MCXD (also known as X-MCD) is not a surface science technique, but is closely related to other techniques in this list such as EXAFS and XANES. As in these techniques, MCXD uses a monochromatic X-ray beam that is directed at the sample and the photon energy of the X-rays is gradually increased such that it traverses one of the absorption edges of the elements contained within the sample. However, MCXD also uses a circularly polarised beam of X-rays. If an atom has a magnetic moment, then the probability of the absorption of an X-ray whose energy is just above an absorption edge can depend on whether the X-ray is left or right circularly polarised. Hence this technique can observe the behaviour of different magnetic atoms as the magnetic field is cycled. It can also derive both the spin and angular momentum contributions to the magnetic field in the sample for each magnetic atom. Hence it is a powerful new technique for the investigation of magnetic materials. The late Theo Thole played a large part in the introduction of MCXD.