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.