Photoemission Electron Microscopy (PEEM)

Andreas Scholl

Advanced Light Source

Bibliography:

Magnetism: From Fundamentals to Nanoscale Dynamics (Springer Series in Solid-State Sciences) by J. Stöhr, H.C. Siegmann

Berkeley, April 2007

Applications of Magnetism

From Hitachi Global Storage Technologies

Challenges

DynamicsMicrostructure/Complexity

poly AFM

The microscopic origin of exchange bias

Magnetization dynamics

Spectroscopy / MicroscopyUsing Synchrotron Radiation

Synchrotron

Polarizationselectingaperture

Grating

Sample

20-50 nm spatial resolution

X-ray Absorption Spectroscopy

500 600 700 800 900

Ni

Co

Fe

O

Inte

nsity

[a.u

.]Photon Energy [eV]

Energy

EF

~~

hν(x-ray)

core level

band

Elemental sensitivity

776 778 780

Co CoO

El

ectro

n Y

ield

(arb

. uni

ts.)

Photon Energy (eV)868 870 872 874

Ni NiO

Photon energy (eV)

Co

met

al/o

xide

Ni m

etal/oxide

Chemical sensitivity(oxidization state, chemical environment)

NiO

Fe

valence

2p 3/22p1/2

X-ray Magnetic Circular Dichroism (XMCD)

EF

2p1/2

2p3/2

3d

4sp

s+

0.6

0.6 0.2

1.0P = 25%l+s

P = -50%l-s

Stoner model of magnetism

770 780 790 800 810

-0.4-0.20.00.20.40.60.81.01.2

0

1

2

3

4

5

6

Diff

eren

ce (a

.u.)

Photon energy (a.u.)

Inte

nsity

(a.u

.)

µ+

µ-

Step Iso

AL3

AL2

Co L2/3 edges circularly polarized x-rays

iso

L2L3Bs A

2A A~m −µ

iso

L2L3Bl A

A A~m +µ

Sum rules

Photoemission Electron Microscopy (PEEM)

Acceleration, focusing and magnification

Magnification

Energy and angle filtering

Image Aberrations and Correction

MirrorLensSpherical aberrations

Chromatic aberrations

Aberration-corrected PEEM-3

CCD

Electrostaticdodecapole

ElectrostaticQuadrupoleElectrostatic/

Magneticdodecapole

DiagnosticCCD

hv

Sample

Electrostaticmirror

Dipoleseparator

• EPU beamline- up to 1000x higher flux- polarization control

• Aberration correction- 5 nm spatial resolution

• Liq. He cooled manipulator- new materials

Spatial Resolution of Uncorrected And Corrected PEEM

Using aberration correction

Some “Nano-Science” Done Using PEEM

Composition maps of PS/PMMA sample exposed to fibrinogen

10 nm iron oxide clusters on SiChemical imaging of Mn nodules

Magnetic phase transition in FeUV exposed polyimide

C. Morin et al., JES&RP 137-140, 785 (2004).

M. Zharnikov et al, J. Phys. Chem. B 108, 859 (2004)

Y. Wu et al., Phys. Rev. Lett. 93, 117205 (2004)

Magnetic domains in NiO/Fe

L. Duo et al., unpublished (2004).J. Rockenberger et al., J. Chem. Phys. 116, 6322 (2002). A.D. Smith et al., J. Phys. IV 104, 373 (2003).

Example:

How X-PEEM Solved The Mystery of Exchange Bias

Exchange Bias

Ferromagnet

Exchange bias

Antiferromagnet

Meiklejohn, Bean, 1956

Unidirectional anisotropy, pinning of the ferromagnet by the antiferromagnet.

Science Questions

• What couples the ferromagnet to the antiferromagnet?

• Can we see antiferromagnetic domains and do they play a role in bias?

Measure microstructure of the ferromagnet/antiferromagnet interface.

Imaging of an AntiferromagnetExample: LaFeO3

TEMTwin domains

SrTiO3(001)

LaFeO3

40 nm c

Fe

ba

O

[110]

AFMaxis

[-110]

[001]

[010][110][100]

X-Ray Magnetic Linear DichroismXMCD

e-

A B

B/A

6 µmlinearly polarized

determineAFM axis

x-rays

705 710 715 720 725 730 Photon energy (eV)

BA

L2

L3

dark domain bright domain

Inte

nsity E

Ratio image: Antiferromagnetic domains

Temperature Dependence of the Antiferromagnetic Order Parameter

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

mea

n fie

ld th

eory

T/TNeel

<M>T

<M>T2

<M2>T

( )

⎟⎠⎞

⎜⎝⎛−⎟

⎠⎞

⎜⎝⎛ ++

=

==

−+=

ϕ−

xJ2

1cothJ2

1xJ2

1J2cothJ2

1J2)x(B

kT/)M(Hx ),x(JBM

)2/xcoth(M1JJM

M)cos31(~I

J

JT

TT2

T22

XMLD

Brillouin function

Numerical solution

Mean Field Approximation

Temperature dependence calculated in mean field approximation for d5

Temperature Dependence Proves: Contrast Due To AFM Domains

290K

565K

490K

290K

2 µm

300 400 500 600 7000.0

0.2

0.4

0.6

0.8

bulk

1st run back to RT 2nd run back to RT

Nor

mal

ized

XM

LD C

ontra

st

Temperature [K]

Fit: TN = 670 K

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

<M>T

<M>T2

<M2>T

Experimental temperature depen-dence well described by theory.

