X-Ray Magnetic Circular Dichroism
XMCD is a variant of XAS in which circular polarized light is used. We acquire two spectra, one with right circular polarized light and one with left circular polarized light. When electrons absorb a photon they are promoted from a core level up to the fermi level. According to the dipole selection rules, electrons of one spin direction will preferentially be promoted to the fermi level according to whether right or left circular polarization is used. With the other polarization, the other spin direction electrons will be promoted to the fermi level. However, in a magnetic material the density of states at the fermi level differs for the two spin orientations. (For a half-metallic ferromagnet, for which the spin resolved density of states is shown below, the density of states of one of the spin orientations is zero.) The absorption observed will then be different for the two light polarizations used and we effectively measure the spin populations of the up and down spin orientations in the sample. By subtracting the two spectra from one another, we obtain the difference in spin populations. A non-zero difference indicates a magnetic sample. Because the technique is element sensitive, the XMCD signature is characteristic of a particular element.
The spin resolved density of states of a half-metallic ferromagnet is shown on the left. On the upper right is the XAS spectra acquired with each of the light polarizations and on the lower right is the "difference" spectrum, the XMCD