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X-ray Photoemission Electron Microscopy

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Contrast using PEEM


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Lawrence Berkeley National Laboratory

Advanced Light Source
Rajesh Chopdekar
last update: 03/30/2020

Tutorial - Contrast using PEEM

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.
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.
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.
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.
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.
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.
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.