advanced chapters:

• imaging by magnetic linear dichroism

• electron energy filtering in PEEM

• time-resolved magnetic imaging

• aberration correction

• imaging x-ray holography

basics:

• x-ray absorption detection schemes

• cathode lens: working principle, resolution

• photoelectron emission microscopy (PEEM)

• low energy electron microscopy (LEEM)

• magnetic transmission x-ray microscopy (M-TXM)

Title & outlineMagnetic imaging by LEEM, X-PEEM,X-ray microscopy, and X-ray holographyWolfgang Kuch, Freie Universität Berlin

magnetic read head

Movie Lesekopf

Layered magnetic systems

Sketch MRAM

Layered magnetic systems

magnetic RAM

soft magnetic layer

tunnel barrier

hard magnetic layer

bit line

word line

G. Reiss et al., Phys. Bl. 54 (1998) 339

giant magnetoresistance (GMR)

(sensor, hard disk read head)

metallic conductivity

tunnel magnetoresistance (TMR)

tunneling current

(sensor, magnetic RAM)

spin torque transfer

momentum transfer byspin polarised e–

(fast switching)

spin transistor

EB

C

spin transistorwith tunnel barrier

EB

C(logical devices, taking advantage of charge and spin)

metallic ferromagnet

non-magnetic metal

insulator

semiconductor

Some modern concepts of spin electronicsLayered magnetic systems

trilayers sketch 2

giant magnetoresistance (GMR)

(sensor, hard disk read head)

metallic conductivity

tunnel magnetoresistance (TMR)

tunneling current

(sensor, magnetic RAM)

spin torque transfer

momentum transfer byspin polarised e–

(fast switching)

spin transistor

EB

C

spin transistorwith tunnel barrier

EB

C(logical devices, taking advantage of charge and spin)

metallic ferromagnet

non-magnetic metal

insulator

semiconductor

Some modern concepts of spin electronicsgiant magnetoresistance (GMR)

(sensor, hard disk read head)

metallic conductivity

tunnel magnetoresistance (TMR)

tunneling current

(sensor, magnetic RAM)

spin torque transfer

momentum transfer byspin polarised e–

(fast switching)

spin transistor

EB

C

spin transistorwith tunnel barrier

EB

C(logical devices, taking advantage of charge and spin)

metallic ferromagnet

non-magnetic metal

insulator

semiconductor

Some modern concepts of spin electronics

XMCD example

Layer-resolved information from XMCD

abso

rptio

n

900880860840820800780760photon energy (eV)

Co Ni

magnetization parallel to x-rays

magnetization antiparallel to x-rays

L3

L2

L3

L2

s– circular polarization6 ML Co/5 ML Cu/15 ML Ni/Cu(001)

CoCuNi

Synchrotron radiation needed

BESSY photographs

hn hn

sample detector

absorptiondetection schemes

– real “absorption”– only very thin substrates

1.) “Total electron yield”

– ≈ proportional absorption– surface sensitive (l ≈ 20 Å)

hn

samplee-

e-

Is

hn

e–

first publication:J. Stöhr et al.,Science 259 (1993) 658

2.) Transmission

hn

Detection methods for x-ray absorption

imaging absorption

e–

electron optics

x-ray optics

Imaging the x-ray absorption

hn hn

sample detector

– real “absorption”– only very thin substrates

1.) “Total electron yield”

– ≈ proportional absorption– surface sensitive (l ≈ 20 Å)

hn

samplee-

e-

Is

2.) Transmission

hn

ideal lens schematic

Optical imaging: Ideal lens

lens

focal plane:beams starting under identical angles meet

image plane:beams starting at same position meet

F F

p q

f

f1

p1

q1

= +

P

Q

QP

= qp

electrostatictetrode lens

G. Schönhense, J. Phys.: Cond. Matt. 11 (1999) 9517

Cathode lens schematic

electrostatictriode lens

H. Seiler, “Abbildung von Oberflächen”, Bibliographisches Institut, Mannheim (1968)

Cathode lens for electron emission microscopy

Cathode lens for electron emission microscopy

electrostatic tetrode lens

sample is part of optical system

Cathode lens calculation

ra

real starting angle

virtual starting angle

†

a0

†

¢ a

†

k|| = k sina0 = ¢ k sin ¢ a

†

k =2mE0

h

†

¢ k =2m(E0 + eUex )

h

a'

