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Tutorial - Contrast using PEEM
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| Depending on sample and wavelength
of the illumination, different mechanisms generate an image
contrast. Topographical contrast appears on rough samples and
is the result of the curvature of the accelerating field around
topographic features of the surface. The result is a general
degradation in spatial resolution and local focusing. Workfunction
contrasts dominates using UV illumination with an energy close
to the workfunction of the surface. Differences in the local
workfunction result in large differences in the electron emission. |
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Synchrotrons are tunable sources
of radiation between the infrared (meV) and hard X-rays (keV).
Soft X-rays ecompass an intermediate region roughly between
200 eV and 2 keV. Many light elements show strong absorption
edges in this energy range that can be used for element-selective
imaging. The graph on the right shows the L absorption edges
of the magnetic 3d transition metals manganese, iron, cobalt,
and nickel. Also shown is the K absorption edge of oxygen. The
strong resonances (white lines) at the absorption edge is due
to electronic transitions into valence states that are sensitive
to the chemical environment of the atom. The study of the electronic
and chemical strucure using X-rays is called NEXFS for Near-Edge
X-ray Absorption Fine Structure. |
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| An example of elemental contrast
imaging is shown on the right. The star pattern was produced
using electron beam lithography. The spokes are made of nickel
with photoresist filling the gaps. A very strong element contrast
appears when the photon energy of the beamline is tuned to the
nickel L3 white line.This sample was used for resolution
testing of the PEEM-2 microscope. The spoke width at the inner
ring is 100 nm and the lithographs fails at about 30 nm spoke
width. PEEM-2 is able to resolve the spokes close to the region
in which the lithography fails, albeit at a much reduced contrast.
Note that the surface of this sample is not perfactly flat,
possibly reducing the apparent resolution of the instrument. |
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Chemical contrast is illustrated
on the left for a manganese nodule from the deep sea floor.
While changing the photon energy different regions of the PEEM
image light up due to a different chemical composition and oxidization
state of the material. Absorption spectra acquired in different
regions show marked differences. This study also illustrates
the concept of spectromicroscopy. While PEEM is an imaging method,
its true strength lies in its ability to perform local spectroscopy
experiment from very small volumes of material. Spectromicroscopy
is performed by acquiring images at incremental photon energies.
Local spectra are generated by post processing this intensity
versus energy dataset using a suitable analysis
software. |
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| X-ray magnetic circular dichroism
(XMCD) is a standard method to study magnetic thin films and surfaces.
The last 15 years have seen great progress following the first XMCD
spectroscopy measurements at the important transition metal L edges
in 1987 by Schütz et al., the first imaging of ferromagnetic
domains in 1993 by Stöhr et al.18 and the first imaging of antiferromagnetic
domains by Stöhr and Scholl et al. Circularly polarized X-rays
measure the direction of the atomic magnetic moment of a ferromagnet
relative to the polarization vector of the X-rays. In the presence
of spin orbit interaction the photon angular momentum is transferred
to the angular momentum of the photo-excited electron, in particular
the electron spin, which senses the different density of empty states
in each spin channel. An imbalance of the occupation of the majority
and minority states is characteristic for a ferromagnet. Strong XMCD
effects (up to 40%) appear at the L edges (2p-3d transition) of the
transition metal ferromagnets Fe, Co, and Ni, since the d states carry
most of the magnetic moment. The XMCD spectrum is defined as the difference
spectrum of two absorption spectrum acquired with either opposite
polarization or magnetization. The XMCD effect is opposite in sign
at the L3 and L2 edge because of the opposite
sign of the spin-orbit coupling in the 2p states: l+s for 2p3/2,
and l-s for 2p1/2. The different coupling gives rise to
a unique feature of XMCD, its ability to separate spin and orbital
moment. The spin momentum is proportional to the difference of the
integrated XMCD intensity at the L3 and the L2
edge, the orbital momentum is proportional to the sum. Sum rules have
been developed, which are used to quantitatively determine the spin
and orbital magnetic moment per atom. |
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| Magnetic materials and thin films
often exhibit magnetic domains, which can be visualized by PEEM imaging
using polarized X-rays. X-ray Magnetic Circular Dichroism (XMCD) at
the L3/2 edges is sensitive to the ferromagnetic domain
direction, scaling with cos2α, the angle between
the ferromagnetic direction and the x-ray polarization direction.
Ratio or asymmetry images originating from images taken either using
opposite polarization or using the same polarization at both L3
and L2 energies show the magnetic domain structure. Other
contrast is suppressed. Antiferromagnets are imaged similarly using
linearly polarized X-rays. A strong x-ray magnetic linear dichroism
(XMLD) is present at the L edges of transitional metal oxides that
depends on cos2α, the angle between the ferromagnetic
direction and the linear x-ray polarization direction. XMLD images
at the appropriate energies show the antiferromagnetic domains structure.
Local spectra of the ferromagnet and the antiferromagnet are shown
on the left. The data was taken on an ferromagnet/antiferromagnet
Co/LaFeO3 bilayer, demonstrating interface exchange coupling
between the two materials |
| Linear dichroism is the result of
an anisotropic electronic charge distribution, which can be
caused either by magnetism or a a lower than cubic symmetry
of the unit cell. Magnetostriction, substrate effects, and the
lattice type can result in a non-magnetic dichroism also called
natural linear dichroism. Temperature dependent measurement
of the dichroism is a straigh-forward way to ascertain the antiferromagnetic
origin of the dichroism. To right the XMLD contrast of a LaFeO3
film is shown. The contrast drops towards zero approaching the
Neel temperature of the film, following a mean field derived
fit of the dichroism contrast which is proportional to <M2>T. |
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Multiferroic materials possess more
than one type of long-range order, e.g., ferromagnetism, ferroelectricity,
and/or ferroelasticity. Coupling between these may provide a
lever to control one type of order, used for example in a storage
device, by a second type, which may be easier to modify. If
strong coupling between a magnetic and ferroelectric material
were found, electrical control of magnetism would be possible.
This would be a significant improvment over current methods
since electric fields are easier to produce and localize in
a device than magnetic fields. BiFeO3 is a room temperature
antiferromagnetic and ferroelectric material. The top row shows
PEEM images whose contrast is dominated by the antiferrommagnetic
domains. The bottom shows piezo-force microscope images, a scanning
probe technique, of the ferroelectric structure. The images
strongly suggest that magnetic and electric degrees of freedom
are strongly coupled. The magnetic origin of the PEEM contrast
was ascertained by temperature-dependent measurement. |
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