X-ray birefringence and dichroism obtained from magnetic materials

In the past decade, synchrotron radiation has triggered a surge in studies of the polarization dependence of X-ray beams passing through non-isotropic materials. Avast range of experimental results concerning polarization-dep endent absorption (dichroism) and dispersion (birefringence, for example) a re available from materials which are either magnetic or exhibit preferred directions due to the local atomic environment. This article aims to bring together the diversity of modern experiments in this field with established methods of optical calculus, in a way that highlights the simplicity of th e underlying physics. A useful framework is formed when observable quantiti es, in the X-ray case, are related to atomic variables of the sample materi al. Atomic descriptions of absorption spectra with various levels of comple xity are considered, and some well documented sum-rules are encountered. Th e framework is the most general allowed within the electric dipole approxim ation. By way of illustration, dichroic X-ray absorption by two materials w ith highly anisotropic properties and magnetic ions with different valence shells are considered; namely, a 3d-transition ion in ferrous niobate, and a lanthanide ion in dysprosium borocarbide. Both materials display interest ing magnetic properties that are challenging to interpret at an atomic leve l of detail, and it is shown how absorption experiments can contribute to r esolving some issues.