Manipulation of spin currents

1
Manipulation of spin currents spin Hall
effects in metallic systems
Institute for Solid State Physics, University of
Tokyo
Takashi Kimura
H. Nanaumi, K. Ohnishi, M. Morota Y. Otani
2
Advantage of planar spintronic devices
Conventional
Planar structures
MRAM
LEAD
MTJ
Mainly two terminal structure ?Obtained
information is only series resistance
Easily expand to multi-terminal devices
Difficult to make multi-terminal devices
Detailed study of spin current diffusion in F/N
hybrid structures
Development of novel functional spintronic
devices.
3
Contents

• Spin diffusion pure spin current
• Temperature evolution of spin relaxation
• Spin absorption effect spin torque
• Spin absorption effect spin Hall effect

4
Transport properties in FM
Nonmagnet
Flow of spin angular momentum is zero.
Ferromagnet
Generation of spin current
Spin current
Spin polarization
5
Electrical spin injection
Ie
Electrons are injected from F into N.
x
6
Detection of spin accumulation
Two Fs are in parallel
NiFe
Ag
Small spin accumulation in N Smooth current flow
? Small resistance
NiFe (Py)
100 nm
Two Fs are in anti-parallel
Permalloy wire 100 nm in width 30 nm in
thickness
Silver wire 100 nm in width 100 nm in
thickness
Large spin accumulation in N ? Large
resistance
7
Magnetoresistance due to spin accumulation
T. Kimura et al. Appl. Phys. Lett 85, 3501 (2004)
18 mW
R (W)
Magnetic field (Oe)
8
Local nonlocal spin injection
Jedema et al. (2002)
Local spin injection
Nonlocal spin injection
V
V-
Driving force for spin is the diffusion
Spin diffusion length
Spin current is superimposed on the charge
current.
Spin current without charge current
Spin current with charge current
9
Nonlocal spin valve measurement
Py
Py
Ag
V/I (mW)
DRS
H
Magnetic field (Oe)
NM
FM
Equilibrium position for FM potential shifts
above or under the middle point.
10
Spin diffusion length for Ag wire
T. Kimura et al. Phys. Rev. Lett. 99, 196604
(2007)
Py1
Py2
Ag
d
PI 0.17 _at_ RT 0.26 _at_ 77K
11
Spin diffusion length for various metals
T. Kimura et al. Phys Rev B (2005) T. Kimura et
al. Phys Rev Lett (2006) T. Kimura et al. Phys
Rev Lett (2007)
12
Spin flip scattering spin diffusion length
Spin orbit interaction with Phonon (Intrinsic)
Impurity, Defect, Grain boundary (Extrinsic)
Mainly study in thin films by conduction ESR
G. Feher and A. F. Kip, Phys. Rev. 98, 337 (1955).
Limited materials, Intricate analysis
Electrical spin injection detection technique
M. Johnson and R. H. Silsbee, Phys. Rev. Lett.
60, 377(1988).
Hanle effect ? Tunnel junction or Ohmic junction
with very long interval
Separation distance dependence of spin
accumulation
Simple evaluation of spin diffusion length
T. Kimura, et al. Phys. Rev. Lett. 100, 066602
(2008).
13
Sample fabrication procedure
Shadow evaporation
SEM image of a typical device
Py thickness 20 nm Cu thickness 30 nm 320 nm
Spin accumulation in a Cu wire is evaluated by
nonlocal spin valve measurements.
Distance between Py1 Py2
200 nm 1600 nm
14
Temperature dependence of spin signal
Cu
V-
I
H
V
Py1
Py2
Spin accumulation signal is found to take a
maximum at a characteristic temperature TM.
15
Spatial distribution of spin accumulation
d
Cu
Py2
Py1
16
Temperature dependence of spin accumulation
D is proportional to the conductivity.
Reduction of l should be due to the reduction of
tS at low T.
Spin diffusion length is found to take a maximum
at a characteristic temperature TM.
Why ?
17
Temperature evolution of spin flipping process
Cu surfaces are known to be easily oxidized in
air. Top side surfaces of the Cu wire may be
oxidized.
T gt TM
tCu
CuOx layer induces stronger spln-flip scattering
than the inside of Cu later
T _at_ TM
S. Yuasa et al, JAP 83, 7031 (1998).
Reduction of the temperature induces the
increment of the mean free path l.
T lt TM
Reduction of l can be caused by the increment of
the scattering in CoO.
18
Cu thickness dependence of spin accumulation
tCu
l is close to tCu at higher temperature.
l is close to tCu at lower temperature.
TM should increase with decreasing the thickness
tCu .
19
Spin current absorption into Py wire
Without middle wire
T. Kimura et al. Appl. Phys. Lett. 85, 3795 (2004)
Py
Py
Cu
600 nm
With middle wire
Py
Py
Py
Cu
550 nm
20
Spin current absorption into Py wire
Without middle wire
T. Kimura et al. Appl. Phys. Lett. 85, 3795 (2004)
Spin current flows almost isotropically in both
side.
With middle wire
Spin currents are preferable absorbed in the
middle Py wire.
Fast spin relaxation in FM Spin current
absorption into middle FM Pure spin current
into the detector
Spin absorption effect
21
Magnetization reversal due to spin injection
Spin injection into FM
Spin torque
Spin current with charge current are utilized.
Only spin current is required for the
magnetization switching
Nonlocal spin injection spin absorption effect
Possibility of magnetization switching by pure
spin current
22
Magnetization reversal due to pure spin current
Nonlocal planar device
T. Kimura et al. Phys. Rev. Lett. 96, 037201
(2006)
However
Only from AP to P
Realization of switching by pure spin current
Nonlocal vertical pillar devices
Back forth switching have been realized.
T. Yang, et al., Nature Phys. in press.
23
Spin Hall effect (Current ? Spin current)
Spin-orbit interaction
Since the scattering direction depends on the
spin, transverse spin current is generated.
Novel way for manipulating spin current without
FM (Charge current ? Spin current)
24
Spin Hall effect (Current ? Spin current)
Spin-orbit interaction
Since the scattering direction depends on the
spin, transverse charge current is generated.
Novel way for manipulating spin current without
FM (Charge current Spin current)
Transition metal (ex. Pt)
Material with strong spin-orbit interaction
25
Electrical detection of spin Hall effect
Nonlocal spin Hall device (Proposed by Takahashi
Maekawa )
Utilizing in-plane spin current
Valid only for the material with long spin
diffusion length (Weak
spin-orbit interaction)
Tiny spin Hall effect
26
Magnitude for spin-orbit interaction for various
metals
S. Takahashi S. Maekawa Physica C 437-438,
309-313 (2006)
27
Spin current absorption due to NM
T. Kimura et al. Phys. Rev. B 85, 3795 (2005)
Without
DRS
Cu
Spin resistance
DR (mW)
Au
Pt
_at_RT
Magnetic field (Oe)
28
Detection of inverse SHE using spin absorption
T. Kimura et al. Phys. Rev. Lett. 98, 156601
(2007)
Ie
x
S
y
Is
z
Distance between 1 3 400 nm
Ie
Ie
Pt 4 nm in thickness Cu 80 nm in thickness Py
30 nm in thickness
29
Inverse SHE observed in Pt wire
A
10 K
DVc/I (mW)
x
B
z
Sweep direction
y
Magnetic field (Oe)
A
B
Direction of the spin axis of generating spin
current is reversed by the magnetic field.
30
Direct SHE observed in Pt wire
A
10 K
DVs/I (mW)
B
Sweep direction
Direction of the generating spin current is not
reversed by the magnetic field.
Magnetic field (Oe)
A
B
Positive voltage appears.
Negative voltage appears.
31
Reciprocal relationship between spin charge
current
DSHE
ISHE
DRDSHE
DRISHE
Magnetic field (Oe)
Magnetic field (Oe)
Spin-Hall conductivity for the charge
current
Charge-Hall conductivity for the spin
current
Experimental demonstration of Onsager reciprocal
relation
means
32
Temperature evolution of SHE
DRSHE (mW)
T (K)
33
Origin of spin Hall effect
Spin orbit scattering due to impurity
J. Smit (1956)
(mWcm)-1
Quadratic term is dominant. ?
Side jump (?)
L. Berger (1970)
34
Intrinsic SHE
S. Murakami et al. Science (2004)
J. Sinova et al. PRL (2004)
Rashba effect
Luttinger model
Y. Guo et al. PRL (2008) H. Kontani et al. PRL
(2008)
Theory for d-electron system
Angular dependence of inter orbital hopping with
spin-orbit interaction
Orbital Hall effect (OHE)
35
Material dependence of intrinsic SHE
H. Kontani et al. PRL (2008)
SH conductivity (SHC) OHC x ltLS couplinggt
OHC is always positive does not depend on the
material so much.
LS coupling
Hunds third rule

