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Current research in current-driven

magnetization dynamics

S. Zhang, University of Missouri-Columbia

Beijing, Feb. 14, 2006

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Outlines

Magentoelectronics started from discovery of giantmagnetoresistive (GMR) effect

Spin-dependent transport in magnetic metal based

nanostructures

Spin angular momemtum transfer: physics and potential

technology

Perspectives and conclusions

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M.N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988).

400

110

H (kOe)-40

H // [ 011]

What is giant magnetoresistance?

R

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Origin of GMRtwo current model

e e e e

EF

A ferromagnet Different numbers of

up and down electrons

R R

Up and down resistances

Low resistance High resistance

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GMR Reading head

Bit width

Bit length

Conductor

lead

JM Spin

valve

Spin valve

OR MNM

M

AF

01

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Concert efforts: theorists, experiments and

technologists on GMR

Theorists: predict, explain, model and design GMR

effects and devices

Experimentalists: design, fabricate, characterize, andmeasure GMR devices

Technologists: produce, evaluate, pattern, integrate, and

deliver GMR devicesIt would be otherwise impossible to push

the information storage so rapidly

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History of magentic tapes and hard disks

Now: 80Gbits/in2

5 years: 1

Terabits/in2

In 1988, giant Magnetoresistance (GMR) was discovered;

in 1996, GMR reading heads were commercialized

Since 2000: Virtually all writing heads are GMR heads

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GND

Magnetoelectronics: Magnetic Tunnel Junctions

High tunneling probability

Low resistanceLow tunneling probability

High resistance

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Al-O barrier

Cu (30)

IrMn

Co-Fe-B(4)

Ta (5)

IrMn (12)

Al-O (0.8)

Cu (20)

Ta (5)

Py (5)

Ta (5)

Co-Fe-B(4)

-1500 -1000 -500 0 500

0

20

40

60

80

100

0

10

20

30

40

50

60

T=4.2 K

Rp=23.4 RS=4.68 km2TMR=95.4%

TMR(%)

H (Oe)

(b)

TMR curves measured at RT (a) and 4.2 K (b) for

the Co-Fe-B/Al2O

3/Co-Fe-B junction after annealing.

Annealed at 265 0CT=300 K

S=10 x 20 m2Rp=22.3 RS=4.46 km2TMR=58.5%

TMR(%)

(a)

VSource: Dr. Xiufeng Han

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Brief History of TMJ

1974, M. Julliere (a graduate student) published anexperiment article which claimed 14% TMR in Fe/Ge/Fetrilayers. A simple model was proposed (the paperbecame a sleeping giant).

1982, IBM reported 2% TMR on Ni/AlO/Ni.

1995, Moodera (MIT) and Miyazaki (Japan) reported10% TMR for Co/AlO/Co.

1998, DARPA launched MRAM solicitation

1999, Motorolas 128kB MRAM demo

2003, IBM, Motolora, 4Mb MRAM chip demo More than 10 startup MRAM companies formed.

MRAM becomes internationally recognized futuretechnology

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Emerging non-volatile memory technologies

Flow

Spin

Quantity FRAM

PCRAM

MRAM

PFRAM SiC Bipolar

PMC

Molecular

Polymer Perovskite

NanoXtal

3DROM

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Current-driven spin torques

GMR/TMR: magnetization states control spin transport

(low-high resistance).

Adverse effect: spin transport (spin current) affectsmagnetization states?

Every action will have reactionspin transfer

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T

spin angular momentum transfer?

Momentum transferelectromigration

Angular momentum transfermagnetization dynamics

An impurity atom receives a force by

absorbing a net momentum of electrons:

electromigration is one of the major failuremechanisms in semiconductor devices.F

The atom receives a torque by absorbinga net spin angular momentum of electrons:

the spin torque can be used for spintronics

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Interaction between spin polarized current and

magnetization (J. Slonczewski, IBM)

m m

out i n

m e B

ou t

m e B

i n P

dMJ J

dt

J PJ M e

J PJ M e

Mp

M

Spin torque on the magnetic layerM

( )

/

J P

e

J B

dM a M M M dt

a PJ e

C t i d d D i ll ti

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0, 0

t t

Current torque on DW(Magnetic field pressure on DW, )

0, 0

t t

Massless motion!!

From Sadamich i Maekawa

Current induced Domain wall motion

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Magnetization dynamics: LLG equation

(micromagnetics)

1;| | | | 1; 1

( )

( )

( )

J P

J J

eff

eff

p

eff

b

a m m m

m mm m c m x x

dm dm m H m

dt dt

dm dm m H mdt dt

m m V

E mH

m

LLG+spin torque

Where

Spin valve

Domain wall

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Novelty of spin transfer torques

Manipulation of magnetization states by currents

Self-sustained steady state magnetization dynamics

Unusual thermal effects

Interesting domain wall dynamics

Dynamic phases: synchronization, modification and chaos

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First observation of current induced

magnetic switching by Spin torques

Co1=2.5nm

Co2=6.0nm

Katine et. al., PRL (2000).

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Self-sustained steady-states precession

2| | ( ) ( )

eff J p p eff

dEm H a M m M m H

dt

The first term is always negative (damping), the second term

could be positive or negative (it even changes sign at

different times).

