## 1 Introduction  
The absorption of a femtosecond laser pulse by a thin magnetic film induces a non-equilibrium and spin-dependent electron distribution that triggers a cascade of microscopic processes and causes a very rapid change in the magnetization [1; 2]. Spin-orbit coupling mediates spin-flips in scattering events and leads to a local loss of the magnetization [3]. Magnon generation quenches the long-range magnetic order [4; 5; 6], and the spin-dependent transport properties of excited electrons trigger spin currents and redistribute spin angular momentum within the sample [7; 8]. One of the most fascinating observations in the field of femtomagnetism is all-optical switching, which was first discovered in the rare earth-transition metal alloy, FeGd [9]. Upon optical excitation, the antiferromagnetically coupled Gd and Fe sites can be promoted into a transient ferromagnetic state, which mediates a reversal of macroscopic magnetization [10; 11]. Microscopically, this is understood by a transient suppression of the inter-atomic exchange interaction due to ultrafast heating of the electrons, enabling an independent response of the rare earth and transition metal sublattices. Upon cooling, angular momentum exchange breaks the symmetry of the system and leads to magnetization switching. Because of the low specific heat of electrons, all-optical switching is inherently energy efficient with obvious potential for applications. Very recently, a completely new mechanism to manipulate the magnetic structure has been put forward theoretically: laser pulse excitation induces an optical inter-site spin transfer (OISTR) in multi-element magnetic systems [12]. This spin manipulation is of a purely optical nature and is therefore one of the fastest ways to manipulate spins by light. The authors of this theoretical study also predicted that the spin-selective charge transfer between distinct sites can lead to a metamagnetic phase transition, switching the magnetic order from an antiferromagnetic to a transient ferromagnetic alignment. In the last two years, a number of experiments demonstrated signatures of OISTR, both in the optical [13; 14] as well as in the extreme ultraviolet (XUV) [15; 16; 17; 18] spectral range. The latter experiments were carried out with high harmonic generation (HHG) sources and exploited the element specificity of resonant core-to-valence state transitions. Here, the sensitivity to the local magnetic moment can be easily explained by a two step model [19]: if circularly polarized photons are absorbed by core level electrons with finite spin-orbit interaction the resulting photoelectrons become spin-polarized. Then, the helicity-dependent absorption strength in the material depends on the spin imbalance of the ferromagnet, with different numbers of available states for majority and minority electrons in the exchange-split valence band. Therefore, the observable, calculated as the difference of the transmitted light intensity for opposite circular polarization, is directly proportional to the magnetization and is called magnetic circular dichroism (MCD). More generally, the optical response around such a resonant transition in magnetic materials can be parameterized by a complex magneto-optical function. While time resolved MCD measurements have been pioneered in the soft X-ray range at the \(L\)-edges of transition metals, in particular, at the femto-slicing beamline at BESSY, HZB, Germany [20], in the last decade a lot of effort has been devoted to transferring resonant probing techniques of magnetization to more shallow resonances in the XUV spectral range, both at free electron laser (FEL) facilities [21] and at laboratory-based HHG sources in reflection [22; 23] or transmission geometry [24].