1

Exchange Bias in Antiferromagnetic-Ferromagnetic Bilayers

Kannan M. Krishnan

Department of Materials Science and EngineeringUniversity of Washington, Seattle

DoE/BES

Condensed Matter Colloquium, Physics Department, UW (11/01)

Broad Research Themes and Projects

Growth and Properties of Thin Film Structures

Nanoscience and Nanotechnology

Advanced Characterization with Electron and Photon Probes

Deposition by Ion Beam Sputtering, Sol-Gel ProcessingExchange Interactions in AFM/FM bilayersGrowth mode of complex oxidesMagnetically actuated shape memory alloy films

Synthesis by chemical, metallurgical and lithography routes (Group)Achieving theoretical coercivity and role of dilution in Fe-Nd-B alloys Self-Assembled Magnetic Nanocrystals (Mike Beerman , Yuping Bao)Patterned Media to overcome the superparamagnetic limit (J.D. Wright, G. Kusinski)

Measurements of Interface roughness and correlation with GMR (M.E. Gomez)Quantitative measurements of elemental segregation in Co-Cr media (Dr. W. Grogger)Detection/Resolution limits of energy-filtered imaging (Dr. W. Grogger, G. Kusinski)Magnetic imaging with electrons and photons - complementarity (G. Kusinski)

UW, UCB/LBNL

2

Magnetic Thin Films and NanostructuresMagnetic Thin Films and Nanostructures

☛ Physical phenomena (magnetoresistance, hysteresis etc.) - fundamental studies☛ Microstructure - unique characterization tools and atomic level control☛ Technology (Magnetic Recording, MRAM, hard magnets, MEMS, NDE ….. )

Vertically integrated ProgramVertically integrated Program

• UHV Ion-beam deposition • Chemical synthesis• Pulsed Laser Deposition (IML)

Synthesis & Processing Properties & Phenomena

Characterization NCEM - Electron microscopy

Mg Fe Mn

- 2

-1.5

- 1

-0.5

0

0.5

1

-4

-3

-2

-1

0

1

2

690 700 710 720 730

MC

D

MC

D Integration

Energy (eV)

MCDMCD Integr.

b0

2

4

6

690 700 710 720 730

Ab

sorp

tion

Energy (eV)

+3T/LC-3T/LC

a

ALS - X-ray magnetic circular dichroism

EELS

Mn-L Sr-M

O-K

• Magnetic• Transport

0

0.05

0.1

0.15

0.2

0.25

100 150 200 250 300ρρρρ

(((( ΩΩΩΩ

))))

T (K)

cm

H = 0

H = 1T

-0.0015

0

0.0015

-150 0 150

M (

emu)

H (Oe)

Spin Valve - Hysteresis Loop

Systems of Reduced Dimensions3D 2D 1D 0D

DoS

EnergyEF

d

DoS

EnergyEF

f(d)

For 2D behaviorOnly ONE level within∆∆∆∆E = + kT (0.026 eV) of EF

1st estimate, set energyof lowest level ~ kT ~ p2/2m

de Broglie wavelengthλλλλ ~ h/p ~ 8 nm (slabthickness)

Ultrathin filmsMultilayersInterfaces

ChemicalSynthesis

+Self

Assembly

And

Lithography

3

Growth & Novel Properties of 2D Structures

Ultrathin FilmsMultilayersInterfaces

UHV Ion-beam DepositionAFM FM

Frustration & Proximity Effects

M/M

s

Field (Oe)

-1.2

0

1.2

-400 0 400

(A) 200 °C(B) 250 °C(C) 300 °C

He

Exchange Bias

Cheng, Ahn and Krishnan, J. Appl. Phys., 89, 6597 (2001).

Perpendicular Interface Anisotropy

Ean = Keff Sin2θ

Keff = -2πMs2 + 2Ks/tm + KvCho, Krishnan, Lucas and Farrow, J. Appl. Phys. , 72, 5799 (1992).

