## 2 Results and Discussions
### Time Resolved Magnetic Circular Dichroism and Helicity-Dependent Absorption  
In the second part of this paper, we present recent results on the hypothesis that tailored chemical heterogeneities in magnetic systems can be used to enhance the ultrafast magnetic response after optical excitation. As outlined in the introduction, in multi-component magnetic systems with sites exhibiting a distinct number of available states above the Fermi energy, OISTR can become an important mechanism for ultrafast demagnetization. To this end, we have compared the ultrafast magnetization dynamics of an elemental Co film and Co\({}_{0.5}\)Pt\({}_{0.5}\) alloy via MCD in transmission geometry [18], as schematically depicted in Figure 2b. We generate high harmonic radiation by focusing intense laser pulses into a neon-filled gas cell, leading to discrete harmonic emission peaks in the spectral  
Figure 4: (a) Measurements of \(I_{\pm}\) for opposite magnetization directions, \(M_{\pm}\), as a function of the time delay at the Gd \(N_{4,5}\) resonance at \(E\approx 149\) eV. Corresponding values of \(M(t)/M_{0}\) (b) for the Gd and (c) Fe sublattice determined according to Equation (5). Solid lines are non-linear least-square fit with a monoexponential function with a time constant \(\tau=(233\pm 16)\) fs and \(\tau=(94\pm 15)\) fs for Gd and Fe, respectively. range between \(\approx\)40-72 eV with a width of \(\approx\)200 meV. The state of polarization is controlled by a four-mirror phase-shifter [24; 51; 52], resulting in a high degree of circular polarization of \(>\)80% in the energy range between 50 and 65 eV. The broadband XUV spectrum is transmitted through the sample, energetically dispersed and measured by a charged coupled device (CCD). As both magnetic films, Co and CoPt, have their easy magnetization direction oriented in the sample plane (cf. schematic in Figure 2d,e), we mounted the sample at \(\theta=45^{\circ}\) with respect to the incoming wave vector of the XUV light to ensure a finite projection onto the in-plane magnetization. The samples were excited in a collinear geometry with laser pulses centered at a wavelength of \(\lambda=800\) nm, a fluence of \(\approx\)15 mJ cm\({}^{-2}\) and a pulse length of 39 fs. The experiment is carried out at a 3 kHz repetition rate. We measure the transmitted intensity, \(I_{\pm}(E,t,M_{\pm})\), for two opposite magnetization directions by toggling the external magnetic field, \(B_{\pm}\). Note that measurements with opposite magnetization directions of the sample but with a fixed helicity of the light is equivalent to measurements with opposite helicities but a fixed magnetization direction [19]. The magnetic asymmetry, \(A\), is approximately proportional to the MCD contrast and is given by:  
\[A=\frac{I_{+}-I_{-}}{I_{+}+I_{-}}\approx\frac{1}{2}ln\left(\frac{I_{+}}{I_{-} }\right)=-\frac{2 Ed}{\hbar c}\Delta\beta\propto M. \tag{6}\]  
To increase the signal to noise ratio, we record the incoming spectrum with a second spectrometer and repeat the time delay scans up to 100 times. The total measurement time per sample amounts to about 4 h for each sample. For more details on the experimental details, we refer to [PERSON] et al. [53]. In Figure 5, we show the magnetic asymmetry as a function of the time delay after optical excitation. The asymmetry for the elemental Co film measured at the Co \(M_{2,3}\) edge is \(A=-6.6\%\), which drops with a time constant \(\tau(\text{Co}:\text{Co}M_{2,3})=(124\pm 4)\) fs by 20%. For the CoPt alloy, we follow the element-specific response by recording the respective sublattices at the Co \(M_{2,3}\) edge as well as at the Pt \(O_{3}\) and \(N_{7}\) edge independently in the same measurement. Here, the asymmetries are smaller and remain below 1%. Note that the small asymmetries in XUV MCD compared to Faraday rotation measurements require a significantly better noise management. The rms value for \(A\) before \(t=0\) fs corresponds to \(<3\times 10^{-4}\), yielding an excellent signal to noise ratio of our data.