Lawrence Berkeley National Laboratory

The soft x-ray spectral region contains important core levels of 3d transition and rare earth elements and so has emerged as a powerful spectral region in which to study magnetism in a variety of materials. While some spectroscopic techniques are relatively mature in this spectral range, others are still under development. This chapter reviews a variety of evolving soft x-ray techniques as applied to the study of magnetism and magnetic materials. Emphasis is given to entirely photon based techniques that, compared to techniques detecting photoelectrons, can probe relatively deeply into samples and are compatible with strong and varying applied fields. Emphasis is also placed on techniques that can resolve magnetic structure either in depth or laterally in samples, rather than just providing spatially averaged properties. The soft x-ray spectral range extends roughly from 100 eV to 2500 eV, and is often defined as that region where the path length of x-rays is insufficient to propagate in air at atmospheric pressure. This strong soft x-ray absorption has made this spectral range one of the last to be exploited to study magnetic materials, since specialized sources, optical elements, and instrumentation are necessary for such measurements. Paradoxically perhaps, this strong absorption indicates large interaction cross-sections, especially at certain core levels, where magneto-optical effects can be larger than in any other spectral range. Synchrotron radiation sources generally provide the polarized soft x-rays needed for these studies, and are now common enough to provide reasonable access. Essentially all optical and scattering techniques common in the near-visible and x-ray spectral ranges have been extended into, or sometimes rediscovered in, the soft x-ray range. The coupling of these large, resonant magneto-optical effects with these various techniques is discussed here. The following sections review fundamental characteristics of resonant magneto-optical spectra at x-ray core levels before introducing different approaches to apply these effects in different ways. Since modern magnetic materials are typically chemically and magnetically heterogeneous, often down to nanometer length scales, it is natural to categorize techniques by their ability to resolve such structure both in depth and laterally. For example, direct measurements of transmitted (forward scattered) beams average both laterally and in-depth throughout the illuminated area. Specular reflection techniques average over lateral structure, but can provide depth resolution in different ways. Diffuse scattering and diffraction, in transmission or reflection geometry, can resolve lateral structure and can also have variable depth sensitivity. Partially coherent scattering provides an ensemble average over structure, while coherent scattering retains details of local structural information. Finally, zone-plate microscopy provides direct images of local chemical and magnetic structure. Recent advances in each of these areas are discussed and compared below.