Neville V. Smith

Quantum well states and oscillatory magnetic interlayer coupling

Some interesting magnetic properties of artificially layered metallic materials are strongly connected with the existence of electron standing waves, or quantum well (QW) states. One such property is the oscillation in exchange coupling between two ferromagnetic materials separated by a nonmagnetic spacer layer of varying thicknesses. This article summarizes the findings of an extended investigation of QW states and their relation to oscillatory magnetic interlayer coupling carried out using angle-resolved photoemission with synchrotron radiation and auxiliary techniques such as magnetic x-ray linear dichroism and surface magnetic optical Kerr effect. A key feature of the measurements was the use of wedge-shaped samples, which, in combination with the small spot size of the synchrotron source, permitted investigation of the entire layer thickness range in a single experiment. Single-wedge samples were used as well as double-wedge samples tapered in orthogonal directions. The systematics of QW formation are well understood in terms of the elementary quantum mechanics of a particle in a box. We treat a single well, a double well and a corrugated well. The work on single wells focused on the elucidation of the long and short magnetic oscillatory periods for a Cu spacer layer, and their relation to the belly and neck regions of the Cu Fermi surface. The effects of interfacial roughness and interfacial mixing were investigated. The studies on double wells focused on the controllable degree of tunnelling between the wells and the avoided crossings that occur when the QW energies in one well are swept through those of the other. Finally, we consider the QW wavefunctions and their envelope modulation. The latter can be understood in terms of Bragg diffraction within a corrugated well. With use of a double-wedge-shaped sample it has proved possible to pass a thin probe across the well and to detect experimentally the envelope modulation.

Soft x-ray circular polarizer using magnetic circular dichroism at the Fe L3 line

Linearly polarized soft x-rays are converted to elliptical polarization at the Fe L3 line (707 eV) using magnetic circular dichroism (MCD) on transmission through thin Fe films. A linear polarizer measured the transmitted polarization at different incidence angles to vary as expected from a model for in-plane magnetization, and also to exhibit a weak MCD effect at normal incidence interpreted to originate from perpendicular interface anisotropy. An MCD signal from a downstream Fe film was produced by switching the helicity of x-rays transmitted through an upstream circular polarizer. Practical considerations for optimizing the production of circular polarization are discussed, and synchrotron radiation applications using these circular polarizing filters are suggested.

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.

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.

X‐ray magnetic microscopy and spectroscopy using a third generation synchrotron radiation source

Applications of x‐ray magnetic circular dichroism (XMCD) to the study of magnetic materials are described. XMCD spectra can be used to quantitatively determine magnetic properties on an element‐specific basis. These spectra are also sensitive to the chemical state and environment of the element being probed. The dichroism effect can also be used to produce images of microscopic magnetic structures and domains. Third generation synchrotron light sources are well suited to these experiments. Current and planned facilities at the Advanced Light Source, the first of the new light sources in the U.S., are described, focusing on a new facility with specialized undulators which will directly produce high flux, high brightness beams of circularly polarized x‐rays. With new beamlines which have been optimized for either spectroscopy or microscopy, this facility will provide the capability to provide detailed information about magnetic materials.

The Advanced Light Source Upgrade

The ALS, a third-generation synchrotron light source at Berkeley Lab, has been operating for almost a decade and is generating forefront science by exploiting the high brightness of a thirdgeneration source in three areas: (1) high resolving power for spectroscopy; (2) high spatial resolution for microscopy and spectromicroscopy; and (3) high coherence for experiments such as speckle. However, the ALS was one of the first third-generation machines to be designed, and accelerator and insertion-device technology have significantly changed since its conception. As a result, its performance will inevitably be outstripped by newer, more advanced sources. To remain competitive and then set a new standard, the performance of the ALS, in particular its brightness, must be enhanced. Substantial improvements in brightness and current have always been feasible in principle, but they incur the penalty of a much reduced lifetime, which is totally unacceptable to our users. Significant brightness improvements can be realized in the core soft xray region by going to top-off operation, where injection would be quasi-continuous and the lifetime objections disappear. In top-off mode with higher average current, a reduced vertical emittance and beta function, and small-gap permanent-magnet or superconducting insertion devices, one to two orders of magnitude improvement in brightness can be had in the soft x-ray range. These improvements also extend the high energy range of the undulator radiation beyond the current limit of 2000 eV. Descriptions of the upgrade and the important new science achievable are presented.

Production and Characterization of Circularly Polarized Soft X-Rays for Materials Research at the ALS

The experimental program at the Advanced Light Source to produce and characterize circularly polarized x-rays is described. A number of beamlines are either operating or under construction which produce circularly polarized light. Bend magnets, multilayer phase retarders, and insertion devices which directly produce circularly polarized radiation are described. These beamlines will have capabilities for both microscopy and high resolution spectroscopy. Measurements of the linear and circular polarization of these beamlines are also described.