EXCHANGE BIAS A comprehensive study Ivan K. Schullerpeople.virginia.edu/~saw6b/summerschool/pdf/schuller.pdf · EXCHANGE BIAS A comprehensive study Ivan K. Schuller Funded by DOE

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EXCHANGE BIASA comprehensive study

Ivan K. SchullerIvan K. Schuller

Funded by DOE and AFOSRFunded by DOE and AFOSR

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COLLABORATORSUCSD:R. Morales

Z-P LiI.V. RoshchinO. Petrasic

X. BatlleJ. Eisenmenger

C. LeightonK. Liu

S. K. SinhaS. Roy

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CollaboratorsLANL:

M FitzsimmonsJ.R. Groves

P. ArendtR. Springer

A. HoffmannP.C. Yashar

LLNLJ. KortrightK. Chesnel

U. MinesottaI. Krivotorov

E. D. DahlbergNIST:

C.F. MajkrzakJ.A. Dura

HMI:H. Fritzsche

ILL:Vincent LeinerHans Lauter

U. A. Barcelona:J. Nogues

RWTHA. TillmannsB. BeschotenG. Guntherodt

LBLE. Arenholz

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PLANPLAN• Introduction: Exchange Bias ?• Expectations: Deceptively Simple• Real Life: More Complicated• Key Ingredients: Obvious• Solution: Hard Work• Summary: Understanding• Open Questions

Physical = Structural Length Scale

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WHEN THINGS BECOME SMALLΨ extends outside the structureB extends outside the structure

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Interesting PhenomenaInteresting PhenomenaLength Scales

•Magnetic 1-100 nm

•Superconducting 1-1000 nm

•Elastic 1-10 nm

•Catalysis 1-10 nm

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EVERYTHING YOU WANTED TO KNOW ABOUT EVERYTHING YOU WANTED TO KNOW ABOUT FERROMAGNETISMFERROMAGNETISM………….but .but ……were afraid to askwere afraid to ask

FM

Free FMSmall coercivity HCExchange field HE=0SymmetricUniform

H

M

HC

Reversal.exe

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VERY INTERESTING

50 100 150

-1.0

-0.5

0.0

0.5

1.0

M /

MS

Temperature (K)TN

AF

FM

H HFC=1kOe

Proximity BetweenProximity BetweenDissimilarDissimilar MagnetsMagnets

Z-P Li et al, Phys. Rev. Lett. 96, 137201(2006)

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Exchange BiasExchange Bias

FM

Free FMExchange field HE=0Small coercivity HCSymmetricUniform

FMAF

Pinned FMLarge HELarge HC

HE

2HC

H

M

H

M

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Reversal Mode is the Same in Both Sides of the Hysteresis

Loop

In all materials systems known to man !!

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CoO-Co particles

Meiklejohn and Bean(1956)

CoO (AF)-Co(F)

Large bias (HE)

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Why Interesting?Why Interesting?Spin ValvesSpin Valves

Read HeadRead Head Magnetic RAMMagnetic RAM

Magnetic TunnelingMagnetic TunnelingExchange BiasExchange Bias Important DevicesImportant Devices

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Deceptively Simple

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Mechanism

HFC

FM

AFM

M

HA

T < T < TCN

W.H. Meiklejohn, C.P. Bean, Phys. Rev., 105, 904(1957).

