shang-fan lee et al- spintronics ----spin transfer torque in magnetic nanostructures and spin...

8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect

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Shang-Fan Lee ()

Y. D. Yao(), Y. Liou( )S. Y. Huang (), F. T. Yuan (), C. Yu ( ), T. W. Chiang (

), L. K. Lin (), L. J. Chang (), Faris B.

Y. L. Chen(), Y. C. Chiu (), Y. H. Chiu ()Institute of Physics, Academia Sinica

J. J. Liang () D. S. Hung()

Dept. of Physics, Fu Jen University Dept. of Info. Telecom. Eng.

Ming Chuan University

Financial support from National Science Council and Academia Sinica

----

Spintronics ---- Spin transfer torque in magnetic

nanostructures and spin pumping effect


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Spintronics :

Electronics with electron spin as an extra degree of freedomGenerate, inject, process, and detect spin currents

Generation: ferromagnetic materials, spin Hall effect, spin

pumping effect etc.

Injection: interfaces, heterogeneous structures, tunnel

junctions

Process: spin transfer torque

Detection: Giant Magnetoresistance, Tunneling MR 38, 898 (2007).

30, 116 (2008).

87, 82 (2009).


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Single Magnetic Domain Wall Resistance

Phase diagram of magnetization reversals

Edge roughness effect on domain wall mobility

Current driven magnetization reversals

Possible applications:

Magnetic sensors, Reading heads Magnetic RAM

Logic operation

Magnetic Nano-structures


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-400 -200 0 200 400

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

MR(%)

0.0 0.1 0.2 0.30.00

0.05

0.10

0.15

0.20

one - step

two - step

wid

th(m)

b/a

calculated one - step

calculated two - step

(a)P1

P3

-400 -200 0 200 400

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

H (Oe)

MR(%)

or

P2P2

Variation of magnetization reversal in pseudo-spin-valve elliptical rings

APL 94, 233103 (2009)


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Exchange bias in spin glass (Fe9.6 at.%Au)/NiFe thin films

APL 96, 162502 (2010)

We investigated the exchange bias in the (Fe9.6 at.%Au)/NiFe.

While the temperature was increased to a compensation

point, sign change in exchange bias was observed in the

thickness range of the FeAu layer from 5 to 100 nm. We

suggest that the inverse bias originates from the magnetic

relaxation of the SG layer induced by the reversible rotation

of interfacial FeAu spins coupled to the magnetic moments of

NiFe. The inverse bias was also found to decrease with the

increasing maximum field of a hysteresis loop


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(tranport of magnetization by an electrical curent)

- fundamentals

- switching of magnetization by spin transfer torque

applications (STT-RAM, reprogrammable devices)

- microwave oscillations by spin transfer and applications totelecommunications

Spin transfer Torque


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Compensation between magnetoresistance and switching

current in Co/Cu/Co spin valve pillar structure

MR ratio and current density for induced magnetization reversal showed compensation

behaviors. In order to achieve maximum efficiency (MR ratio) and minimum

consumption (critical current) in a practical device, the thickness of the injection layer

should be around the spin diffusion length for optimum performance.

APL 96, 093110 (2010)


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Transport geometry

CIP resistance can be measured easily, CPP resistance

needs special techniques.

From CPP resistance in metallic multilayers, one canmeasure interface resistances, spin diffusion lengths, andpolarization in ferromagnetic materials, etc.

lead

CIP geometry

~

CPP geometry

~

lead


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Magnetic switching Generation of microwave oscillations

SPIN TRANSFER

H


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Transverse

component

The transverse spin component is lost by the

conduction electrons, but is actually transferred to the

global SPIN of the layer rotation ofS S

S

F1 F2

0.1 m

S

Concept of spin transfer(Slonczewski 1996)


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Au

4 nm

10 nm

Free magn. layer

CuPolarizer

Trilayered pillar or tunnel junction

Metallic pillar 50x150 nm

x

1) Magnetization switching by spintransfer

2) Sustained precession of themagnetization of the free layerand generation of radio-frequency

oscillations

Two regimes of spin transfer

Applications: writing a memory, etc

Applications: spin transfer nano-oscillators (NSTOs) for

communications (telephone, radio,radar)

Zero or low field

Appl. field

H

Polarizermagnetization

Free layer magnetization

70 nm

Au

CoFeB

MgO

CoFeB

Tunnel junction 50x170 nm


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Experimental observations

A. Giant Magnetoresistance (Disc. 1988)

Change Magnetic Order Change Resistance (or Current)

F

N

F

P

AP

Read Heads; Sensors; MRAM (Tunneling)

B. Current-Driven Switching

F

F

N

J

First F Polarizes J. Polarized J exerts Torqueon second F. +Jc Flips to AP; -Jc Flips to P.

Write in MRAM? Write on MR Media?

