Efficient Spin Injection and Absorption Using CoFe-Based Alloys
Takashi KIMURA
Department of Physics Kyushu University
Fukuoka, Japan
KMS meeting
Main Contributors & Outline
1.Concept of Spin Current
2.Efficient electrical spin injection
3.Efficient thermal spin injection
4.Spin current absorptions in hybrid systems
Kazuto Yamanoi
SSP Lab D2SSP Lab D2
Tatsuya Nomura
ShaojieHu
Lab PD
Main Contributors
Outline
Spin current : Flow of spin-angular momentum
Manipulation of magnetization
Charge & Spin
Flow of Charge Electric Current
Flow of Spin Spin Current
Two important innovations for development of spintronics
Concept of spin currents
Manipulation of electric current Spin
current
j j j
0Sj j j
Charge current (Flow of charge)
Spin current (Flow of spin angular momentum)
j j j
0Sj j j
Conduction electrons in NM & FM
Charge current (Flow of charge)
Spin current (Flow of spin angular momentum)
Ferromagnetic (F) metal
Nonmagnetic (N) metal
Electrical spin injection
Spin current appears over the spin diffusion length.
x
F NIe
Applying electric field across a F/N junction
x
Spin diffusion length (l)
exp /x l Non-equilibrium spin accumulation
Highly polarized
Conventional
Spin reabsorption effect in F/N interface
x
Is
Electrical spin injection
CPI
P
0S CI PI
Original spin current is proportional to spin polarization.
Spin polarization cannot
maintain the original value.
Highly polarized
Conventional
Spin reabsorption effect in F/N interface
x
Is
Backflow can be suppressed
by high spin polarization.
2 N N
F F
1 1I
P
P
l
l
Electrical spin injection
0 0.2 0.4 0.6 0.8 110-3
10-2
10-1
100
P
21 1I
P
P
Spin reabsorption effect in F/N interface
I
10
Drastic increasedue to spin polarization
Highly polarized
Conventional
x
Is
Spin polarization is key.
N N
F F
l
l
• Increase of original flow• Suppression of backflow
純スピン流の検出法:非局所スピンバルブ測定法Evaluation of spin injection efficiency
Nonlocal spin valve P
AP
S SR I
RS : Spin signalNiFe : P~0.3, CoFeAl : P~ 0.6
-100 0 100
-5
0
5
Cu
e
HPy
e
CFA
500 nm
H
Py/Cu LSV
CFA/Cu LSV
500 nm
Evaluation of electrical spin injection efficiency
H (mT)
CFA/Cu
Py/Cu
5mΩΔR
(mW
)4.8mW
0.33mΩ
V
V
Py
CFA
Junction size 130 × 130nm
S. Hu et al. NPG asia mater. (2014)
Comparison between Py/Cu and CFA/Cu LSV
Over 10 fold
Distance dependence
R
S(m
W)
V
FM FlF PF
Py0.5
fWm2 0.35 0.015
CFA1.2
fWm2 0.67 0.10
S. Hu et al. NPG asia mater. (2014)
Transparent interface
Outline
1.Concept of Spin Current
2.Efficient electrical spin injection
3.Efficient thermal spin injection
4.Spin current absorptions in hybrid systems
スピン流の生成
Charge current 𝐼C = (𝜎↑ + 𝜎↓)𝛻𝑉
Spin current 𝐼S = (𝜎↑ − 𝜎↓)𝛻𝑉
Spin current 𝐼S = (𝜎↑𝑆↑ − 𝜎↓𝑆↓)𝛻𝑇
Charge current 𝐼C = (𝜎↑𝑆↑ + 𝜎↓𝑆↓)𝛻𝑇
Electrical Spin Injection𝜎↑ , 𝜎↓: Electrical conductivity
Spin current generation due to 𝝈↑ ≠ 𝝈↓
𝑆↑ , 𝑆↓: Seebeck Coefficient
Electrical and Thermal Spin Injection
Thermal spin injection
Spin current generation due to 𝑺↑ ≠ 𝑺↓
スピン流の生成
Charge current 𝐼C = (𝜎↑ + 𝜎↓)𝛻𝑉
Spin current 𝐼S = (𝜎↑ − 𝜎↓)𝛻𝑉
Spin current 𝐼S = (𝜎↑𝑆↑ − 𝜎↓𝑆↓)𝛻𝑇
Charge current 𝐼C = (𝜎↑𝑆↑ + 𝜎↓𝑆↓)𝛻𝑇
Electrical Spin Injection𝜎↑ , 𝜎↓: Electrical conductivity
Spin current generation due to 𝝈↑ ≠ 𝝈↓
𝑆↑ , 𝑆↓: Seebeck Coefficient
Electrical and Thermal Spin Injection
Thermal spin injection
Spin current generation due to 𝑺↑ ≠ 𝑺↓
S
C
IP
I
02 2
ST S
C
S SI P PP P
I S
スピン流の生成Thermal Spin Injection
Spin current can be injected even at an open circuit condition !!
