X-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD) is the difference in X-ray absorption between left and right circularly polarized light in magnetic materials. It is the X-ray counterpart of the magneto-optic effect for visible light but shows a magnetic contrast up to three orders of magnitude higher. The exploration of XMCD using high-flux, monochromatic and polarization-variable synchrotron sources has advanced the understanding of magnetism and magnetic materials, in particular, when combined with spectral analysis based on powerful sum rules that enable the quantification of spin and orbital moments with elemental, even chemical, selectivity and high sensitivity. As an essential cornerstone of techniques to probe magnetic nanostructures and spin textures as well as their dynamics, XMCD has become an indispensable tool for the study of magnetism at the nanoscale and atomic scale. This Primer provides an overview of the principles and physics underlying XMCD, the experimental techniques used to measure it and its application to the study and understanding of fundamental and technologically relevant magnetic phenomena.