Q: Physics; Minimize Jc

1.40

1.45

1.50

1.55

1.60

1.65

dV/dI()

AP

P295K

Py/Cu/Py

1.40

1.45

-0.2 0 0.2

dV/dI()

H (kOe)

P

AP

1.0

1.1

1.2

1.3

1.4

-6 -4 -2 0 2 4 6

dV/dI()

I (mA)

4.2KP

AP

1.0

1.1

-0.4 0 0.4d

V/dI()

H (kOe)

AP

P

P AP


16/46

Compensation between magnetoresistance and switching

current in Co/Cu/Co spin valve pillar structure

MR ratio and current density for induced magnetization reversal showed compensation

behaviors. In order to achieve maximum efficiency (MR ratio) and minimum

consumption (critical current) in a practical device, the thickness of the injection layer

should be around the spin diffusion length for optimum performance.

APL 96, 093110 (2010)

)2(2

sksc MHtM

p

eJ


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The Definition of Spin Polarization

Spin polarization ():

NN

NNNP

normal metal

E

metallic ferromagnet

E

4s3d

half-metallicferromagnet

E

Uex

P = 0 P = 10 < P < 1

Spin polarization of current: Ballistic or diffusiveII

IIP

Mazin, PRL 87, 1427 (1999)


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How to Determine the Spin Polarization

NNNNP

E

D(E)

h

Spin-polarizedphotoemission

Point-contact

Andreev reflection

S tip

FV

~

Spin-LED

Substrate

FSemi-conductor spacerQW

~

Spin-polarizedtunneling

H

S

F

I

V

~

Efficiency of spin

injection. Effects ofthe interface and

spacer are included.

P is barrier

dependent and

junction dependent

ff

ff

NN

NN

P


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Andreev Reflection : A Probe of Spin Polarization

Andreev reflection:A conversion of normal current

to supercurrent occuring at ametallic N/S interface.

N SE

N(E)

D

DEF

eVE

N (E)N(E)

The suppression of Andreevreflection due to spinpolarization serves as aprobe of the degree ofspin polarization.

When N is ferromagnetic,

only part of the electronsare paired.


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Tip-Sample Approach: Differential Screw

differential

screw

The turning shaft

The tip

The sample

The sliding tank

net

movement

0.79375 mm

0.75mm

0.04375mm

Differential screw gives abetter control of theplacement of the tip of 40 mper revolution.


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Modified BTK Theory

Superconducting gapSpin polarizationPInterface barrierZ

Pdependence Z dependence T dependence

-4 -2 0 2 40.0

0.5

1.0

1.5

2.0

P=0.4

P=0

Norm

alizedconductance

V (mV)

T=1.5K

DmV

Z=0

P=1

-4 -2 0 2 40.0

0.5

1.0

1.5

2.0

Z=0.4

Z=0

V (mV)

T=1.5K

DmV

Z=0

Z=1

Three parameters :

G. E. Blonder et al., PRB 25, 4515 (1982).

Y. Ji, G. J. Strijkers et al., PRL 86, 5585 (2001).

-4.0 -2.0 0.0 2.0 4.0

1.0

1.5

2.0

T=5K

T=7K

V (mV)

T=3K

Note : ballistic transport assumed


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Domain wall

Another spin transfer effect:

displacement of a wall between magnetic domains

Magnetic film

Magnetization to the right Magnetization to the left


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Edge Roughness effect on the magnetization reversal

process of spin valve submicron wires

-40 -20 0 20 40

1.000

1.005

1.010

1.015

-40 -20 0 20 40

1.000

1.005

1.010

1.015

-40 -20 0 20 40

1.000

1.005

1.010

1.015

-40 -20 0 20 40

1.000

1.005

1.010

1.015

(d)(c)

(b)

MRRatio

no spike(a)

spike 26nm (pitch 200nm)

MRRatio

H(Oe)


H(Oe)


Submitted to APL


24/46

Non-local measurement

Spin diffusion lengthHanle effect

Spin Hall effect & Inverse Spin Hall effect

Spin Pumping (spin battery)

Pure spin current

Spin Hall effect & Inverse Spin Hall effect


25/46

The extrinsic SHE is due to asymmetry in electron scattering for up and

down spins. spin dependent probability difference in the electron

trajectories

The Intrinsic SHE is topological band structures,


26/46

26

Pure Spin Currents: The Johnson Transistor

F1 F2

N V

e-

L

F1 N F2

Emitter Base

or

Collector

M. Johnson,

Science 260, 320 (1993)

M. Johnson and R. H. Silsbee,Phys. Rev. Lett. 55, 1790 (1985)

0

F2

F2

First Experimental Demonstrations

I+

I- V

Jedema et al., Nature 410, 345 (2001)

Cu film: s = 1 m (4.2 K)


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27

Spin Pumping

F N

Spin accumulation gives rise to spin current

in neighboring normal metal

IS

t

m

mgI rpump

S

4

In the FMR condition, thesteady magnetization

precession in a F is maintained

by balancing the absorption of

the applied microwave

and the dissipation of the spin

angular momentum --thetransfer of angular momentum

from the local spins to

conduction electrons, which

polarizes the conduction-

electron spins.