HVa0

contrast aperture

sample+–

virtualsample

Uex

†

fi sina0sin ¢ a

=eUexE0

+1

†

a0¢ a

ªeUexE0

†

DW µ1

E0accepted solid angle

Photoelectron spectrum using a cathode lens

†

a0¢ a

ªeUexE0

†

DW µ1

E0accepted solid angle

300

250

200

150

100

50

0

aver

aged

imag

e in

tens

ity (

arb.

uni

ts)

806040200kinetic energy (eV)

80 60 40 20 0binding energy (eV)

Fe 3p Fe 3dW 4f

¥ 25

10 ML Fe pattern on W(001)

hn = 95 eV

†

Uex = 3.4 keV

†

2ra = 150 mm

†

a0 : 18° 8° 2°

full electron spectrum

chromatic aberration

spherical aberration

diffraction error

†

ds

†

dc

†

dD

†

Cs

†

Ccmagnetic

electrostatic

†

ª 10f

†

ª f

†

ª 4f

†

ª f

†

ds =12

Csa3

†

dc = CcDEE

a

†

dD ª12

l

a

aberration formulas

†

a

†

a

†

a

Aberrations in optical imaging

theoretical resolution

(magnetic triode, 25 kV/3 mm, E =2.5 eV, DE = 0.25 eV)

E. Bauer, Surf. Rev. Lett. 5 (1998) 1275

†

d = ds2 + dc

2 + dD2

†

a µ rA

chromatic aberration

spherical aberration

diffraction error

†

ds =12

Csa3

†

dc = CcDEE

a

†

dD ª12

l

a

resolution limit

Resolution limit

d/nm

S. A. Nepijko et al., Ann. Phys. 9 (2000) 441

Cathode lens: Flat samples required

roughness image deterioration

Electrostatic photoelectron emission microscope (PEEM)

CCD camera

fluorescent screenchannelplate

photonsHV +–

PEEM sketch

projection lenses

first use:E. Brüche, Z. Phys. 86 (1933) 448;J. Pohl, Zeitschr. f. techn. Physik12 (1934) 579

PEEM contrast: work function

work function contrastfrom coarse-grained Au

H. Seiler, “Abbildung von Oberflächen”, Bibliographisches Institut, Mannheim (1968)

Hg lamp (hn = 4.9 eV)

PEEM work function contrast

PEEM contrast: topographic

J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535

PEEM topographic contrast

PEEM contrast: spectroscopic

J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535

elemental

chemical

PEEM spectroscopic contrast

PEEM contrast: spectroscopic

magnetic

PEEM XMCD contrast (movie)

Co/Ni/Cu(001)

†

I(s +)

†

I(s -)

†

I(s +) - I(s -)I(s +) + I(s -)

W. Kuch, FUB,K. Fukumoto, J. Wang,MPI-MSP,C. Quitmann, F. Nolting,T. Ramsvik, PSI-SLS,unpublished.

XMCD-PEEM: separate magnetic and topographic information

PEEM asy images

20 µm

Vergleich Würmer

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

cont

rast

60 40 20 0 -20incidence azimuth (deg)

azimuth series +25°

XMCD-PEEM: vectorial information by variation of incidencedirection

W. Kuch, F. Offi, L. I. Chelaru,J. Wang, K. Fukumoto,M. Kotsugi, MPI-MSP(unpublished)

Co/FeMn/Cu(001)Co L3

hn

10 mm

hn

[110]

hnhn

Cu(001)

15 ML Ni6.5 ML Cu3 ML Co

Ni Co

1st example Co/Cu/Ni

Layer-resolved magnetic images

†

INi µ e- ¢ t / lNid ¢ t = (1- e-tNi / lNi )0

tNiÚ

abso

rptio

n

900880860840820800780760photon energy (eV)

Co Ni

magnetization parallel to x-rays

magnetization antiparallel to x-rays

L3

L2

L3

L2

s– circular polarization6 ML Co/5 ML Cu/15 ML Ni/Cu(001)

Ni

†

tNi

†

e-tCo / lCo

Co

†

tCo

†

e-tCu / lCu

Cu

†

tCu

†

INi µ

Attenuation of secondary electron yield

attenuation formulas

†

l ª 2 nm

1

10

Co la

yer t

hick

ness

(nm

)

1 10Cu layer thickness (nm)

2

3

5

2

0.5

2 3 50.5

Co/Cu on Ni

10000 3000 1000 500 200 100 50

20 10

5 4 3

2

1.5

1e5

attenuation calculation

Attenuation of secondary electron yield

exposure time for same noise level as without overlayers

NiCuCo

PEEM domainwalls as-grown

5 ML FeNi15 ML Co 15 ML FeMn

Cu(001)