Cool T<TN

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What to expect

• Maximum Uncompensated Surfaces• Zero for Compensated Surface• Negative• Reversal Symmetric

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CRYSTALLINE ORIENTATION

17J. Nogues, et al, Appl. Phys. Lett. 68, 3186 (1996)

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ROUGHNESS

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Field Cooling(110)MgF2/FeF2(500Å)/Co(38Å)/Al(25Å)

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

-1.0

-0.5

0.0

0.5

1.0

T=10K

HFC=0.1 kOe

Mag

netiz

atio

n (1

0−4 e

mu)

Magnetic Field (Oe)-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

-1.0

-0.5

0.0

0.5

1.0

T=10K

HFC=0.1 kOe

HFC=30 kOe

Mag

netiz

atio

n (1

0−4 e

mu)

Magnetic Field (Oe)-4000 -3000 -2000 -1000 0 1000 2000 3000 4000

-1.0

-0.5

0.0

0.5

1.0

T=10K

HFC=0.1 kOe

HFC=2 kOe

HFC=30 kOe

Mag

netiz

atio

n (1

0−4 e

mu)

Magnetic Field (Oe)J. Nogues et al, PRL 76, 4624 (1996), I. V. Roshchin et al, EPL 71,297 (2005)

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Big Problem

Why is the loop shape asymmetric?

Co/CoO

Meiklejohn & Bean,1956.

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Polarized Neutron Reflectometry

Spin-flip

cross-sections

• M⊥ H

• vs Q (vs z)

M. Fitzsimmons et alPhys. Rewv. Lett. 84, 3986 (2000)

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0

0.2

0.4

0.6

0.8

1

0.05 0.1 0.15 0.2 0.25

++--SF

Ref

lect

ivity

0

0.2

0.4

0.6

0.8

1

0.05 0.1 0.15 0.2 0.25

++--SF

Ref

lect

ivity

Q [nm-1- ]

-1

-0.5

0

0.5

1

-600 -400 -200 0 200 400 600

M/M

S

H[Oe]

Domain nucleation and wall motion.

Asymmetric reversal

64±4% of M rotated 90°

Q [nm-1- ]

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Moke longitudinal and transverse

s -polH ext p -pol

M LMT

B. Beschoten, G. Guntherodt et al, unpublished

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Kerr EffectHext

ML

s-pol.

p-pol.-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2

50°-pol

p-pol

s-pol

Ker

r rot

atio

n (a

.u.)

field (a.u.)

Longitudinal

Hext

s-pol.

p-pol.

MT

Transverse

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2

45°-pol

s-pol

p-pol

Ker

r rot

atio

n (a

.u.)

field (a.u.)

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Sp-pol.exe

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Reversal.exe

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Always the SameCoherentRotation

Domains

Asymmetric Magnetization Reversal

-1

-0.5

0

0.5

1

-1000 -500 0 500 1000

M/M

S

H [Oe]

? ? ?

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What to expect

• Maximum Uncompensated Surfaces• Zero for Compensated Surface• Negative• Reversal Symmetric

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Rich Phenomenology

• Spin Orientation: Uncompensated,Compensated• Sign: Negative or Positive• Asymmetry• Domains: F and/or AF• Training

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SYSTEMATIC STUDIES ESSENTIAL

• Quantitative (Structure, )• Systematic (One parameter at a time• Different measurements on same samples• Reevaluate (Even well accepted “facts”)

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Exhaustive System

•XF2 – Ferro X=Fe, Mn, Zn, Co, NiFerro=Fe, Co, Ni, Py

• GrowthHV e-beam evaporation

• Characterizationin-situex-situ

Polycrystaline, twinned, epitaxial

Thank you David Lederman

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ADVANTAGES

Easy growthControlled Crystallography

poly, twinned, single(110), (100), (211)

Different Anisotropies Non Magnetic Control (ZnF2)Low TN (~ 70K)

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PREPARATION

Polished(001) MgO

single crystal

200-300°C

Ag Fe FeF2

MgO

XF

Fe

Ag

2

10nm

90nm

MBEMBE

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Structure-Complicated

MgOFeFFeAl

2

glass

Untwinned TwinnedIn-plane

polycrystalline

FeF2

FeAl

ZnF2

bulk FeF2

MgOFeFFeAg

2

110 FeF110 FeF2 2

35SUPREX:

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Characterization

• RHEED Electron diffraction epitaxy• HA XRD High angle X-ray growth direction• GIXR X-ray reflectivity interfaces• AFM Scanning probe surface• GID In-plane X-ray in plane epitaxy

Bottom Line:

Poly, twinned, epitaxial fluoridesPolycrystalline Fe

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F Reversal Uniform?

