## 2 Results and Discussions
### Time Resolved Magnetic Circular Dichroism and Helicity-Dependent Absorption
Non-linear least square fits (solid lines) reveal an equivalent response with a demagnetization amplitude of 80% and time constants of \(\tau(\text{CoPt}:\text{Co}\;M_{2,3})=(86\pm 3)\) fs, \(\tau(\text{CoPt}:\text{Pt}\;O_{3})=(84\pm 7)\) fs, \(\tau(\text{CoPt}:\text{Pt}\;N_{7})=(92\pm 11)\) fs. A priori, identical local demagnetization times for Co and Pt are an unexpected result, because it is generally assumed that an important process responsible for ultrafast demagnetization relies on spin-flip scattering processes, which are mediated by spin-orbit interaction. With an approximately 10-times larger spin-orbit coupling in Pt compared to Co, we would anticipate a larger demagnetization rate localized at the Pt sites. This is, however, not what we observe. Additionally, we recall that CoPt compared to Co exhibits a significantly higher demagnetization rate and efficiency. This strongly suggests that an additional demagnetization channel is active in the two-component alloy CoPt. In the following section, we will demonstrate that this process can be identified as OISTR. In Figure 6, we show a calculation of the laser-driven spin-dependent \(3d\) electron occupation, \(\Delta n_{\text{min/maj}}(t)=n_{\text{min/maj}}(t)-n_{\text{min/maj}}(t<0)\) around the Fermi energy of Co in the CoPt alloy and of the elemental Co film. The simulations are performed using a fully non-collinear version of TD-DFT as implemented within the Elk code [54]. Upon laser pumping, electrons are excited to available states above the Fermi energy. Because the density of states of the spin-split \(3d\) band of Co is dominated by available minority states, minority electrons are reshuffled more efficiently. In elemental Co, we observe a small loss of majority electrons (spin up) and a small increase in minority electrons (spin down), which we can attribute to spin-flips, leading to the observed loss of magnetization. However, much more prominent is the very large gain in minority electrons in CoPt above the Fermi energy, significantly exceeding the loss of minority electrons below the Fermi energy (red dashed line). It is worth also noting that this is much more pronounced compared to the minority response of elemental Co (blue dashed line). The additional minority electrons stem from the Pt sublattice and are transferred in an optical transition between the respective \(d\) minority states. To experimentally demonstrate that the minority 3\(d\) states of Co in CoPt are indeed efficiently filled in an OISTR process, we examine the helicity-dependent absorption channels individually. We calculate the absorption as \(\mu_{\pm}=1-I_{\pm}\), neglecting the small reflectance in the XUV spectral range. Figure 5: The magnetic asymmetry, \(A\) (MCD), of the Co film and the CoPt alloy as a function of the time delay for three different photon energies corresponding to the Co \(M_{2,3}\) edge at 60.3 eV and to the Pt \(O_{3}\) and \(N_{\mathcal{P}}\) edge at 54.1 eV and 72.6 eV, respectively. The lines are non-linear least square fits. The rms value for values before \(t=0\) fs corresponds to \(<\)3 \(\times\) 10\({}^{-4}\) yielding an excellent signal to noise ratio in spite of small asymmetry values. Figure 6: We show a calculation of the laser induced changes in the spin-dependent electron occupations in the 3\(d\) band, \(\Delta n_{\text{min}/\text{maj}}\) around the Fermi energy for Co in the CoPt alloy (red lines) and for elemental Co (blue lines). Laser excitation promotes electrons from occupied states below the Fermi energy to available states above the Fermi energy. Minority states (spin down) of Co in the CoPt alloy are filled much more efficiently than in the elemental Co, indicating a transfer of minority states from Pt 5\(d\) to Co 3\(d\) states, significantly increasing the efficiency of the demagnetization dynamics.