Giant Magnetoresistance

H

R

∆R/R ~ 20-80%

Cyrille, Kim, Gomez, Santamaria, Leighton, Krishnan and Schuller, Phys. Rev. B. 62, 15079-15083 (2000)

H

200 nm

50 nm

Nanoscale materialsby Self-assembly

Chemical method --Size-selective precipitationSurfactant controls shapeBottoms-up approachMagnetism in systems of reduced dimensions

Puntes, Krishnan & Alivisatos,Science, 291, 2115 (2001)

Small Particles

Metallurgical ApproachAchieving theoretical coercivity limits & understanding mechanisms

Girt, Krishnan, Thomas and Girt, J. Mag. Mag. Mat., 231, 219 (2001)

4 µm

“Patterned” Media

Ion-implantation throughStencil Masks; Top-down approachOvercoming the super -paramagnetic limit

Kusinski, Krishnan, Denbeaux, Thomas, Weller, Rettner. Terris Appl. Phys. Lett. 79, 2211(2001)

4

Eij = -2 Jij Si Sj = Jij |S|2 θθθθ 2 for small θθθθ

• No fundamental limit on strength• Isotropic, Amorphous magnets• Determines magnetic orderJij =

3 kB Θc2 Z S(S+1)

θ

Jij

Mn

Fe

Co

Nirabrd

Bethe- Slater Curve

Factors affecting magnetic behaviour: Exchange Interactions

The spin-orbit-lattice interaction

sin2 sin4 Emcuniaxial = Ko + Ku1 θ + Ku2 θ + ..Cobalt Ku1 = 4.1x10

5 J/m3 Ku2 = 1.0x105 J/m3

Factors affecting magnetic behaviour: Anisotropy

Determines Magnetic Orientation

5

Magnetic Behavior of Ferromagnets & Antiferromagnets

IronΘc ~ 790°C (1063 K)Strong magnetic Order

Weak Orientation

Superparamagnetic limit

KV

~25kBT

Rc ~ kBT

K

13

AFM + FM heterostructures

Exchange Bias : Unique Behavior not Observed in Bulk FM

ΘN

χ

T

Weak Magnetic OrderStrong Orientation

Antiferromagnet

Angle (Theta °)

He(B)= -111Oe [cos(ϑ )-0.15cos(3ϑ )-0.01cos(5ϑ )+0.01cos(7ϑ )+...]Hc(B)= 44Oe [1+0.82cos(2ϑ )+0.24cos(4ϑ )+0.05cos(6ϑ )+...]

He(A)= -26Oe [cos(ϑ )-0.34cos(3ϑ )+0.13cos(5ϑ)-0.07cos(7ϑ )+...]Hc(A)= 80Oe [1+0.46cos(2ϑ )-0.04cos(4ϑ )+0.01cos(6ϑ)+...]

-100

-50

0

50

100

0 100 200 300

H (

Oe)

He(B)

Hc(B)

Hc(Fe)

Hθ

M/M

s

Field (Oe)

-1.2

0

1.2

-400 0 400

(A) 200 °C(B) 250 °C(C) 300 °C

Unidirectional CouplingUnidirectional CouplingSample

HysteresisHysteresis

Exchange Bias in AFM/FM Bilayers

6

Frustration , Interface and Proximity Effects

• Phenomenon poorly understood• Interface effects dominate• Need element-specific probes• Growth of ideal structures - epitaxy• Technology

H

R0 RH

∆∆∆∆R/R ~ 20 %

R

H

“Spin”Valves

AFMFM

20nm 20nm

MnFe

Free Energy:

AFM

FMJint

θ

F = H MFM tFM Cos θ - Jint Cos θ + KFM Sin2θ

Solution in terms of an effective field

H’ = H - Jint / (MFM tFM ) = H - HE

HE = Jint / (MFM tFM )

If typical values for JFM are usedthen observed HE are too small

Exchange Bias: Basics

Easy Direction

7

Uncompensated

Compensated

Exchange Bias: Basics ….

Mauri (1987) Planar Domain Wall

Malozemoff (1987, 1988) Random Field Model

AFM Domain

Koon (1997) Spin Flop Coupling Model

Frustration Spin-Flop

a-axis normal

3.58Å

4.07Å

4.07Å

Spin Uncompensated

3.58Å

4.07Å

4.07Å

c-axis normalSpin Compensated

Pal et al, Jour. Appl. Phys. 39, 538. 1968

Crystal and Spin Structure of AFM Mn50Pd50

8

Ion-beam Deposition

Vacuum System

ThicknessMonitor

Substrate Heater (1000°C)

