## 2 Experiment  
The sample used in our experiment is a Ni(20 nm)/Fe\({}_{50}\)Ni\({}_{50}\)(10 nm)/Ni(20 nm) trilayer deposited via magnetron sputtering on top of Ta(3 nm)-covered Si\({}_{3}\)N\({}_{4}\)(30 nm) membranes. A final Pt(3 nm) capping was used to prevent oxidation (Figure 1). The magnetic layers are polycrystalline and exhibit an in-plane anisotropy with a square magnetic hysteresis loop and a coercive field of 8 mT. Note that the presence of the Fe\({}_{50}\)Ni\({}_{50}\) layer was required for a different femtomagnetism study but is not relevant for the outcome of the present work. Indeed, we would like to emphasize that Ni atoms in the alloy layer make up a total equivalent thickness of only 5 nm, to be compared with 40 nm of pure Ni. Alloyed Ni thus only contributes \(\simeq\)10% to the total absorption signal. For comparison, we used NiFe layers with equal thickness placed at different depths (e.g., Ni(10 nm)/NiFe(10 nm)/Ni(30 nm)). Measurements on these samples yielded the same absorption time traces. The experiments, based on our recently developed X-ray streaking technique [29], were performed at the DiProl end-station [38] of the seeded XUV-FEL at FERMI in Elettra, Trieste [39]. The setup employed in our study is shown in Figure 1. As described in detail previously [29], our approach is based on the use of a nearly collimated XUV beam with circular polarization, which illuminates an off-axis zone-plate. The converging first order emanating from the zone-plate passes through the sample and its intensity distribution is collected by a charge-coupled device (CCD) camera (_sample camera_). As shown in Figure 1, the photons detected at the bottom of this camera (yellow line) have traveled a longer path than the ones arriving at the top (green line). This results in a 2.6 ps time difference at the zone plate focus, which becomes spatially encoded in the sample camera. Note that optical geometry-induced contributions to the temporal resolution are as low as 2.5 fs and can be neglected here. To measure the shot-to-shot variations of the incident X-ray pulses, a second CCD detector (_reference camera_) is used to record the diverging first negative order. By dividing the sample camera with the reference camera picture, we obtain a measure for the transmission of the sample \(T=I/I_{\text{in}}\), where \(I\) is the photon intensity after, and \(I_{\text{in}}\) the one before the sample. Angular integration of the intensity ratio pictures [29] eventually yields the transmission time traces shown in Figure 2a. In order to be sensitive to the in-plane magnetization of the Ni layer, the sample was tilted by 18\({}^{\circ}\) with respect to the incident beam. To ensure saturation of the magnetization for each pump-probe measurement, an in-plane magnetic field of \(\pm\)20 mT was applied by using an electromagnet. The sample was excited by an 800 nm infrared (IR) laser delivering 60 fs pulses using an incident fluence of 5 mJ/cm\({}^{2}\), and probed by circularly polarized 70 fs XUV pulses, with an intensity at the sample plane well below the excitation regime [40]. In the sample plane, the size of the IR spot was \(\simeq\) 250 \(\times\) 165 \(\upmu\)m\({}^{2}\) (FWHM), significantly larger than the XUV spot size \(\simeq\) 130 \(\times\) 70 \(\upmu\)m\({}^{2}\) (FWHM). The experiment was performed at 50 Hz. To probe the absorption and MCD around the Ni \(M_{2,3}\) resonances, we used five probing XUV wavelengths with energies of 64.6, 65.7, 66.3, 66.7 and 68 eV. For each energy, we recorded CCD images by accumulation of 150 to 250 shots with and without an IR pump for both applied magnetic field directions and both XUV helicities. Note that the CCDs employed in our study cannot be read out at 50 Hz, and that we use the same number of accumulated shots on the reference and the sample camera to perform the normalization. For symmetry reasons, reversing the XUV probe beam helicity is equivalent to a reversal of the applied magnetic field direction.