# Ultrafast Electron Dynamics in Magnetic Thin Films
## 4 Discussion
Therefore, our results point towards the indirect mechanism as the most likely hypothesis. A lingering question remains: Do individual photon energies play a significant role towards the measured demagnetization effect? Indeed, this indirect mechanism critically depends on the presence of a high integrated photon energy to raise electron temperatures above phonon temperatures. Comparing the photon energies from the optical (eV energy scale) range to the THz (meV energy scale) range, we conclude that this effect is not directly possible without exponentially increasing the number of THz photons. In contrast to this claim, the experimental results do not reveal a relationship between individual photon energy and demagnetization. First, there is no high fluence threshold for demagnetization; A nearly linear relationship between fluence and demagnetization is observed in our experiments (Figure 1f). Secondly, with comparisons between our results and a previous report on a similar sample under optical excitation [2], the absorbed fluence needed for demagnetization from optical and terahertz pulses are of the same order of magnitude. The differences in energy between individual optical and THz photons are three orders of magnitude apart, and thus confirms a negligible dependence of demagnetization on individual excitation photon energy, ruling out any significant direct interaction of the THz electric field frequency on demagnetization in our experiments, with evidence suggesting that only the net fluence absorbed influences the demagnetization in the sample. In light of these results, we extended our search of the mechanisms behind demagnetization by including charge and lattice degrees of freedom. The ultrafast excitation of thin films leads to rapid heating and excitation of electrons to higher non-equilibrium states. This absorbed energy is eventually transferred to the lattice through electron-phonon interactions, leading to spatio-temporal strain pulses and coherent acoustic phonons which reverberate inside the film. This model has been used to explain ultrafast optical generation of lattice strain waves of coherent acoustic phonons, which can manipulate and coherently control the magnetisation orientation in ferromagnetic films [8,11,12,13], thereby justifying our investigation into these additional dynamics. The spin dynamics (as shown in Figure 2b) exhibit a rapid \(\sim\)1 ps response, which decays over time, concurring with the results shown in Figure 1 Additionally, the charge-lattice-combined response (Figure 2c) is characterised by a sharp transient, followed by a slow relaxation with a fast damping oscillations. Here, the oscillations represent the coherent acoustic phonons and are independently extracted in Figure 2d, where \(\sim\)ps to a few ps oscillations are observed. To further justify the significance of these additional dynamics, the constant bias experiments was performed again at excitation fluence of 48 mJ/cm\({}^{2}\) and 85 mJ/cm\({}^{2}\) as shown in Figure 3a,c. Where the isolated spin and charge dynamics, corresponding to the magnetic and electronic responses are shown in Figure 3b,d. It is shown that the magnetic response lags behind the electronic response for all values of fluence. Furthermore, it is shown that a sub picosecond magnetic response exists from a THz pulse excitation.