KaufmanIon Gun

Multiple Target Holder

Ar

Cathode

Anode

GlowDischarge

Magnets

2.5 cm diameter 1-2 keV20-50 mAmps0.5 Å/sec sputtering

UHV Reactive Ion-beam Deposition SystemUHV Reactive Ion-beam Deposition System

ComputerCotrolled Process &Diagnostics

Portable Platform

Accurate control of gas flowrates (O2, N2, Ar)

Load-lock Gate Valve

Sample-transferChamber

Transfer Rod

Kauffman-type ionsource

Target(4)rotation

Targetblock

Shutter

Substrate heater feed through (1000°C)Thermocouple

Thickness Monitor

Residual gas analyser

Versatile, Portable (ALS)

9

Epitaxial growth and exchange biasing of FM/AFM bilayers

Normal Structure

PdMn (~280Å)

Fe (~60Å)MgO(001)

10 20 30 40 50 60 70 80

inte

nsity

(a.u

.)1

theta

a-growth (002)MgO

(200)MnPd

(004)Fe-0.8

0

0.8

-500 0 500

mom

ent(

mem

u)

H(Oe)

He= 9 Oe

theta

inte

nsity

(a.u

.)

10 20 30 40 50 60 70 80

(001)MnPd

(002)MnPd

(002)MgO

(004)Fe

c-growth

-0.5

0

0.5

-500 0 500

mom

ent(

mem

u)

H(Oe)

He = 30 Oe

a-axis normal

3.58Å4.07Å

4.07Å

Spin Uncompensated

3.58Å

4.07Å

4.07Å

c-axis normal

Spin Compensated

X-ray TEM

GIXRD

Plan View

X-sectionθθθθ−−−−2222θθθθ

Scattering Geometries:

Complementary information for thin films

Electron Diffraction in a TEM

10

SAD[100]a-axis growthannealed

Chemical ordering of a-axis grown PdMn by annealing

a -ordereda -disordered

SAD[100]a-axis growth as-grown

10 20 30 40 50 60 70 80

inte

nsity

(a.u

.)

2theta (degree)

(002)MgO(200)

Fe( 002)

He ~ 0 He ~ 8-10 Oe

Cheng, Ahn and Krishnan, JAP (2001)

PdMn/Fe/MgO : Summary of growth & magnetic properties

-10

0

10

20

30

40

0 100 200 300 400 500

He

(Oe)

Growth Temperature (°C)

Annealed

As-grown

a-axis growth c-axis growth

Disordered

Ordered

11

Epitaxial growth and exchange biasing of FM/AFM bilayers

-1

0

1

-500 0 500

Mom

ent(

mem

u)

H(Oe)

He ~ 010 20 30 40 50 60 70 80

inte

nsity

(a.u

.)

theta

(002)MgO

(200)MnPd

(004)Fe3.58Å4.07Å

4.07Å

a-axis normal

-1

0

1

-500 0 500

Mom

ent (

mem

u)

H(Oe)

He = 72 Oe10 20 30 40 50 60 70 80

inte

nsity

(a.u

.)

theta

(002)MgO

(002)MnPd

(001)MnPd

(004)Fe

3.58Å

4.07Å

4.07Å

c-axis normal

Inverse Structure

PdMn (~280Å)

Fe (~60Å)

MgO(001)

Phillips 200/FEG with Image Filter

d band

2p3/22p1/2

1s

e-

Diplole Selection Rule ∆l = + 1 apply

High Resolution EELS Studies in a TEMElement-specific spectroscopy and imaging at high spatial resolution

550 600 650 700 750 800

100K

200K

300K

400K

500K

600K

700K

Energy Loss (eV)

12 nm

Cr L 3,2 Fe L 3,2

Element-resolvedlocal electronic structure

Spatially-resolved measurements

12

Energy-filtered Imaging in a TEM: Detection Limits

Phillips 200/FEG with Image Filter

d band

2p3/22p1/2

1s

e-

Diplole Selection Rule ∆l = + 1 apply

Spatially-resolved measurements

Si

PdMn

Mn

PdMn

PdMn

PdMn

PdMn

PdMn

PdMn

Mn

Mn

Mn

Mn

Mn

Mn

10 nm

arb.

uni

tsnm

0

1

2

3

4

5

6

arb.

uni

ts0 1 0 2 0 3 0 4 0 5 0

nm

0.4 0.9 1.3 1.7 2.0 2.4 nm1.1

4 Å

8 Å

12 Å

16 Å

20 Å

24 Å

c)

