Magnetic thin films: from basic research to spintronics Christian Binek 11/18/2005 Physics 201H Why thin films Length (and time) scales determine the physics.

Magnetic thin films: from basic research to spintronics

Christian Binek11/18/2005

Physics

201H

Why thin films

Length (and time) scales determine the physics of a system

all macroscopic properties

Electronic states

Quantum mechanics tells us: Confinement of electrons by lowering dimensions affects the electronic states

3D bulk 2D film 1D wire 0D quantum dotsartificial atoms

Size matters

11/18/2005

Physics

201HWhen can films considered to be thin

or

thin with respect to what

Thin in comparison with the characteristic length scale

Examples:

-Superconducting thin film thickness correlation length

-optical thin film like dielectric mirrors Length scale /4 500nm/4

dcharacteristic length

d

d

11/18/2005

Physics

201H-Magnetic thin films approach the ultimate extreme

ferromagnet

ferromagnet

spacernonmagnetic

Spacer thickness d in # of atomic layers

thickness quantum mechanical exchange interaction length a few atomic layers

d=8 monolayer

J(d=8)>0

Ferromagneticcoupling

d=10 monolayerJ(d=10)<0

Antiferromagneticcoupling

Exchange J(d)

How to grow magnetic heterostructures

?

> 250 000

Molecular Beam Epitaxy

•Thin film growth @ low deposition rate•Ultra high vacuum condition

)Pa(mbar 810 1010

deposited material

Important growth modes in heteroepitaxy

Reflection High-Energy Electron Diffraction RHEED

Layer-by layer (Frank van der Merwe)

3D islands (Volmer weber)

Monolayer followed by 3D islands (Stranski Krastanov)

Electron gunup to 50 keV

3o

sample RHEEDscreen

Eyecamera

specific free energy

B A

B A substrate

interface

What are the magnetic heterolayers good for

Basic components of modern spintronic devices

•Conventional electronics has ignored the spin of the electron

•Advantages using spin degree of freedom:

magnetic field sensors M-RAM

?

Spin-transistor

semiconductor

Quantum-information

1960 1970 1980 1990 2000 201010-3

10-2

10-1

100

101

102

103

104

105

Are

al d

ensi

ty [M

b/in

2 ]

Year

Superparamagnetic effect

inductive read head

Magnetoresistive heads

GMR

Evolution of magnetic data storage on hard disc drives

•Impact of GMR based field sensors on magnetic data storage

rotating sensorlayer FM1

fixed layer FM2

How to pin FM2 while the sensor layer FM1 rotates?

Exchange Bias!Pinning of the ferromagnet

by an antiferromagnet

-40 -20 20 40

-10

-5

5

10

m [1

0-7A

m2]

0H [mT]

TN

AF

T

H

Hfc

field cooling: from T>TN

to T<TN

FMFM0

FMAFEB t M

SJS:H

Meiklejohn Bean: uniform magnetization reversal of a pinned FM

coupling constant: J

FM interface magnetization: SFM

MFM :saturation magnetization of FM layer

tFM

KFM, H

MFM

0AF F

EM

FM FM

J S SH

M t

Exchange bias field:

AF interface magnetization: SAF

20 FM FM FM FMF - M t cos K t s nH i

A FMF S S-J cos

Stoner-Wohlfarth AF/FM-interface coupling

F2

0 FM FM FM M FAF MF -( M t J ) coH s K t sinSS

0 FM FMM tFM F

AF

M

FMHSJ

M t

S

Cr2O3(0001)/Pt0.67nm/(Co0.35nm/Pt1.2nm)3/Pt3.1nm

Electric control of the Exchange Bias

Investigated multilayer system:

perpendicular magneticanisotropy

tPt=1.20nm

tCo=0.35nm

FM thin film with

EαM II

Magnetoeletric effect of Cr2O3

MagnetizationM=m/V

electric field E=U/d

Cr2O3 (0001)

U

Co

Co

Co

Pt

Pt

Pt

Cr2O3 (0001)

Cr2O3: Magnetoeletric AF, TN=308K

U

FMFM

FMAFE0 tM

SJSH

E M contributes to SAF

Idea:

SQUID-magnetometry @ T=290K *

*A. Hochstrat, Ch.Binek, Xi Chen, W.Kleemann, JMMM 272-276, 325 (2003)

M

-300 0 300

-0.04

0.00

0.04

(µ

0HE)

[mT

]

E [kV/m]

CoPt

Cr2O3 (0001)

U=Ed

Change of the exchange bias field as a function of the electric field at T = 150K

Magnetoelectric Switching of Exchange Bias*:2 Control via field-cooling*P. Borisov, A. Hochstrat, Xi Chen, W. Kleemann and Ch. Binek, PRL 94 117203 (2005)