Crystallography Determines Magnetic Structure

no miscut

2° miscut

c A

ba

c

γ=45o

γ

AcTEM PEEMStructure Magnetism

E

2 µm

In-plane projection of AFM axis parallel to c- axis

Microscopic Images of Exchange Bias

Ferromagnet

Image interface magnetic structure

XMCDXMLD

Antiferromagnet

Domain Correlation Shows Coupling on a Microscale

Microscopy

775 780 785 790 795 800

L2

L3

Photon energy (eV)

TEY

(a.u

.)

Co

705 710 715 720 725 730

TEY

(a.u

.)

Photon energy (eV)

720 722 724 726

BA

LaFeO3

Spectroscopy

LaFeO3

Co FerromagnetXMCD

s

E

XMLD

F. Nolting, Nature 2000

Local Hysteresis Loops

Co domain switching

Local loops

-223 Oe 92 Oe

103 Oe 223 Oe

XMCD

XMLD

[100]

[010]

10 mm

12

-200 -100 0 100 200

Field (Oe)

1 2

LaFeO3Determine the local! switching field.

Local Exchange Bias

LaFeO3

Co

Co/LaFeO3

0.5 1.0 1.5 2.0 2.5 3.0

8

10

12

14

Bia

s - σ

(Oe)

Diameter-1 (µm-1)

+30 Oe

-30 Oe

Bias map

Bias scales inversely with domain size.

A. Scholl et al., APL 2004

Random Field Model

LaFeO3

Co

Large domain - surface spins compensated

Small domains - larger probability that surface spins are uncompensated

Hex ~ √N/A ~ 1/ d

Proposal: Exchange bias is the result of a statistical surplus of pinned interface spins.Malozemoff, PRB 1987

Takano, PRL 1999Miltenyi, PRL 2000

Imaging Uncompensated Interface Spins, Example NiO

[1 0 0 ]

[0 0 1 ]

[0 1 0 ]

3 possible "S" domainsOne of 4 "T" domains

<0 1 1 >

_

< 1 1 0 >_

<1

01

>

_

<2 1 1 >

_< 1 2 1 >

_

<1

12

>_

x'

NiO

W.L. Roth, Phys. Rev. 111, 772 (1958)W.L. Roth, J. Appl. Phys. 31, 2000 (1960)

Interface Spins Couple Ferromagnetand Antiferromagnet

NiOCo

Linear dichroism Circular dichrosim

sE

H. Ohldag et al., PRL 2001

Interface ferromagnetic structure

A Simple Microscopic Model

ideal AFM real AFM

Time & Length Scales in Magnetism

Example: P-sec Vortex Dynamics

Kerr microscopyshows magnetostatic mode

Magnetic vortex

Nanometer-size vortex core

Direction of magnetization

From J.P. Park et al., PRB 67, 020403

Obtain time-resolved images of ps-ns dynamics after a field pulse.

Pump-Probe Microscopy Using X-rays

~ psfs laser125 MHz

pump

probe

adjustable delay

samplex-raymicroscope

X-raye-

synchrotron500 MHz

70 ps10 µm focus

SampleG

aAsH

Ground plane

waveguide 10 mm

Waveguide: 200 nm Cu Pattern: 20 nm Co90Fe10

Field Production

Magnetic field

Current

Conducting wireMagnetic Patterns

10 µm

Sample Deposition-sputter deposition -e-beam evaporation

Waveguide Structure- photo-lithography, lift-off

Patterning-Focused Ion Beam (FIB) etching Photoconductive switch

Substrate: GaAs

Magnetic Field Pulse

Before pulse

At pulse

wave guide

0 bias

wave guide

+/- bias

ground

E

Current pulse

0 500 1000 1500

0

50

100

150

200U = 15 VR = 50 ΩIpeak = 200 mA

I (m

A)

Delay (ps)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

470 ps

Fie

ld (

mT

)

Peak field: 13 mTPulse rise time <100 psPulse width: 0.5 ns

Experiment

time

• Repeat experiment 5-10 times• Align and sum all images• Generate a movie

Total acquisition time: 24 hours

Vortex Acceleration

?Field pulse H

What is the direction of the core acceleration in response to a p-sec field pulse?

Movies of Vortex Dyanmics

1 x 1 µm2 2 x 1 µm2

100 nm

100

nm

100 nm

100

nm

100 nm

100

nm

1.5 x 1 µm2

1) Acceleration parallel to field pulse.

3) But: vortex cores gyrate oppositely although domain structures identical.

2) Gyrotropic (spiraling) motion of core causes oscillation.

S.B. Choe et al., ALS, Science 304, 420 (2004)

Micromagnetic Model

Right-handedvortex

Spins precess around field

Vortexcore

Handedness Acceleration

Right-handed Parallel

Left-handed AntiparallelVortex moves parallel to field

Vortex dynamics determined by the structure of the nanometer-size core.

Vortex Rotation

H

Hdemag

Excitation

Rotation

Summary

Applications of PEEM:• Chemical and elemental imaging of surfaces and thin films.• Magnetic domain imaging of ferromagnets and antiferromagnets• Time-resolved imaging at high temporal and spatial resolution

Forefront science: Imaging of ferroic materials, dynamics.

Interface spins

Magnetization dynamics