20 nm

-1

0

1

- σσσσ

+ σσσσ

Simple Roughness Measurements Using Energy-filtered Images

MgO

NiMnTEM Bright Field Image

Fe-L2,3 jump ratio image

Trace or Mean Position

13

Fe Layer Roughness in Fe/MnPd bilayers

2/12

1

2 ))1

1()( ( >

14

x0

ξξξξ////////

xL

σσσσL(x0)z(x)

x

h(x)

h

Interface Profile h(x)Average Value or hHeight Deviation z(x) = h(x) - h

Interface width ~ rms roughness σ(L) ∼ < σ L(x0) >x where the averaging is done over all points x within L and

σL(x0) = < |z(x) - zav(L)|2>L1/2 zav(L) is z(x) averaged over L

Correlation length, ξξξξ || is extractedby fitting

σσσσ((((L)))) ∼∼∼∼ σσσσsat [ 1- exp (-L/ξξξξ ||)]

Definitions: Rough interface with various characteristic length scales

Xray Magnetic Circular Dichroism

d band

2p3/22p1/2

1s

EE

( or )

L3

∆l = + 1XAS & EELS

L2

XMCD∆l = + 1

∆mj = + 1

left

right

spinorbital Sum Rules

- 2

-1.5

- 1

-0.5

0

0.5

1

-4

-3

-2

-1

0

1

2

690 700 710 720 730

MC

D

MC

D Integration

Energy (eV)

MCDMCD Integr.

b

0

2

4

6

Ab

sorp

tion

+3T/LC-3T/LC

a

morb / mspin = 0.03

morbmspin

= 2q

(9 p - 6 q)

Element Specific, Spin/Orbital Resolved Magnetic Measurements

15

circularlypolarizedx-ray photon

silicon window

silicon nitridemembrane

NiMnFe(Co)

Au cap

Magnetic structure of AFM-FM bilayer interfaces

690 710 730 750

Absorption

680 700 720 740 760

Dichroism

Cheng, Krishnan, Farrow, Young, Girt , .JAP (in press)

M/M

s

Field (Oe)

-1.2

0

1.2

-400 0 400

(A) 200 °C(B) 250 °C(C) 300 °C

Fe L3,2

660 680 700 720 740 760 780

C

B

A

He

He ( B > C > A ) MFe (A > C > B)

• Iron Moment Correlates inversely with Exchange field• The films show (111) texture• (111) is spin compensated• Iron diffuses into MnNi and orders antiferromagnetically• The decrease in iron moment suggests ~ 4MLs of diffusion• Fe-Mn is a well known disordered AFM• Need to investigate chemical and structural roughness

MicrospectroscopyMicrospectroscopy & & SpectromicroscopySpectromicroscopy with Photons (ALS) with Photons (ALS)

PhotoEmission Electron Microscope

16

3, RT 18, 170 C 27, 220 C

39, 280 C 48, 3.9, 300 C 63, BKRT

PEEM : Temperature Dependence

HB

Cheng, Ohldag, Krishnan (2001)

Interface is key to understanding such anisotropy.

K = - 2 ππππ Ms+ + K 2 Kt

s

MV

2

effE = K sin ΘΘΘΘan eff

2

Perpendicular Interface Anisotropy

GaAs (001)

Ag

[110] Pt, Ag

GaAs (001)

Ag

[001] Pt, Ag

GaAs (111)

Ag

[111] Pt, Ag

Co

Kef

f tm

CoxPd18

FexPd18

x(Å)4 8

Recent discoveries in artificially structured magnetic films

17

Exchange Bias in Systems with Out-of-Plane Coupling ?

3.58Å

4.07Å

4.07Å

MnPd

FePdFePd

Malozemoff (1987, 1988)

Random Field Model

AFM Domain

MgOFe

MnPd

Lithographically Patterned Antiferroamgnetic Elements ?

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Acknowledgements

Ning Cheng - Growth & Magnetic measurementsDr. J.-P. Ahn - Ion Beam DepositionDr. Werner Grogger - EFTEM Imaging & AnalysisChris Nelson - Philips CM200 OperationDr. H. Ohldag - PEEM Beamline ScientistDr. Tony Young - XMCD measurements

DoE/BESMax Kade Foundation