exp /x l Spin current diminishes with the diffusion.
Spin current 𝐼S = (𝜎↑𝑆↑ − 𝜎↓𝑆↓)𝛻𝑇
Charge current 𝐼C = (𝜎↑𝑆↑ + 𝜎↓𝑆↓)𝛻𝑇
𝑆↑ , 𝑆↓: Seebeck Coefficient
Thermal spin injection
Spin current generation due to 𝑺↑ ≠ 𝑺↓
Simplification of device structure
First demonstration of thermal
spin injection using NiFe
S S
Seebeck coefficient in FM
Generated pure spin current
can be injected into NM
A. Slachter,et al, Nature Physics 6, 879 (2010).
~20 nV
@ RT
Generated spin current is very small because of the small difference in
the Seebeck coefficients for NiFe.
NiFe/Cu lateral spin valve
Thermally driven spin current in conventional FM
𝐷↓(𝐸) 𝐷↑(𝐸)
𝐸
𝐷↓(𝐸) 𝐷↑(𝐸)
𝐸High T Low T
FM
F
,
,
E
dD ES
dE
Small spin splitting yields a tiny difference between S↑ and S↓ .
Inefficient situation for
generating spin current
Conventional FM such as NiFe, Ni, etc..
,SS S S S
Small spin-dependent Seebeck coefficient
Thermally driven spin current in a favorable FM
𝐷↓(𝐸) 𝐷↑(𝐸)
𝐸
𝐷↓(𝐸) 𝐷↑(𝐸)
𝐸High T Low T
FM
FM with a large exchange splitting
Signs for S↑ and S↓ may be different each other
Highly efficient situation
for generating spin current
0, 0S S
1S
S SP
S S
CoFe-based alloy
F
,
,
E
dD ES
dE
1µm HCu
CoFeAl/Cu lateral spin valve for thermal spin injection
CoFeAl
43 wt% Co
54 wt% Fe
03 wt% Al
Cu
CFA1 CFA2
Temperature distribution under thermal spin injection(COMSOL simulation)
2nd harmonic detection
Joule heating
I
V
2T I
T (K)
∇T=64 K μm−1
at I=0.78 mA
𝑉 = 𝑅1𝐼𝐴𝐶𝑠𝑖𝑛𝜔𝑡+𝑅2𝐼𝐴𝐶
2𝑠𝑖𝑛2𝜔𝑡 +⋯
-100 0 100-62
-61
H(mT)
∆𝑉𝑠2𝑓
= 0.873µV
-100 0 100-64.4
-64.3
-64.2
-64.1
H(mT)
Evaluation for thermal spin injection property
𝑉↑↑2𝑓
≠ 𝑉↓↓2𝑓
Anomalous Nernst effect S. Hu and T. Kimura,
Phys. Rev. B 87, 014424 (2013).
CoFeAl/Cu LSV NiFe/Cu LSV
NiFe detector
0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
Spin voltage
0 0.2 0.4 0.6 0.8 1
-80
-60
-40
-20
0
𝐼(𝑚𝐴)
Electric voltage
Confirmation of thermally excited spin current
Both of 𝑉𝑠2𝑓
and 𝑉02𝑓
are well
reproduced by the parabolic curves.
