Introduction by F. Wilhelm, ESRF

X-ray Magnetic Circular Dichroism (XMCD) is inherently an element specific and orbital selective experimental technique that has greatly contributed to our understanding of magnetism. This tool is now widely used to unravel the microscopic origin of magnetism in complex magnetic materials: crystals, multilayers, clusters and impurities. Recently, technological applications have stimulated the interest in new magnetic heterostructures at the nanometre scale (spin valves, magnetic tunnel junctions, etc), which exhibit unusual magnetic properties. However, there will be no further progress in the design of such new magnetic nanostructures without a deeper understanding of the mechanisms that control their magnetic properties and govern their magneto-optical response.

It is often considered that itinerant magnetism originates mainly from the spin contribution. However, the orbital moment can be larger than 10% of the total magnetic moment (as in the case of 5d and 5f elements). XMCD experiments have shown that the orbital moment plays an important role in magnetic properties. Typically, orbital magnetism is at the origin of the magnetic anisotropy and therefore determines the easy axis magnetisation direction. The dimensionality strongly affects the orbital magnetism due to reduction of symmetry at the surfaces or interfaces. XMCD spectroscopy is one of very few experimental techniques that allow one to disentangle the spin and orbital contributions to the total magnetic moment carried by an absorbing atom. The effect of the local structure on orbital magnetism is nicely illustrated by the XMCD studies of strained superlattices under high pressure.

The effect of dimensionality on the orbital magnetic moment of transition metals is another hot topic in magnetism research. A wide variety of magnetic behaviours has been revealed in one-dimensional magnetic chains that are strikingly different from those observed in two-dimensional structures and bulk materials. Unexpectedly large orbital magnetic moments have been observed in the case of transition metal impurities in diamagnetic cubic single crystals. Such results cannot be easily understood using existing theories and certainly call for a development of more sophisticated models of magnetism in low-dimensional systems.

Another important axis for research that came out of XMCD studies is the magnetism of "non-magnetic" atoms induced via hybridisation with their magnetic neighbours. Particularly of interest are U-based multilayers in which a small induced magnetic moment on uranium is responsible for the strongly-enhanced magneto-optical response observed with Kerr rotation at room temperature.

The use of the resonant-scattering technique adds spatial sensitivity to the X-rays research in magnetism. These experiments were shown to be very efficient to unravel the magnetic structure of complex antiferromagnetic oxides. Moreover, X-ray scattering techniques can be extended to study charge and spin density waves in thin films and heterostructures.

Last but not least, continuing efforts are invested into the development of multi-element, energy-resolved detectors, which could be operated in the soft X-ray range as well as at high energies. The first experimental results obtained with this type of detector have revealed a great potential for recording high-quality XMCD spectra of magnetic nanostructures.