Most soft x-ray magnetic dichroism studies are carried out with light of mixed polarization. The effect of incomplete circular polarization is usually taken care off by linearly extrapolating the data to P_circ=1. Using a classical model which includes the polarization factor we show that the MCD-effect, which is the normalized quantity 2(I~+ -I~-)/(I~+ +I~-), is highly non-linear with regard to P_circ. With a specially designed quadruple reflection phase shifter we measrued the MCD effect with P_circ<1 as well as with P_circ approx 1. MCD spectra of Fe around the M_2,3 transition verify our model calculationsof the polarization dependence. The data indicate that the observed non-linear response of the MCD-effect to P-circ can generally introduce large errorsin the quantitative analysis of MCD data where spin and orbital moments are extracted by means of sum rules.
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