Keith W. Goodman

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 dichroism effect of binary alloys using a circularly polarized x ray

We have studied the magnetic properties of CoNi binary alloy films with various atomic compositions using the soft x-ray magnetic circular dichroism technique. The alloy films were deposited on a single Cu(100) crystal in situ using our well established epitaxial growth technique to achieve a layer-by-layer growth and a metastable face-centered-cubic structure, with all the films exhibiting an in-plane magnetic anisotropy. The high density, circularly polarized x-ray beam was supplied by the elliptically polarizing undulator at the Stanford Synchrotron Radiation Laboratory. Utilizing the element-specific ability and nanostructure magnetization sensitivity of this technique, we were able to perform the absorption measurements at L2 and L3 edges of Co and Ni atoms and we observed large dichroism signals. The extraction of spin moment and orbital moment for varying elemental stoichiometry using magneo-optical sum rules is discussed.

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.

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.

Element-specific magnetometry with linear dichroism in photoemission

In this paper, we investigate the magnetic linear dichroism in the core-level photoemission spectra of the binary alloys CoxNi1−x and FexNi1-x /Cu(100).These epitaxial films have fcc structures, but very different magnetic behaviors. We show that the x-ray magnetic linear dichroism in photoemission (XMLD) signal tracks the magnetization in these alloys. Comparison with recent SQUID data provides a quantitative check and endorses the view that XMLD monitors the element-specific magnetometry.

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.

Magnetic instability of ultrathin fcc FexNi1-x films grown on Cu(100)

The {open_quotes}invar effect{close_quotes} in Fe{sub x}Ni {sub 1{minus}x} alloys occurs when the Fe content approaches 65{percent}. At this point, the magnetization falls to zero, and a martensitic structural transformation from a fcc to a bcc lattice occurs. This paper addresses the question: {open_quotes}What happens if the structural transformation is suppressed in an ultrathin alloy film?{close_quotes} We present results to this effect, showing the variation of the magnetization with changing composition in ultrathin films grown on Cu(100). We find a new low-spin, ferromagnetic phase of matter, which is a sensitive function of the atomic volume. {copyright} {ital 1997} {ital The American Physical Society}

Fermi Surface Mapping Using A Third Generation Light Source

The electronic states at the Fermi surface determine diverse properties such as magnetism, chemical bonding, and phonon-electron coupling. Using a conventional hemispherical analyzer at the ultraESCA beamline 7.0 of the Advanced Light Source, the authors have measured Fermi contours of the bulk and surface states of Cu(001) and Ag(001). For bulk states, the authors used uniform sampling in k-space by varying both the electron takeoff angle as well as the photon energy. Three-dimensional plots (in k-space) of bulk and surface states at the Fermi level can easily be achieved within one or two synchrotron shifts. Surface states, whose momentum is independent of k-perpendicular, are easily mapped if sufficiently dense angular sampling is performed. The states crossing the Fermi level at {bar X} in the surface Brillouin Zone of Cu(100) and Ag(100) are presented as examples.