# Ultrafast Electron Dynamics in Magnetic Thin Films
## 2 Materials and Methods  
For our experiment, a 15 nm sputtered Ni thin film on Si substrate was chosen as the sample. This is due to the many extensive studies performed upon the ultrafast magnetism of the material, first reported by [PERSON] et al. in 1996.  
The setup is a tightly focused terahertz bullet scheme, where the THz pulse is focused at the diffraction limit to reach the maximum possible intensity. The THz pulse is generated by the optical rectification of near infrared pulses (1550 nm central frequency, 100 Hz, 50 fs, 3.5 mJ, light conversion OPA system) and an organic crystal DAST (Swiss Terahertz GmbH), 350 \(\upmu\)m thick, and is modulated via a mechanical chopper at 37 Hz. The beam is expanded then focused on the sample (three off axis parabolic mirrors scheme). The probe beam is 800 nm centred, 70 fs, 100 Hz, and collinear with the THz pump. The sample was placed to allow for an incident angle of 45\({}^{\circ}\). Both probe and pump were linearly polarized, and the full setup is illustrated in Figure 1a. The THz-induced magnetisation dynamics lead to birefringence measured with a collinear 800 nm probe (observed as an electrical signal on the balanced detector, which is linearly proportional to the amount of birefringence). The temporal and spectral contents of the THz pulse are shown in Figure 1b,c. All measurements of the ratio between the magnetisation and its saturation value M(t)/M\({}_{\text{s}}\), as shown in Figure 1, were performed by modulating the external magnetic field at 40 mT and 25 Hz. This frequency was used as a reference for our acquisition system to eliminate any non-magnetic contribution to the measured signal.  
The magnetic field modulation is locked to the laser clock frequency, and its magnitude is larger than the saturation magnetic field of the sample. Therefore, the measured birefringence angle without the THz pump can be characterised as complete magnetisation of the sample. When the sample is pumped, the birefringence changes linearly with the sample magnetisation, which can then be extracted accordingly. This experimental setup was also characterised in-depth in a previous work [27].  
The THz source contains significant spectral components up towards \(\sim\)18 THz (as characterised through electro-optical sampling of a GaP crystal and ABCD detection technique); these components were eliminated through a set of low pass filters with cut-off frequencies at 3, 6 and 9 THz. This results in a set of pulses that are well defined in spectral content; we expect these pulses to contain several oscillation pulses in the time domain. To calculate the pulse fluence, we refer to a previous work on the same THz source: [28]. In the cited text, a one-to-one correspondence between the pulse fluence and the peak electric field strength was found through the Kerr effect in diamond, allowing us to infer the fluence of a filtered pulse by measuring the pulse duration and spot size (measured previously in [29]).  
In order to decouple the charge, spin and phonon dynamics without a detailed analysis of their specific relationships with one another, we exploit the timescale differences between them. This was achieved by modulating the THz pump excitation and repeating the experiment in Figure 1 with static magnetic fields of B+ = +40 mT and B\(-\) = \(-\)40 mT, corresponding to biases which are parallel and antiparallel to the THz pulse magnetic field, thus maintaining a constant magnetic bias during our experiments.