Fig. 3: Fluence-dependence of the helicity-dependent dynamics.

a Demagnetization amplitudes (D) measured upon pumping with circularly polarized extreme ultraviolet (XUV) pulses with opposite helicities (σ_±, red circles and green squares) and (b) their difference ∆M (yellow diamonds) as a function of incident fluence and XUV photon energy. All values are normalized to the magnetization in the unexcited state and ∆M was obtained as shown in the inset of Fig. 2. The solid lines in (a) correspond to sigmoid functions fitted as guide to the eye; the shaded areas denote the experimental uncertainty by a 90% confidence interval. The dashed lines in (b) indicate linear trends fitted to the different fluence regimes, in which the magnitude of the helicity-dependence ∆M increases upon increasing the XUV fluence and subsequently saturates or even decreases for high fluences (white and gray shaded background). For clarity, only the fluence-dependence of the σ_±-polarized excitation is shown (for linear polarization, see Supplementary Fig. 6). The error bars of the D values correspond to the uncertainty of the fitted demagnetization amplitudes (y-axis) and the standard error of the mean fluence values obtained by sorting the data after the free-electron laser (FEL) pulse energy (x-axis). The error bars of the ∆M values are calculated from the error propagation.