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.