Distinct spectral response in M-edge magnetic circular dichroism

Experimental investigations of ultrafast magnetization dynamics increasingly employ resonant magnetic spectroscopy in the ultraviolet (XUV) spectral range. Besides allowing one to disentangle the element-specific transient response of functional magnetic systems, these techniques also promise to access attosecond to few-femtosecond dynamics of spin excitations. Here, we report on a study of transient magnetic circular dichroism (MCD) on the transition metals Fe, Co, and Ni as well as on a FeNi and GdFe alloy after optical excitation and reveal a delayed onset between the electronic and the MCD response for measurements with narrow bandwidth XUV pulses tuned to a specific energy around the respective elemental ${M}_{2,3}$ resonances. We argue that for this experimental implementation, the MCD observable does not necessarily reflect the initial femtosecond magnetization dynamics of the sample, but instead---supported by ab initio calculations---an effective delay can be attributed to a transient energy shift of the MCD spectra due to nonequilibrium changes of electron occupations.