Energy damping and pumping:

Limit cycle: the energy change is zero in an orbit

[ ( ) ( )] 0eff J p

E

E dm m H a m m

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Calculated limit cycles

2 2 2sin cos 2 sin cosE K H

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Kiselev et al., Nature (2003)

Experimental identification of limit cycles

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Unusual Thermal effects

Eb

P AP

Neel-Brown relaxation:

( , )exp( / )b B

f T E E k T

( , )f T Mwhere is algebraicdependent on T, E

Question: in the presence of thespin torque, how do we formulate

the relaxation time?

Thermal activation

Difficulty: the spin torque is not conservative: ( )J p m

a m m F m

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LLG equation at finite temperatures

( ) ( ) ( )

0

( , ) ( ', ') 2 ( ') ( ')

eff J P

i j

i j

dm dm m H h m a m m m dt dt

h

h r t h r t D r r t t

random field

( )

( )

eff m

eff

J p m

M

H E m

m H

a m m F m

Dm P

The magnetization receives following fields

Precessional conservative field

Non-conservative damping field

Non-conservative spin torque field

Diffusion field

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Solution of Fokker-Planck equation

( ) [ ( ) ( )] ( ) 0eff J p M

E E

B

P E dm m H a m m dm D P m

D k T

is diffusion constant (dissipation-fluctuation relation)

The probability energy density is:

'

'

( ) exp

( )' ' ( ')

( )

eff

B

J p

E

eff

E Eeff

E

EP E A

k T

a dm m m E E dE E dE C E

dm m H

where

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Experimental data interpretation

Telegraph noise

P

AP

P A P

P AP

H

P A PJ

P A P

J P A P

H+

J

R

Field alone Current alone

P A PH

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H-I phase boundary of equal dwell times.

Comparison with experiments

Equal dwell timesfor P and AP states

P A P

By simultaneously changing

H and J, one can always have

( )(1 )bc

IE H Const I

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Synchronization, modification and chaos

Limit cycle

+ 1. Another oscillator

2. AC external field

3. AC external currentLinear oscillator

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Calculated limit cycles

2 2 2sin cos 2 sin cosE K H

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Observation of synchronization by an AC current

Rippard et al, PRL (2005)

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Observation of mutual synchronization

Kaka et al., Nature (2005); Mancoff et al, Nature (2005)

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Observation of mutual synchronization

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Narrower spectrum width at synchronization

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Dynamic phases due to AC currents

M

M

M

M

20( )

0.02

200( )

0

aca Oe

H Oe

K

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Synchronization spectrax1

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Modification spectra (beating)

x2

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Synchronization and modification

agree well with experiments

x3

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Chaos spectrax3

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Theories of spin torques in ferromagnets

Me

Berger, domain drag force, based an intuitive physics picture

Bazaliy,

et al,

Waintal and Viret, a global pressure and a periodic torque

Tatara and Kohno, spin transfer torque and momentum transfer torque.

Zhang and Li, adiabatic torque and non-adiabatic torques

Barnas and Maekawa, non-adiabatic torque relates to the damping of the

adiabatic torque

within a ballistic transport model for half-metallic materials

MM Mx

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Spin torques in a domain wall

1

ex s sf

m mJ m M

t M

Equation of motion forconduction electrons

( )e J Jff

bM M

M H MM M

M M c M x xt t

/ 0.01ex

J J

sf

c b where

Interaction between conduction electrons and magnetization:

ex

H m M

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( , , )xm m x v t y z / /xm t v m x

If the wall is in steady motion, the current driven wall

velocity is independent wall structure and pinning potentials

extJ

xWHcv

ext ext x H H eSteady state wall motion

Steady state wall velocity is thus

xssejj

eff J J m m m m m H m b m m c m t t x x

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Wall velocity for different materials in a perfect

wire

Ms (A/m) P Wall velocity

(m/s)Co 14.46x105 0.35 1.41

Permalloy 8x105 0.7 5.1

Fe2O3 4.14x105 1.0 14.0

CrO2 3.98x105 1.0 14.6

27/101 cmAjs

Observed Domain wall motion in a nanowire

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Observed Domain wall motion in a nanowire

Yamagushi et al.,

PRL (2004)

Observed Wall velocity

8 2

3 /

1.2 10 /

v m s

j A cm

for

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Vortex domain wall motion driven by current

05.0,01.0/108

28

cmAje

Wall transition: from vortex all to transverse wall

xv

yv

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Magnetic tunnel Junction

1 0

Goal: using a reasonable currentto switch magnetization,

ideally less than 106 A/cm2

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Conductor

lead

J

Oscillation ofM (GHz)by a DC current

Application 2: local AC magnetic field oscillators

(generators)

Local AC field (1000 Oe) with spatial resolution 10nm!

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Application IV: concerns of CPP reading

heads

Bit width

Bit length

Conductor

lead

JM Spin

valve

01

The large current density in CPP

reading heads may produce

unwanted switching!

Goal: eliminates current-induced

switching for current density

larger than 107A/cm2

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Acknowledgement

Students: Dr. Yu-nong Qi,

Mr. Zhao-yang Yang, Mr. Jie-xuan He

Postdoctoral: Dr. Z. Li (Postdoctoral)

Collaborators: P. M. Levy (NYU)

A. Fert (Orsay-Paris)

Funded by: NSF-DMR, NSF-ECS, DARPA, NSFC