5 ML Cu

[100][100]

[010]

[010]

as grown

10 mm

Fe L3 Co L3

hn

XMCD-PEEM: layer-resolved magnetic imaging

L. I. Chelaru, F. Offi, M. Kotsugi, and W. Kuch, MPI-MSP (unpublished)

PEEM domainwalls 25 Oe

5 ML FeNi15 ML Co 15 ML FeMn

Cu(001)

5 ML Cu

[100][100]

[010]

[010]

25 Oe

10 mm

Fe L3 Co L3

H

hn

XMCD-PEEM: layer-resolved magnetic imaging

L. I. Chelaru, F. Offi, M. Kotsugi, and W. Kuch, MPI-MSP (unpublished)

PEEM domainwalls 340 Oe

5 ML FeNi15 ML Co 15 ML FeMn

Cu(001)

5 ML Cu

[100][100]

[010]

[010]

340 Oe

10 mm

Fe L3 Co L3

H

hn

XMCD-PEEM: layer-resolved magnetic imaging

L. I. Chelaru, F. Offi, M. Kotsugi, and W. Kuch, MPI-MSP (unpublished)

• element specific, can be used for layer-specificity

• needs synchrotron radiation

• good resolution

• parallel imaging

• moderately surface sensitive (≈ 20...100 Å)

• sensitive to external magnetic fields

• in vacuum

• vectorial information by rotating sample

• quantitative spectroscopic informationavailable (“sum-rule microscopy”)

PEEM features

XMCD-PEEM

sample

objective lensmagnetic sector field

spin-polarizedelectron gun

illuminationcolumn

imagingcolumn

imaging unit

CCD cameraspin-manipulator

spin-polarized low energy electron microscopy (SPLEEM)

SPLEEM sketch

(Elmitec LEEM 3)

SPLEEM photograph

spin-polarized low energy electron microscopy (SPLEEM)

SPLEEM

spin manipulation magnetic contrast

Th. Duden and E. Bauer, Surf. Rev. Lett. 5 (1998) 1213

SPLEEM detail & mechanism

T. Duden and E. Bauer, PRL 77 (1996) 2308

SPLEEM: example

magnetization “wrinkle” in Co/W(110)

in-plane out-of-plane tilt angle

topography

SPLEEM example Duden

K. L. Man et al., PRB 65 (2001) 024409

2.13 ML

2.20 ML

2.33 ML

2.87 ML

3.13 ML

3.29 ML

3.60 ML

3.83 ML

SPLEEM: another example

during deposition ofFe/Cu(001),growth rate: 0.080 ML/minE = 1.8 eV

SPLEEM example Man

2018

1614

1210

86

42

0 20181614121086420

W. Kuch, K. Fukumoto, J. Wang, MPI-MSP,C. Quitmann, F. Nolting, T. Ramsvik, PSI-SLS, unpublished.

LEEM Cu(001)

Topographic LEEM contrast

atomic steps at the surface of Cu(001)

• surface sensitive

• fast

• vectorial measurement without turning sample

• small field of view possible due to electron beam focusing

• topographic information simultaneously available

• conditions for best contrast depend on sample

• sensitive to external magnetic fields

• needs UHV

• not element specific

SPLEEM features

SPLEEM

2d(r1)

l

r

l

}}

2d(r2)

Zone plate as x-ray lens

diffraction of x-rays ofwavelength l to one spot

block areas of destructiveinterference

Æ slits of width d

†

d(r) ªlf2r

zone plate schematics

from: homepage of Center for X-ray Optics,Lawrence Berkeley National Laboratory

inner part of a zone plate lens.diameter: 45 µm,outermost zone: 35 nm wide.

Zone plate as x-ray lens

zone plate SEM image

G. Denbeaux et al., IEEE Trans. Mag. 37 (2001) 2764

Transmission x-ray microscopy (TXM)

