## 2 Experiment
This allows us to average the measured curves, resulting in two transmitted intensities \(T^{\pm}=\frac{I^{\pm}}{I_{\text{in}}}\). We grouped the data such as to obtain a data point every 30 fs, and the error bars presented in all our figures reflect this grouping and averaging procedure. Typical transmitted intensity time traces are plotted in Figure 2a for an XUV energy of 66.7 eV and both external field directions \(H^{+}\) (dark green) and \(H^{-}\) (light green). As expected, the thin film is excited on sub-ps timescales; the difference between \(T^{+}\) and \(T^{-}\) is quickly decreasing after \(t=0\), which reflects the strong transient modifications of the magnetic properties of the system. Figure 1: Experimental setup and sample composition. A circularly polarized extreme ultraviolet (XUV) beam is diffracted using a zone plate. The positive, first diffraction order is focused on an in-plane magnetized sample, which is pumped using linear infrared pulses. The sample camera records the intensity transmitted through the sample \(I\), resulting in a spatial encoding of the time traces. The reference camera records the incident intensity \(I_{\text{in}}\). To quantify these changes, \(T^{\pm}\) data can be used to calculate the absorption \(\mu\) and the \(MCD\), which are related to the electronic occupation and to the spin state of our magnetic layer, respectively. We therefore use \(\mu^{\pm}\propto-\ln T^{\pm}\), \(\mu=\frac{\mu^{+}+\mu^{-}}{2}\propto\frac{\ln(T^{+},T^{-})}{2}\) and \(MCD=\mu^{+}-\mu^{-}\propto\ln\frac{T}{T^{+}}\)[41]. Quantities extracted from pumped data will be denoted \(T^{\pm}(t)\), \(\mu^{\pm}(t)\), \(\mu(t)\) and \(MCD(t)\), while static data will be labeled \(T^{\pm}_{0}\), \(\mu^{\pm}_{0}\), \(\mu_{0}\) and \(MCD_{0}\). In order to compare data measured using different XUV energies, we need to look at the relative changes \(\frac{\Delta\mu^{\pm}(t)}{\mu^{0}_{0}}\), \(\frac{\Delta\mu(t)}{\mu_{0}}\) and \(\frac{\Delta MCD(t)}{MCD_{0}}\). Here, the prefix \(\Delta\) is used as a short notation for a subtraction of the static signal, e.g., \(\Delta\mu(t)=\mu(t)-\mu_{0}\). Due to different IR filters used in front of the cameras, experimental transmission data are only known up to a prefactor. This can be seen in Figure 2a, where the transmission is superior to 1, which is of course at odds with the fact that the beam intensity is weakened _via_ absorption. To correct this artifact, we use the calculated transmission \(T_{\text{th}}\) shown in Figure 2b (dark curve) to rescale our data and obtain the proper relative absorption. The calculated transmission and MCD curves presented in Figure 2b have been obtained using tabulated values (CXRO website [42]) for Si\({}_{3}\)N\({}_{4}\), Ta, and Pt, which were combined with literature data for Ni and Fe [43]. Comparison of the calculated MCD signal (green curve in Figure 2b) with our static experimental \(MCD_{0}\) (colored dots) yields good agreement for each energy scrutinized in the present study (Figure 2b) (note that the experimental MCD spectrum does not need to be rescaled since the constant pre-factors cancel out as shown in Equation (1)). The following equations summarize our approach, and all data presented in the remainder of this work are based on these calculations:  
\[\frac{\Delta\mu^{\pm}(t)}{\mu^{\pm}_{0}}=\frac{\ln\left(\frac{T^{\pm}_{0}}{T^ {\pm}(t)}\right)}{\ln\left(\alpha\cdot T^{\pm}_{0}\right)}\]  
\[\frac{\Delta\mu(t)}{\mu_{0}}=\frac{\ln\left(\frac{T^{+}_{0}\cdot T^{-}_{0}}{T ^{+}(t)\cdot T^{-}(t)}\right)}{\ln\left(\alpha^{2}\cdot T^{+}_{0}\cdot T^{-}_ {0}\right)}\quad\text{with}\ \ \alpha=\frac{2T_{\text{th}}}{T^{+}_{0}+T^{-}_{0}} \tag{1}\]  
\[\frac{\Delta MCD(t)}{MCD_{0}}=\frac{\ln\left(\frac{T^{+}_{0}\cdot T^{-}(t)}{T ^{+}(t)\cdot T^{-}_{0}}\right)}{\ln\left(\frac{T^{-}_{0}}{T^{+}_{0}}\right)}\]  
Figure 2: Characteristic pump-induced changes of \(T^{+}\) and \(T^{-}\) on fs timescales and static energy-dependent transmission and magnetic circular dichroism (MCD).