Coster-Kronig contributions to magnetic circular dichroism in theL2,3x-ray fluorescence of iron

High-resolution x-ray fluorescence spectroscopy has been performed on magnetically oriented polycrystalline iron samples. Circularly polarized synchrotron radiation was used to excite the Fe ${\mathit{L}}_{2,3}$ x-ray emission, either directly from a bending magnet (white radiation), or from an asymmetric wiggler followed by a monochromator tuned to the ${\mathit{L}}_2$ edge. Large differences in the magnetic circular dichroism (MCD) were observed. Spin-polarized relativistic linear muffin-tin orbital calculations and a simple qualitative model demonstrate that the ${\mathit{L}}_3$ x-ray emission dichroism is partially canceled by the ${\mathit{L}}_2$-${\mathit{L}}_3$ ${\mathit{M}}_{4,5}$ Coster-Kronig process. The Coster-Kronig process is large enough to make MCD x-ray absorption experiments, performed in the fluorescence mode, unreliable.