# Ultrafast Electron Dynamics in Magnetic Thin Films
## 1 Introduction  
The study of ultrafast magnetism is crucial in advancing our understanding of magnetic systems, as well as developing ultrafast memory devices. The difficulty of this subject lies in spin dynamics, and its role as a part of a larger picture of the barely understood coupled structural dynamics. As an example, when a ferromagnetic film is excited by a femtosecond (fs) laser pulse, partial demagnetization of the material occurs within \(\sim\)100 fs, followed by a remagnetisation to the original state on a longer time scale. This was first observed in 1996 for optical laser pulses [1], and later also for THz laser pulses [2; 3; 4; 5]. While this phenomenon has led to intense investigations in the field by both experimentalists and theorists, it has now become evident that this process is mediated by a complex puzzle of strongly entangled pieces [6; 7]. To elaborate on this, it is essential to understand the physical mechanisms that can influence the spin angular momenta of the electrons responsible for magnetisation. In order to simplify the almost unfeasible task of tackling coupled structural dynamics, we examine the phenomena responsible and their respective timescales. This allows us to decouple mechanisms which have infinitesimal effects on each other. As [PERSON] and [PERSON] pointed out, a separation of atomic nucleus dynamics and the surrounding electrons can be made. This is because electrons will adiabatically follow any comparatively slow changes of the lattice. That is, typical dynamics of phonons are on the picosecond timescale, whereas electron dynamics in metals can occur in as short a time span as the attosecond timescale. Similarly, a separation of electron charge and spin degrees of freedom is reasonable, due to the slower time scales that are observed for the dynamics of the spins, \(\sim\)100 fs, than that for the electron scattering, \(\sim\)10 fs. The dynamics of magnetism in a stimulated material could be affected by any number of combinations of the mechanisms described above. In particular, there may be different channels of light-matter interactions; here, we elaborate on the two extremes: direct coupling of the laser pulses to electronic spins, and intermediate charge and/or phonon excitations which induce magnetisation dynamics [1; 8; 9; 10; 11; 12; 13]. In the direct scenario, the local electro-magnetic field of the stimulus directly couples with the electronic spin angular momenta [10; 14], such that there is minimal heating of the electronic structure. In the indirect channel, the laser photons do not primarily change the magnetisation. Instead, photon energy is transferred to the system in the form of an increased electronic and/or atomic temperature. This process can excite electron-hole pairs, lattice phonons and magnons, which will non-selectively spin-flip scatter with the electrons responsible for magnetisation, modifying their angular momenta [15; 16; 17; 18]. Demagnetization effects in past reports with optical laser pulses are mediated through this indirect channel. This results in a non-equilibrium state, where the electronic temperature is raised above the phononic temperature, and an imbalance exists between chemical potentials of electrons with differing spins. This difference is the driving force for ultrafast demagnetization [19; 20]. The system then evolves through the above discussed spin-flip scatterings, leading to changes in the orientations of atomic magnetic moments. As a result of this, a temporary demagnetization is observed, followed by remagnetisation to the original state through the balance of the chemical potentials, and the thermalisation of the electrons and phonons. Furthermore, in samples with metallic substrates, there is also the contribution of a superdiffusion process [21]. In this process, the excited mobile spin carriers are transferred onto a conducting substrate. Lastly, the total effective magnetic field exerts a Zeeman torque onto the atomic magnetic moments.