  The magneto-optical Kerr effect (MOKE) in the extreme ultraviolet (EUV)
regime has helped to elucidate some of the key processes that lead to the
manipulation of magnetism on ultrafast timescales. However, as we show in this
paper, the recently introduced spectrally-resolved analysis of such data can
lead to surprising experimental observations, which might cause
misinterpretations. Therefore, an extended analysis of the EUV magneto-optics
is necessary. Via experimental determination of the dielectric tensor, we find
here that the non-equilibrium excitation in an ultrafast magnetization
experiment can cause a rotation of the off-diagonal element of the dielectric
tensor in the complex plane. In direct consequence, the commonly analyzed
magneto-optic asymmetry may show time-dependent behaviour that is not directly
connected to the magnetic properties of the sample. We showcase such critical
observations for the case of ultrafast magnetization dynamics in Ni, and give
guidelines for the future analysis of spectrally-resolved magneto-optical data
and its comparison with theory.