TXM sketch

P. Fischer et al., Rev. Sci. Instrum. 72 (2001) 2322

M-TXM: example

magneto-optical storage media

50 nm Tb25Fe56Co19

M-TXM example MO media

T. Eimüller et al., J. Phys. IV 104 (2003) 483

M-TXM: example

[Fe/Gd] nanostripes

M-TXM example Eimüller

W. Chao et al., Nature 435, 1210 (2005)

15-nm zone plate SEM image

New high-resolution zone plate

images of test object with 19.5 nm linesand spaces

images of test object with 15.1 nmlines and spaces

25-nm zoneplate 25-nm zoneplate15-nm zoneplate 15-nm zoneplate

W. Chao et al., Nature 435, 1210 (2005)

15-nm zone plate examples

New high-resolution zone plate

• high resolution

• element specific, can be used for layer-specificity

• needs synchrotron radiation

• insensitive to magnetic fields

• parallel imaging or scanning

• only in transmission

• does not need UHV

M-TXM features

M-TXM

LEEM image of steps on Si(100), FOV: 4 µm

G. L. Kellogg, Sandia Natl. Lab., Albuquerque

When samples start looking at you...

...it’s time for a break!

3d

eg

t2g

eg

t2g

octa

hedr

alcr

ysta

l fie

ld

mag

neti

zati

on

E

Linear magnetic dichroism in soft x-ray absorption (XMLD)

XMLD & crystal field

D. Alders et al., Phys. Rev. B 57 (1998) 11623

5

4

3

2

1

0

abso

rptio

n

Co L3

-0.06-0.04-0.020.000.020.040.06

diffe

renc

e

785780775770photon energy (eV)

¥ 30

hn

linear dichroism in absorption

Cu(001)6 ML Co

15 ML FeMn

EÆ

W. Kuch et al.,Phys. Rev. Lett. 92 (2004) 017201

Ni L2

NiO/MgO(001) Co/Cu(001)

metaloxide

Linear magnetic dichroism in soft x-ray absorption (XMLD)

XMLD spectra

Co L3

hn

10 mm

XMCD XMLD

6 ML Co/Cu(001)

[110]

Images with XMLD (Co)

6 ML Co/Cu(001)

XMLD as contrast mechanism in PEEM

W. Kuch, F. Offi, L. I. Chelaru, M. Kotsugi, J. Wang, and K. Fukumoto,MPI-MSP (unpublished)

H. Ohldag et al., Phys. Rev. Lett. 86 (2001) 2878

Ni XMLD Co XMCD

8 ML Co/NiO(001)

PEEM example Ohldag

XMLD as contrast mechanism in PEEM

e– with Ekin > e⋅UG can pass grid G (highpass)

imaging energy filter

L1 L2 G

MCP

scre

en

retarding field analyzer

Imaging electron energy analyzers

highpass filter

bandpass filter

energy filters schematic

EF

EVac

2p3/2

hn

EF

EVachn

3p

Ekin

elec

tron

ener

gy

elec

tron

ener

gy

minmaj

Photon absorption vs. electron emission spectroscopy

photon absorption electron emission

absorption vs. emission sketch

Magnetic linear dichroism in photoemission (MLDAD)

W. Kuch et al., Phys. Rev. B 51 (1995) 609

MLDAD spectrum

comparison PEEMXMCD/MLDAD

PEEM: absorption vs. photoemission

XMCDabsorption

MLDADphotoemission

sensitivity:

Fe(001)

W. Kuch et al., unpublished(see also: W. Kuch et al., J. Vac. Sci. Technol. B 20 (2002) 2543)

ON

OFFONOFF

focal plane

image plane

real space k space

PEEM: imaging of the diffraction plane

sample

Fermi surface mapping by PEEM

M. Kotsugi et al., Rev. Sci. Instrum. 74 (2003) 2754

Fermi surface mapping

photon energy 95 eV

Timescales in magnetic materials

Magnetic storage technology

Thermally activated processes(Domain wall propagation / nucleation)

Spinprecession

Spin-orbitinteraction

10 -15

Read/Write Storage

10 -12 10 -9 10 -6 10 -3 1 109

Photoelectric interactions

Time (s)

timescales

Time-resolved PEEM

A. Kuksov et al.,J. Appl. Phys. 95 (2004) 6530

TR-PEEM example Schneider

C. M. Schneider et al.,Appl. Phys. Lett. 85 (2004) 2562

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

TR-PEEM example Choe

Time-resolved PEEM

H. Stoll et al., Appl. Phys. Lett. 84 (2004) 3328

Time-resolved M-TXM

TR-TXM example Stoll

e–

BESSY

Dt1

Dt2

Dt3

PEEM

800 ns

Dt

PulseSupply

time

photon pulse from BESSY(50 ps width)