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tmmgI r

pumpS

4


29/46

29

Direct Detection of Spin Pumping via Inverse Spin

Hall Effect

FMR

Spin Current

in adjacent

normal metal

Transverse

Charge Current

E. Saitoh, et al., Appl. Phys. Lett. 88, 182509 (2006).

The spin-orbit interaction bends

these two electrons in the

same direction and induces a

charge current transverse to Js,

The surface of the Py layer is of a

1*1 mm2 square shape. Two

electrodes are attached to both

ends of the Pt layer.


30/46

The microwave mode with a frequency

off =9.45 GHz is exited in the cavity,

and The microwave power is 100 mW.

The FMR spectrum shows that the

magnetization in the Py layer resonates at

HFMR=130 mT.

A possible small discrepancy in the

sample position from the center of the cavity

may cause a microwave electric field at the

sample position and may generate dc AHE in

cooperation with FMR.

the ISHE contribution dominates the observed

V signal


31/46

31

E. Saitoh, et al., Appl. Phys. Lett. 88, 182509 (2006).


32/46

32

Spin Pumping

Ferromagnetic Resonance results in time-

dependent interfacial spin accumulation

This spin accumulation diffuses away from the

interface

Results in net dc spin current perpendicular to

interface

Additional spin current gives rise to additional

damping

Quantify spin current from

linewidth broadening

F N

IS


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33

Combine Spin Pumping and

Inverse Hall Effect

Use Spin Pumping to Generate Pure Spin Current

Quantify Spin Current from FMR

Measured Voltage Directly Determines Spin Hall Conductivity

Key Advantage: Signal Scales with Device Dimension


34/46

34

Determine Spin Hall Angle for Many Materials

Pt Au Mo

= 0.01200.0001

= 0.00250.0006

= -0.000960.00007


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38/46

Enhancement of magnetic damping in NiFe thin

films by structural defects

H = H0 + 4f/

= g|e|/2mc


39/46

H = H0 + 4f/ = g|e|/2mc


40/46

Summary

GMR effect charge can be controlled

by magnetization (spin).

STT (spin transfer torque) effect

-- magnetization can be controlled by

spin polarized current.

-- New materials with high spin polarization,low saturation moments,

high Curie temperature are needed.

Pure spin current will it be realized in circuits?


41/46

The Definition of Spin Polarization

Spin polarization ():

NN

NNNP

normal metal

E

metallic ferromagnet

E

4s3d

half-metallicferromagnet

E

Uex

P = 0 P = 10 < P < 1

Spin polarization of current: Ballistic or diffusiveII

IIP

Mazin, PRL 87, 1427 (1999)


42/46

How to Determine the Spin Polarization

NNNNP

E

D(E)

h

Spin-polarizedphotoemission

Point-contactAndreev reflection

S tip

FV

~

Spin-LED

Substrate

FSemi-conductor spacerQW

~

Spin-polarizedtunneling

H

S

F

I

V

~

Efficiency of spin

injection. Effects ofthe interface and

spacer are included.

P is barrier

dependent and

junction dependent

ff

ff

NN

NNP


43/46

Andreev Reflection : A Probe of Spin Polarization

Andreev reflection:A conversion of normal current

to supercurrent occuring at ametallic N/S interface.

N SE

N(E)

D

DEF

eV E

N (E)N(E)

The suppression of Andreevreflection due to spinpolarization serves as aprobe of the degree ofspin polarization.

When N is ferromagnetic,

only part of the electronsare paired.



44/46


differential

screw

The turning shaft

The tip

The sample

The sliding tank

net

movement

0.79375 mm

0.75mm

0.04375mm

Differential screw gives abetter control of theplacement of the tip of 40 mper revolution.

M difi d BTK Th


45/46

Modified BTK Theory

Superconducting gapSpin polarizationPInterface barrierZ

Pdependence Z dependence T dependence

-4 -2 0 2 40.0

0.5

1.0

1.5

2.0

P=0.4

P=0

Nor

malizedconductance

V (mV)

T=1.5KDmV

Z=0

P=1

-4 -2 0 2 40.0

0.5

1.0

1.5

2.0

Z=0.4

Z=0

V (mV)

T=1.5K

DmV

Z=0

Z=1

Three parameters :

G. E. Blonder et al., PRB 25, 4515 (1982).

Y. Ji, G. J. Strijkers et al., PRL 86, 5585 (2001).

-4.0 -2.0 0.0 2.0 4.0

1.0

1.5

2.0

T=5K

T=7K

V (mV)

T=3K

Note : ballistic transport assumed

Point contact Andreev reflection


46/46

Point contact Andreev reflection

Superconducting tip -10 -5 0 5 10

1.00

1.05

1.10

MBTK:

T=4.20K

z=0.00

p=0.475

D=1.00 meV=0.030

3D BTK:

T=4.20K

z1=0.155

z2=1.05

z3=11.5

p=0.385

D=0.950 meV=0.030

Co

nductance

V(mV)

data

MBTK

3D BTK

Our new BTK model

D dEeVEfEZZZPFGG

NN

NS )(),,3,2,1,(

IIIII ddcczaazzz )**(3)*(21

II

I

III

N S

Modified BTK theory

D dEeVEfEZPFGG

NN

NS )(),,,(

shang-fan lee et al- spintronics ----spin transfer torque in magnetic nanostructures and spin...

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