title & outline magnetic imaging by leem, x-peem, x-ray...
TRANSCRIPT
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
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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