40 ns

Dt

magnetic pulse

Stroboscopic time scheme

stroboscopic scheme

Time and layer resolved PEEM imaging

sampleCu foil

x-rays

J. Vogel et al., Appl. Phys. Lett. 82 (2003) 2299

5 mm5 nm Fe19Ni814 nm Cu5 nm Co

Movie 28 V monopolar

Time and layer-resolved PEEM imaging

fi importance of domain wall energyfor local domain wall speed

W. Kuch et al., Appl. Phys. Lett. 85 (2004) 440

Starting configuration

start (static)

Fe Co

20 mm

4 nm Fe19Ni812.5 nm Al2O37 nm Co

tunnel junction

field

(m

T)

25

Fe

20 mm

Layer-resolved stroboscopic magnetic microscopy

4 nm Fe19Ni812.5 nm Al2O37 nm Co

tunnel junction

DICTIONARY OF PHOTOGRAPHY1889, Londonby E.J. Wall

“In fig. 23 I am enabled, by the kindness ofMessrs. Perken, Son, & Rayment, to give a sketchof the Euryscope lens, which is composed of twosymmetrical combinations of flint glass, andworks at an aperture of f/6, a great gain for rapidwork. These lenses are perfectly free fromspherical and chromatic aberration...”

“... some of the finest lenses of the day; andin figs. 21 and 22 are shown two more ofSteinheil's lenses, which work at f/2.5, No.21 being for groups, No. 22 for portraits.”

Aberration correction in light optics

aberration correction (Foto lens)

Aberration correction in electron optics

O. Scherzer, Optik 2 (1947) 114

aberration correction (Scherzer)

Round convex lenses

Chromatic aberration

Spherical aberration

focalpoint

focalpoint

focalpoint

focalpoint

outer electrodeat -3750 V

inner electrodeat 15000 V

electron trajectory

Equipotential surfacesin a diode mirror

electrostatic mirror

Aberration correction by electrostatic mirror

H. Rose and D. Preikszas, Nucl. Instr. & Meth. A 363 (1995) 201R. Fink et al., JES 84 (1997) 231

aberration correction sketch

sample transfer optics

energy filter

projector

E/DE = 150.000

screen

corrector(tetrode mirror)

electrongun

final lateral resolution: Dx = 2 nmenergy resolution: DE = 100 meV

SMART sketch

LEEM/ PEEM: improved resolution by aberration correction

“SMART” project

H. Rose and D. Preikszas, Nucl. Instr. & Meth. A 363 (1995) 201R. Fink et al., JES 84 (1997) 231

Energy filter

Detector

Mirror corrector

90° sector fieldMeasurement chamber

Vibration dampedframe

electron gun

“SMART” project: set-up at BESSY

SMART photograph

D. Preikszas and H. Rose, J. Electr. Micr. 1 (1997) 1Th. Schmidt et al., Surf. Rev. Lett 9 (2002) 223

LEEM/PEEM: improved resolution by aberration correction

“SMART” target parameters:

SMART resolution graph

DE a2

+ DE2 a

DE a + …Chromatic aberr.

1/a1/aDiffraction

a5a3 + …Spherical aberr.

withcorrection

withoutcorrection

Resolution limit

holography general sketch

Coherent x-ray diffraction (speckles)

Lensless domain imaging using coherent soft x-rays

M. Lörgen et al., BESSY-Highlights 2003, p. 32

holographic image[Co/Pt] multilayer

1200 nm sample hole200 nm reference hole

holography pinhole sketch

Difference (RCP – LCP) FFT (Difference)

Convolution theorem applied to diffraction: FT(diffraction) = Autocorrelation (Object)

saturated lin. scale

X-ray holography: Digital image reconstruction

image reconstruction

*

W

B

*

*

Resolution30 - 40 nm

S. Eisebitt et al., Nature 432 (2004) 885

Reference hole∅ 100 nm

W. F. SchlotterY. Acremann

FT hologram STXM image

Image reconstruction from speckle hologram

[Co(4 Å)/Pt (7Å)]50

comparison reconstructed image -MTXM

FEL principle

dipole bend:wiggler:undulator:FEL: †

µ N

†

µ nN

†

µ n2N

†

µ n2N2

N - number of electrons

n - number of undulations

Free electron laser (FEL)

FEL parameter

FEL performance

FEL project at BESSY

FEL aerial view

µm

nm

mm ns

ps

fs

magnetic domains

recorded bits

grains, nanoparticlesmolecules

atoms

exchange interaction

magnetic anisotropy/ spin–orbit coupling

spin precession anddamping

thermal activation

Exploring the nano- and femtoworld

space time

time- and lengthscales