Lateral spin transport

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Lateral spin transport

• Jaroslav Hamrle
• Fachbereich Physik und Forschungsschwerpunkt
  MINAS, Technische Universität Kaiserslautern
  Kaiserslautern, Germany

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Spin polarized current
? value of j? and j? can be different
j?
? jchj?j? charge current,
transport of charge scalar
quantity ? jspj?-j? spin polarized current
transport of angular momentum L
vectorial quantity
j?
j?
j?
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Balistic diffusive transport
only change of k during collision(scattering)
change of both spin and k during collision

• ? Description of spin transport
• balistic model (semiconductors)
• diffusive model (metals)

E
lmean_free_path
spin-flip length lsf
for noble metals
F.J. Jedema, PhD thesis, (2002) F.J. Jedema, PRB
67, 085319 (2003)
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Two channel diffusion model
Density of states of bcc-Fe
? Electrical conductivity
down-electrons
DOS at Fermi
Meam free path of e
Fermi velocity
? Diffusion of spin-polarized current
up-electrons
Electrochemical potential m
Valet, Fert, PRB 48, 7099 (1993)
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Injection of spin current from FM

• How to create spin current jspj?-j??

There are several ways of creating jsp, but
here we discuss only spin injection by flow of
jch from FM.
? Local spin injection
? Non-local spin injection

• jch is driven from FM to local branch of
  non-FM
• jsp diffuses from FM to both local and
  non-local branch of non-FM

• non-FM contains both
• spin current jspj?-j?
• charge current jchj?j?

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Two current model for local spin injection

• Two current model consists of resistances of ?, ?
  channels amd of shortcutting resistances between
  both channels.
• FM has different resistivities for ? and ?
  channels
• ? both channels hold different channels ?, ?
• ? spin relaxing current ?I has to relax from ?
  to ? channel.
• the spin relaxing current ?I
• ? relax partly in both FM and non-FM
• ? creates ????-?? on the interface

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Simplified two current Model
All shortcutting resistances can be expressed by
one spin resistance R?.
S cross-sectional area ? spin-diffusion
length ? conductivity
spin polarization
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Spin injection efficiency
? Ineffective spin injection
Conduction mismatch
? Effective spin injection
? Spin resistance
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Values of spin resistances R?
? 1/?m ? nm ? R? ? (S1e4 nm)
Py 7.3 e6 4 0.7 0.11
Co 4.2 e6 40 0.36 0.98
Cu 48 e6 350 0 0.72
Al 31 e6 600 0 1.9
GaAs 500 2000 0 4 e5
organic ? ? 0 ?
Usually ? poor spin
injection ?very poor diffusive injection
for semiconductors
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Non-local spin injection

• Spin current is presented in both local and
  non-local branches.
• jch is driven by voltage drop
• jsp is driven by diffusion from source
  (interface)

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Lateral spin-valve structure
? RIKEN's lateral devices

• device consists of two identical Py wires
  bridged by Cu wire.
• Py wires have different coercivity due to
  their different endings.

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? Current flow

• current flows from the first Py wire to Cu.
• then, there is a non-local spin accumulation
  in Cu wire without charge current.
• this spin accumulation is detected by second
  Py wire as a voltage between Cu and second
  Py wire.

Hamrle et al, PRB 71, 094402 (2005)
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Electrical detection of spin current
FM-detector
Non-FM
R?
I?
I?

• Due to contact to FM, jsp is shortcutted in
  non-FM ? reduction of ?? (spin-sink effect)
• the remaining ?? (and jsp) is shortcutted in
  FM-detector
• due to different R? ? R? in FM, ??? appears
• ??? is then measured and called non-local spin
  signal.

R?
Shortcut of spin current
?
???
x
Kimura et al. PRB 72, 014461 (2005)
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3D distribution of jsp
detection within first 60nm from Py-det side.
Both jsp and jch injected within first 30nm from
side of Cu
jsp spreads funlike
jch makes a whirl
Hamrle et al, PRB 71, 094402 (2005)
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3D distribution of jsp
arrows jspj?-j? color mapping ?µµ?-µ?
arrows jchj?j? color mapping µ(µ?µ?)/2
Spin current attracted by Py-det (spin sink)
wcu100 nm
Hamrle et al. PRB 71, 094402 (2005)
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Injection efficiency on junction size
? Effective spin injection

• Reduction S increase RlF
• Increase of RlF increases spin-injection
  efficiency
• small S is in nano-contacts, but reduced
  critical current

Kimura, PRB 73, 132405 (2006)
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Conclusion / overview

• Spin current described by diffusion transport

• Local and non-local spin injection.

• Condition for effective spin injection

• Injection and detection in lateral spin valve
  structures

• 3D current distribution

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BLS tracing spin current I
Py thickness 30nm Cu thickness 80nm Space
between Py wires 200nm
?? Project observe spin-current phenomena
(jsp or Dm) by BLS
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BLS tracing spin current II
?? detected BLS signal at 1w
rf pumped charge current _at_3.8GHz
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BLS tracing spin current II
?? detected BLS signal at 1w
rf pumped charge current _at_3.8GHz
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BLS tracing spin current III
?? Detected BLS signal at 2w
rf pumped charge current _at_3.8GHz
decay again corresponds to lsf of Cu
effect of Cu edge is not so pronounced
?? 2w BLS signal probably originates from
2-magnon processes in Py
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BLS tracing spin current IV
the same branch observed, but without charge
current
decay length is longer, as now there is no heating
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BLS signal from substrate
?BLS signal observed even in substrare!
Possible origins

• back-side reflections of the laser beam
  probing Py

• rf induced ?? (that would be a great
  achievment)

• higher lobes of probing laser beam (that would
  be bad)

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New devices under fabrication
injection mechanism and efficiency
pumped spin-current in confined object
generation of spin-waves using
spin-current (spin-gun)
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Conclusion / questions
?? Conclusion

• for the first time, we can trace spin current
  phenomena in metals using optical means

?? Questions

• what BLS is sensitive for? (jsp or Dm but
  probably to Dm, as Dm can be understand as an
  induced magnetization).
• BLS signal from the substrate?
• origin of 2w BLS signal?
• wrong impedance matching between device and rf
  generator (difficult to measure BLS intensity
  on pumping frequency)

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Prof. Y. Otani (group leader)
Dr. T. Kimura

• Institute for Solid State Physics
• University of Tokyo, Japan
• Institute for Solid State Physics
• University of Tokyo, Japan

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Switching by non-local spin current
R m?
jch mA
Py particle 160 x 80 x 5 nm

• observation of the transition from anti-parallel
  to parallel states due to non-local spin-current.
• magnetization reversal in opposite direction was
  not observed.

Kimura, PRL 96, 037201 (2006)
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3D distribution of jsp
arrows jspj?-j? color mapping ?µµ?-µ?
arrows jchj?j? color mapping µ(µ?µ?)/2
wcu300 nm
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Conclusion / overview

• magnetization reversal by non-local spin current