Magnetic Field Cooling (MFC)

cooling from T>TN in 0Hfr = +0.6 T and Efr=-500 kV/m

Magneto

Field

Electric

Cooling

(+,-)

cooling from T>TN in 0Hfr =+0.6 T and Efr=+500 kV/m

Magneto

Field

Electric

Cooling

(+,+)

Magneto-optical Kerr measurements @ T = 298 K after cooling from T>TN in 0Hfr = 0.6 T

The sign of the Exchange bias follows the sign of EfrHfr

-0.2 0.0 0.2

-1.0

-0.5

0.0

0.5

1.0

M /

MS

0H [mT][T]

(+,-)EfrHfr<0

(+,+)EfrHfr>0

H

R

Spintronic applications*

ME

FM 1

FM 2V V

ME

FM 1

FM 2

*Ch. Binek and B. Doudin, J. Phys.: Condens. Matter 17 (2005) L39–L44

ME

V

ME

FM1

FM2

NM

V

FM1

FM2

NM

U

H

R

U

-He-Hi He-Hi

H

R

Voltage

Input

X:=0

1

+H

-H

Exclusive Or

magn. fieldY:=

-V

+V

1

0

Output

R high

R low

0

1

Example:0+V

-H 0

x | y | xORy 0 | 0 | 00 | 1 | 11 | 0 | 11 | 1 | 0

finite anisotropy KAF≠0

22

333

08 AFFMFMAF

FMAF

FMFM

FMAFe

ttMK

SSJtMSSJ

H

Basic research with magnetic heterostructures

generalized Meiklejohn Bean approach

J :coupling constant

SAF/FM :AF/FM interface magnetization

tAF/FM :AF/FM layer thickness

MFM :saturation magnetization of FM layer

Experimental check of advanced models

understanding the basic microscopic mechanism of exchange bias

Exchange bias is a non-equilibrium phenomenon

new approach to relaxation phenomena in non-equilibrium thermodynamics

reduction of the EB shift upon

subsequent magnetization reversal

of the FM layer

Training effect:

- origin of training effect

- simple expression for

0 EBH vs. n

The training effect: a novel approach to study relaxation physics

Relaxation towards equilibrium

Landau-KhalatnikovF

SS

:phenomenological damping constant

Training not continuous process in time, but triggered by FM loop

discretization of the LK- equation

AF AFAF

S (n 1) S (n)S

Discretization:

LK- differential equation difference equation

1st& 9th hysteresis of NiO(001)/Fe

Comparison with experimental results on NiO-Fe

NiO

12nm Fe

(001) compensated

experimental data

recursive sequence

232 eEB 0 EB EB EB

n

f H (n) H (n) H H (n 1) min.

f0

and

0.015 (mT)-2

0 EB

f, 0

( H )

e

0 EBH 3.66 mTe

Magnetic NanoparticlesCollaborations

25nm25nm25nm25nm

self-assembled Co clusters

~5nm

Transmissionelectron microscopicimage

I thermally decompose metal carbonyls in the presence of appropriate surfactants

You want to know what I am doing?

Fundamental questions

Which magnetic interactions dominate the system

What kind of magnetic order can we observe

For large particle distances the dipolar interaction will dominate

Here is a real fundamental question:

Do dipolar systems still obey extensive thermodynamics

What does this mean:

Magnetic moment ,T,H = 2 Magnetic moment ,T,H

Simulations suggest:

Yes: for a 2 dimensional array of dipolar interacting particles

but

No: for a 3 dimensional array of dipolar interacting particles

Modifications of conventional thermodynamics required

Summary

MBE is a technology at the forefront of

modern material science

magnetic heterolayers are basic ingredients for

spintronic applications

magnetism of thin films and nanoparticles

provides experimental access to fundamental questions in statistical physics

25nm25nm25nm25nm

V(X)

x

x

dVF

dx

equilibriumdV

0dx

Mechanical analogy

equilibrium dF0

d

F()

eq-eq

xeq

Damped harmonic oscillator:

mmx x Dx 0

2dV d 1Dx

dx dx 2

Solution for:

2 22 2

0 01 1t t t

2 22x(t) e A e B e

with0

x(0) 0

x(0) x

22

0

1 D:

2 m

22

0

1

2

22 2

0 0

1 1...

2 2

00 02

20

x1A x x

2 12

2

002

20

x1B x

2 12

2

0

A 0

B x

20 t

0x e

22

01t t

220x(t) x e e

20 t

0x(t) x e

also derived fromintegration of:

dVx

dx where

m/ dV

Dxdx

0 5 10 15

0,0

0,2

0,4

0,6

0,8

1,0

X(t

)

t

Temporal evolution of X with increasing damping:

20 t

0x(t) x e

0

x t

x 0

dx Ddt

x

0 dVx x

dx m

Near earth outer space:610 (100 )P mbar Pa

384,400 km

1110 (1 )P mbar nPa