Clear evidence of thermal spin injection
Bias current dependence
𝑉02𝑓
0 500 1000 1500
0
1
2
L (nm)
𝜆𝐹 ≈ 2𝑛𝑚,𝛻𝑇𝐹 ≈ 64𝐾/𝜇𝑚(@ 𝐼 = 0.78𝑚𝐴)λ𝐶𝑢 = 500 𝑛𝑚
RT
𝑆𝑠 ≈ −72.1𝜇𝑉/𝐾,
𝑆↑ = −35.7𝜇𝑉/𝐾, 𝑆↓ = 36.4𝜇𝑉/𝐾
𝑆0 =𝑆↑𝜎↑+𝑆↓𝜎↓
𝜎↑+𝜎↓
= −22𝜇𝑉/𝐾
Δ𝑉𝑠 =𝑃𝐹𝑅𝐹𝑅𝐶𝑢𝜆𝐹𝛻𝑇𝐹𝑆𝑠
2𝑅𝐹 𝑅𝐹 + 𝑅𝐶𝑢 (cosh 𝐿/𝜆𝐶𝑢 + sinh 𝐿/𝜆𝐶𝑢 ) + 𝑅𝑁2 sinh(𝐿/𝜆𝐶𝑢)
𝑃 =𝜎↑−𝜎↓
𝜎↑+𝜎↓= 0.62
CuF F
L
Confirmation of thermally excited spin current
Comparison of thermal spin injection efficiency
CoFeAl/CuSlachter et al.
Nature Physics (2010)NiFe/Cu S. Hu et al.
NPG asia mater. (2014)
22 nV
𝑆𝑠 = −3.8 𝜇𝑉/𝐾 𝑆𝑠 = −72.1 𝜇𝑉/𝐾𝑃𝑆 =𝑆𝑆𝑆𝐶
= 0.19 𝑃𝑆 =𝑆𝑆𝑆𝐶
= 3.27
@ RT
Spin current density7.46×108 A/m2
Spin current density3.56×107 A/m2
Outline
1.Concept of Spin Current
2.Efficient electrical spin injection
3.Efficient thermal spin injection
4.Spin current absorptions in hybrid systems
Phys. Rev. B 72, 014461 (2005)
•With spin current absorber
Spin current is preferably absorbed
into spin absorber because of
the strong spin relaxation.
•Without additional contact
Spin current absorption
Symmetric flow with respect to
the injecting junction
600 nm
550 nm
Without middle wire
With middle wire
Py Py
Cu
Cu
PyPyPy
T. Kimura et al. Appl. Phys. Lett. 85, 3795 (2004)
Spin current absorption
0.2 mW
0.05 mW
Magnetic field (Oe)
Magnetic field (Oe)
-800 -400 0 400 800-0.4
-0.2
0
0.2
-600 -300 0 300 600-0.4
-0.2
0
0.2
V
/I(m
W)
V
/I(m
W)
R
R=
R=
Spin current absorption into Superconductor
K. Ohnishi et al. Sci. Rep. (2014)
Nanopillar-based Lateral Spin Valve
Influence of Joule heating is perfectly removed.
Spin transport in Cu/Nb bilayer
H (mT)
Spin signal at 10 K is strongly
reduced by spin absorption into Nb.
After the transition, it seems no spin absorption.
0. 8 mW
~0. 8 mW
~0. 2 mW
Longitudinal spin current Transverse spin current
A 2
21
1
LRSP
l
A
2 TRS
l
Relaxation lengths for longitudinal spin
Spin penetration depthSpin diffusion length
Longitudinal and transverse spin current absorptions
The absorption efficiency depends on the relative angle between the injecting
spin and the localized magnetic moment.
02 5 nmL sfD 02 / 1 nmT D J
Relaxation lengths for transverse spin
S. Nonoguchi et al. PRB (2012)
Longitudinal configuration
Transverse configuration
Spin current is moderately absorbed into the FM3.
Spin current is strongly absorbed into the FM3.
By controlling these two states, the spin signal can be modulated.
Spin absorption into a FM/NM bilayer
Modulation efficiency depends on
& PFM3𝜆 𝐿 / 𝜆 𝑇
Nonlocal spin signal with Cu/CoFeAl channel
Spin signal shows the significant reduction but is still detectable.
Cu channel Cu/CoFeAl channel
@RT
H (Oe)
R(m
W)
Spin signal modulation due to anisotropic spin absorption
A
B
C
D
RS
RM
A
B
C
D
Δ𝑅M
(Δ𝑅𝑠2)×100 = 43.2%
Modulation efficiency
Summary
1. Spin polarization improves not only the generationefficiency but also the injection efficiency.
2. CoFe-based alloy is an excellent material for the electricaland thermal spin injection because of its favorable bandstructure.
3. Superconductor is found to be a insulator for spin current.
4. Anisotropic spin absorption has been demonstrated.