X-ray magnetic circular dichroism (XMCD)
...shows up as dependence of the absorption of circularly polarized x-rays at elemental absorption edges on the sample magnetization direction. It is a consequence of the spin polarization of the excited electronic transitions (for example 2p --> 3d), caused by the circular polarization, which probes the spin polarization of the unoccupied electronic states just above the Fermi level (caused by the ferromagnetism of the sample).
By tuning the photon energy of x-rays coming from a synchrotron radiation light source to the elemental absorption edges, different magnetic layers in multilayered samples can be measured or imaged independently. The figure shows the absorption spectrum of an ultrathin trilayer consisting of two ferromagnetic layers (cobalt and nickel) separated by a nonmagnetic copper layer.
The use of synchrotron radiation is necessary because the employed effect requires the wavelength of the x-rays to be tuned to elemental absorption edges. In addition, the x-rays have to be circularly polarized.