Anthony Warwick

The development of electron spectromicroscopy

Abstract Three closely related techniques, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray fluorescence spectroscopy, have become widely accepted as important tools for the study of the chemical composition and electronic properties of surfaces, overlayers, and interfaces. There is now a major effort to push these spectroscopic techniques into a new realm of applications with very high spatial resolution, at and below 1 μm. This results in a new set of probes which can create images of chemical composition with great subtlety. The field is growing rapidly as high brightness sources of X-rays become available. The goals and methods used in electron spectromicroscopy and related X-ray microscopies are discussed, and recent applications of instruments developed in the last few years are used to illustrate the strengths of these new microscopes.

Magnetic x‐ray linear dichroism in the photoelectron spectroscopy of ultrathin magnetic alloy films

The magnetic structure of nanoscale alloy films has been probed using the magnetic x‐ray linear dichroism in photoelectron spectroscopy. FeNi and CoFe epitaxial films were grown on Cu(001), in situ and using molecular beam epitaxy techniques. The magnetic x‐ray linear dichroism measurements were made at the Spectromicroscopy Facility of the Third Generation Advanced Light Source. Because soft x‐rays were used to generate photoemission from the 3p core levels, both elemental selectivity and magnetic sensitivity were achieved simultaneously.

Magnetic x-ray dichroism in the spectroscopy of ultrathin magnetic alloy films

The magnetic structure of nanoscale alloy films has been probed using the magnetic x‐ray linear dichroism in photoelectron spectroscopy and magnetic x‐ray circular dichroism in x‐ray absorption spectroscopy. FeNi and CoFe epitaxial films were grown on Cu(001), in situ and using MBE techniques. Because soft x‐rays were used to probe the 2p and 3p core levels, both elemental selectivity and magnetic sensitivity were achieved simultaneously. Correlation of these magnetic techniques with compositional and structural information will be done. Ultimately, from studies such as this a complete determination of the structure‐property relationships will be achievable.

Direct extraction of exchange splittings from magnetic x-ray dichroism in photoelectron spectroscopy

It will be demonstrated that core-level exchange splitting can be extracted directly from normalized difference curves in magnetic X-ray circular dichroism (MXCD) in angle-resolved photoelectron spectroscopy (PES). Although high resolution is a requirement for the method, this determination can be performed without resorting to time-consuming and difficult spectral simulations. For well-defined cases, it will be shown empirically that this method may also work for the analysis of magnetic X-ray linear dichroism (MXLD). Applying this approach, it will be possible to use MXCD and MXLD in PES for direct surface magnetometry with full elemental specificity.

Comparison of magnetic linear dichroism in 4f photoemission and 4d–4f photoemission from Gd on Y(0001)

Magnetic linear dichroism (MLD) in 4d–4f resonant and 4f nonresonant photoemission (PE) is studied from thin epitaxial gadolinium films. In an angle resolved and high-energy resolution mode, experiments were conducted with the electric-field vector of the incident light perpendicular to the sample magnetization. Our results show a significant difference in behavior of MLD in resonant PE as compared to that in nonresonant PE. Off-resonance, the MLD signal is dominated by a negative feature at the low binding energy side of the peak. Near the 4d–4f resonance maximum, the MLD displays a plus–minus shape, with a negative signal at the low binding energy side of the 4f peak and a positive signal at the high binding energy side. Analysis of MLD in 4d–4f resonant PE may provide insight into interactions of the 4d core hole with the 4f core level in the intermediate state.

Generalized description of magnetic x-ray circular dichroism in Fe 3p photoelectron emission

Extending a single electron picture previously developed by Venus, it will be shown that a generalized prediction of the magnetic x-ray circular dichroism in the angle-resolved photoelectron spectroscopy of the Fe 3p can be obtained. This determination can be performed without resorting to time-consuming and difficult spectral simulations.

Bend magnet beamline for scanning transmission x-ray microscopy at the Advanced Light Source

A bend magnet at the advanced light source is sufficiently bright to illuminate a scanning transmission x-ray microscope, with a zone plate lens to focus the soft x-ray beam at the diffraction limit. The beam line must be carefully optimized for this one purpose of high count-rates, of the order of 1MHz, are to be achieved in the microscope. Such a design is described. The nominal resolving power is 2000 from 150eV to 600eV using a single spherical diffraction grating. Twice the resolving power is available at reduced flux, and the intensity can be traded independently against the spatial and spectral resolution.

Effects of symmetry on circular and linear magnetic dichroism in angle-resolved photoemission spectra of Gd/Y (0001) and Fe-Ni//Cu (001)

We have observed circular and linear magnetic dichroism in angle- resolved photoemission spectra of 50-monolayer Gd film grown on Y(0001) and 6-monolayer Fe-Ni alloy films grown on Cu(001). The 4f level of Gd and the Fe 3p level of the Fe-Ni alloy were measured. A different geometry was used for the magnetic circular dichroism than was used to measure the magnetic linear dichroism. The geometries were chosen so that the shape of the magnetic circular dichroism is predicted to be equal to the shape of the magnetic linear dichroism for four-fold symmetric Fe-Ni/Cu(001) but not for three-fold symmetric Gd/Y(0001). Experimental results are presented. In this paper we examine the effect of symmetry (experimental geometry and sample geometry) on magnetic linear and circular dichroism in angle- resolved photoemission. In particular we chose separate geometries for measuring magnetic circular and magnetic linear dichroism. The geometries were chosen such that samples with four-fold symmetry about the sample normal may have magnetic circular and magnetic linear dichroism of the same shape. But samples with three-fold symmetry should not exhibit circular and magnetic linear dichroism of the same shape. The samples studied are three-fold symmetric Gd films grown on Y(0001) and four-fold symmetric Fe-Ni alloy grown on Cu(001). After presenting the methods of the experiment, we briefly review parts of a model of magnetic dichroism developed by Venus and coworkers and our specialization and extension of it, particularly for FeNi/Cu(001). We then show the results of our measurements.

Correlation Of Magnetic Dichroism in X-Ray Absorption and Photoelectron Emission using Ultrathin Magnetic Alloy Films

We have begun a program to characterize magnetic alloy overlays using both magnetic x-ray circular dichroism (MXCD) and magnetic x-ray linear dichroism (MXLD). This will allow a direct comparison of MXCD-absorption and MXLD-photoelectron emission. First results from the Advanced Light Source will be presented.