
  injection and detection

Spin transport in Cu wires
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Outline
Spin transport in Cu wires
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Outline

• Theory

• Sample fabrication

Spin transport in Cu wires
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Outline

• Theory

• Sample fabrication

• Measurements

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Outline

• Theory

• Sample fabrication

• Measurements

• Discussion

• Conclusions (overview)

• Further work

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Time scales
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Spin relaxation mechanisms
(nonmagnetic metals)
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Spin relaxation mechanisms
(nonmagnetic metals)
magnetic materials magnons interfaces surface
magnons enhanced magnetic scattering
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Spin transport F-N junction
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Spin valves
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Spin valves
CHANGE IN RESISTANCE BETWEEN ALIGNED AND
ANTI-ALIGNED CONFIGURATIONS
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TMR vs Nonlocal Geometry
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Nonlocal Geometry
As opposed to TMR, in the absence of spin effects
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Nonlocal Geometry
As opposed to TMR, in the absence of spin effects
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Sample fabrication

• 2 levels of standard
• e-beam lithography
• ion-milling
• thermal evaporation
• lift-off

1st level Co Al2O3 tunnel barrier 2nd level Cu
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Samples
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Spin transport
current
electrochemical potential
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Spin transport
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Spin transport
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Spin transport in Cu wires
Spin transport in Cu wires
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Spin transport in Cu wires
where lN is the spin diffusion length
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Outline

• Theory

• Sample fabrication

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Characteristic switching
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Hanle effect (spin precession)
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Temperature dependence
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Temperature dependence
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Outline

• Theory

• Sample fabrication

• Measurements

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T dependence analysis
Previous experiments could only measure RA since
RTMR included a large offset not related to spin
injection
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T dependence analysis
samir what about temp dep of D?
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T dependence fit
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T dependence PA, PS
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T dependence PA, PS
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Can we explain this?
N
F2
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Can we explain this?
interface spin-flip scattering
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Physical meaning
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Temperature dependence conclusions

• Temperature dependent measurements reveal a new
  component of the nonlocal resistance.

• Data analysis shows that the new component can
  be fit well with a model related to temperature
  activation with T1227 K (surface magnons?,
  enhanced magnetic scattering?).

• The existence of the new signal is explained by
  extending the previous model to include interface
  spin-flip scattering.

• A physical interpration of the new defined
  quantities PS and PA is given. Ps gives
  information on the differential spin-flip
  scattering at the detector, while PA describes
  the differential spin conserving transport.

• The different character of injector and detector
  is clear in the nonlocal geometry.

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Cross checks temperature dependence
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Cross checks length dependence
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Conclusions (overview)

• Performed comprehensive spin injection and
  detection experiments in Cu-Co spin valves.

• Measured spin precession in Cu and extracted the
  spin diffusion length and the current spin
  polarization P.

• Spin diffusion length measurements are
  consistent with each other and with previous
  measurements.

• Found a temperature dependent symmetric
  component in the nonlocal resistance RS that is
  consistent with the hypothesis of interface
  spin-flip scattering.

• Made various cross checks to make sure RS did
  not come from capacitive leakage, electrostatic
  geometric effects, and heating combined with
  thermoelectric effect.

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Further work

• Study the microscopic origin of interfacial
  spin-flip scattering.

• Nonlocal measurements with MgO tunnel barriers,
  which has been shown to enhance the
  magnetoresistance in MTJ, should be used for
  comparison.

• For device applications, high frequency
  measurements of spin injection and detection
  might be of importance.

• A direct measurement of the spin polarization
  and relaxation lengths, not requiring a transport
  model for data interpration can be useful (MFM).

• Use electron statistics such as shot noise to
  further study interfacial spin transport, even in
  the absence of charge current.

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Experimental setup
osc. out
ref. in
osc. out
sig. out
in
out
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Cross checks Geometric effects
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Cross checks heating and Seebeck effect
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Why heating and Seebeck effect?
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Current leakage?
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Time scales
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What does small B mean?
(typical time interval between spin changing
events)
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Conductivity mismatch
Spin Transport in Cu wires