AF

FM

R. Morales et al - In press

Confirmed by Mossbauer, Synchrotron, Neutrons

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Magnetic CharacterizationSQUID MOKE

(Magneto-optical Kerr effect)Depth dependence

(tprobed~ 30 nm)

FM

AF

Whole sample

70 nm30 nmFM

AF

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Magnetic CharacterizationSQUID MOKE

(Magneto-optical Kerr effect)Depth dependence

(tprobed~ 30 nm)

FM

AF

Whole sample

70 nm30 nmFM

AF

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-2000 -1000 0 1000-1.0

-0.5

0.0

0.5

1.0 SQUID MOKE

Py-side FeF2-side M

/ M

s

H (Oe)-200 -100 0 100 200

-1.0

-0.5

0.0

0.5

1.0

M /

Ms

H (Oe)

SQUID and MOKE

• Py/FeF2 bilayerT (150K) > TN (78K) T (10K) < TN

-2000 -1000 0 1000-1.0

-0.5

0.0

0.5

1.0 SQUID MOKE

Py-side FeF2-side M

/ M

s

H (Oe)

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-2000 -1000 0 1000-1.0

-0.5

0.0

0.5

1.0 MOKE loops Py-side FeF2-side

M /

Ms

H (Oe)

Reversal-Py/FeF2

• Incoherent rotation of a spring wall in the whole sample and parallel to the interface.

Py

FeF2

FMAF

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-2000 -1000 0 1000-1.0

-0.5

0.0

0.5

1.0 MOKE loops Py-side FeF2-side M /

Ms

H (Oe)

Reversal- Py/FeF2

• Incoherent rotation of a spring wall in the whole sample and parallel to the interface.

Py

FeF2

FMAF

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AntiFerromagnetAntiFerromagnetSURFACE OR BULK

Py

FeF2

Ni

Confirmed by Mossbauer, Synchrotron, Neutrons

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200 nm

20 40 60 80 100

T (K)

FM/AF/FMParallel

PyFeF2

Ni

PyFeF2

Ni

Anti-parallel

20 40 60 80 100

0

100

200

300

400

HE

B (O

e)

T (K)

Ni

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• Coupled across the interface

• Uncompensated spins at interface

• Uncompensated spins in the bulk

• Grains, inhomogeneity, roughness

• Anisotropy

3-LAYER MODEL

FM

Interface

AF

FM

AF

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KEY INGREDIENTS• Uncompensated Spins (AF)(Vertical Shift, Dichroism, Bulk AF, dAF

• Coupling Across Interfaces(+ & - HE, AF & F Domains, Mossbauer,• Anisotropy (F, AF): Uni, Bi, Four(Crystallography, Angular Dependence,• Inhomogeneity(Crystallography, Dichroism, • 3 Layers: F, Interface, AF(Neutrons, Dichroism,

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OPEN

• Training(4 fold Anisotropy, • Origin of the Uncompensated Spins(Structure, Dichroism, Universal ?????• Magnitude (Fe2O3,• Nanostructures(Competition of Length Scales,

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REFERENCESREFERENCES• “Exchange Bias”J. Nogues and IKS (JMMM 192, 203(1999))

• “The Exchange Bias Manifesto-2002”I.K.Schuller, G.G.Guntherodt,http://ischuller.ucsd.edu

• “Asymmetric reversal”B. Beschoten, G. Guntherodt et al (in press)

• “Length Scales and Nanostructures”I. Roshchin, et al (EPL 71, 297(2005))

• “Depth Profile....”S. Roy, et al (PRL 95,047201(2005)

• “Spontaneous Reversal….”ZhiPan Li et al (PRL 96, 137201(2006))

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