# Ultrafast Electron Dynamics in Magnetic Thin Films
## 4 Discussion  
We perform a rough calculation to compare the sample temperature at various fluences with the Curie temperature of Ni. The heat equation of the system is: \(Q=m\times c\times dt\), where \(m\) is the mass of the object, \(c\) is the heat capacity, and \(dt\) is the induced temperature change. Bulk nickel has a heat capacity of 0.44 J/gK and a density of \(8.908\times 10^{6}\) g/m\({}^{3}\). In our experiments, the sample thickness is 15 nm and we assume that about 10% of the incident THz pulse is absorbed by the sample. Using a low pass filter of 6 THz, we observe partial demagnetization at a fluence of 16.7 mJ/cm\({}^{2}\), corresponding to a temperature increase of  
Figure 2: (**a**) The THz-induced birefringence in Ni using an external mechanical chopper under and external magnetic bias B+ & B– with a fluence of 190 mJ/cm\({}^{2}\). (**b**) The extracted magnetic response (demagnetization) obtained from B+ \(-\)B–. Blue, green, black, and magenta refer to the fluences of 47, 85, 123, & 190 mJ/cm\({}^{2}\). (**c**) the total electronic and structural response is obtained using (B+) + (B–) which contains both thermal excitation and acoustic phonons. The latter is independently shown in (**d**). 280 K. Complete demagnetization was observed at 47.3 mJ/cm\({}^{2}\), corresponding to a 800 K temperature increase. Taking into account that the experiment was performed at room temperature, this gives us the temperature range of 500 K to 1070 K, which is comparable to the Curie temperature of Ni at 627 K.  
This identifies that the demagnetization observed at very large fluence is due to the existence of hot electrons at the Fermi level (whereby electrons are excited by the intense sub ps heating out of the Fermi level. These electrons are prevented from thermalising with the Fermi sea due to the non equilibrium nature of the system, and therefore dominates the subsequent dynamics [30; 31]). This is similar to what has been previously reported for optical laser sources, but has not been proven previously for THz sources. Previous results on a similar sample [2] showed a maximum demagnetization of 58% at 89 mJ/cm\({}^{2}\) before the sample becomes permanently damaged. This is in contradiction to the results we present here, where higher demagnetization was achieved at lower THz fluence without noticeable damage. We offer an explanation for this discrepancy: The MOKE probe in the previous report likely lacked the spatial resolution to pinpoint the demagnetised area exactly. Hence, both the areas of demagnetization and the areas that are unaffected contributed to the measurement, giving a lower demagnetization percentage. In this paper, we limit the spectral contents to <6 THz; this increases the diffraction-limited excitation spot size. This allows the area of demagnetization to be fully resolved, and we therefore achieved higher measurable demagnetization at lower fluence. We use this quantitative result to address the main question of the role of photons in demagnetization; specifically, whether demagnetization was achieved through the direct or indirect channel discussed above. Conventionally, it was assumed that the direct mechanism was associated with excitations of low frequency electro-magnetic radiations, such as microwave radiation. In contrast, high frequency optical pulse excitations have been shown to demagnetise materials through the indirect mechanism dominantly. However, it is not impossible for both mechanisms to coexist. It is generally considered that the higher the excitation pulse frequency, the higher the excitation field amplitude is needed in order to induce changes in the magnetisation. Therefore, to achieve magnetic switching in the THz regime, it would require practically unrealistic pulse intensities (>10 Tesla). In this limit, undesired structural changes would inevitably take place. Our experiments show that THz excitations are only capable of inducing small amplitude precessions, as is confirmed in Figure 1, where no precessions are observed in the measured signal.