• is positive for shells that
• are less than half filled

• is negative for shells that
• are more than half filled

Sign change of SH conductivity
H. Tanaka et al. PRB (2008)
36
Experimental study of SHE for various materials
Ie
x
S
y
Is
z
Middle wire
H
Pt, Pd, Nb, Mo, Au
37
Inverse SHE for various transition metals
Pd
Pt
DVc/I (mW)
DVc/I (mW)
DRSHE 80 mW
DRSHE 300 mW
Magnetic field (Oe)
Magnetic field (Oe)
Nb
Mo
DVc/I (mW)
DVc/I (mW)
DRSHE 60 mW
DRSHE 100 mW
Magnetic field (Oe)
Magnetic field (Oe)
38
SH conductivity spin Hall angle
Material
s (Wm)-1
sSHE (Wm)-1
sSHE /s
Pt (10K)
8.0 x 106
3.3 x 104
4.1 x 10-3
1
Pd (10 K)
2.2 x 106
1.1 x 103
0.5 x 10-3
Au (10 K)
2.0 x 107
2.0 x 104
1.0 x 10-3
Cu (10 K)
5.0 x 107
2.0 x 103
0.4 x 10-3
Nb (10 K)
2.7 x 106
-2.0 x 103
-0.7 x 10-3
Mo (10 K)
2.8 x 106
-1.4 x 103
-0.5 x 10-3
2
1. L. Vila et al. PRL (2007) 2. M. Morota et
al. Submitted 3. Ta also shows negative sign.
39
Comparison with theory
Numerical calculation intrinsic SHE
Experimentally obtained Hall angle
10-3
5d
Pt
4d
Au
sSHE /s
Pd
Ta
Mo
Nb
Number of electron
The experimental results seem to reproduce the
numerical results.
H. Tanaka et al. PRB (2008)
40
Conclusion

• Temperature evolution of spin relaxation process
  in Cu wire is investigated. The scattering due
  to the surface oxidation layer is found to be the
  dominant mechanism of spin-flip scattering in low
  temperature.
• The influence of the additional contact on spin
  current is investigated. The additional contact
  with fast spin relaxation is found to strongly
  absorb the spin current.
• Using spin absorption effect, the magnetization
  reversal due to pure spin current is realized.
• Using spin absorption effect, the spin Hall
  effects in the materials with strong spin-orbit
  interaction are detected efficiently.
• Experimentally obtained results seems to be
  consistent with the theoretical calculation of
  intrinsic SHE in d-electron system.