# Ultrafast Electron Dynamics in Magnetic Thin Films
## 1 Introduction
This minute contribution on the magnetisation M(t) is a coherent oscillation in time, following the action of the electro-magnetic wave. However, excited electrons with the above discussed spin flip scattering alter M(t) incoherently in time, leading to fluctuations of the phase; dephasing the system. Therefore, the precessional motion is not typically observable after optical laser pulse excitations. Although the effects of Zeeman torque have been disproven to influence electronic spins to the point of demagnetization in the Mott insulator NiO [22], this effect has not been shown in metals. Optical pulse/thin film demagnetization has been extensively examined, and the majority of these studies report results on indirect and indiscriminate spin excitations [23; 24; 25]. Therefore, to increase the coherency and control of the interaction, experiments have been preformed with THz laser stimuli. The THz field cycle oscillates on a similar timescale as the natural speed of electronic spin motions, as opposed to optical pulse stimuli, which oscillate at a much faster timescale. At low field amplitudes, THz lasers are therefore expected to directly and coherently couple with electronic spin dynamics. This interaction leads to a precessional motion of M(t) due to the Zeeman torque, allowing for selective control of the magnetic phase. Moreover, THz photon energies are three orders of magnitude smaller than optical photon energies, inducing significantly less heating at low field amplitudes, and reducing the possibility for spin flip scattering between electrons and excited particles. This direct coupling of THz and demagnetization was reported for ferromagnetic cobalt films [26], where a coherent, phase-locked demagnetization response was observed under excitations from THz pulses. With the recent advances of THz source technology, THz laser pulses with field amplitudes of several Teslas are now accessible. This opens new venues for exploring the complete precessional reversal of M(t). From a technological perspective, obtaining such a reversal would be a milestone, as the time scales involved would be significantly shorter than the Larmor precession obtained in present-day technology. Clearing a potential pathway to increase processor speeds. However, high field amplitudes come at the cost of a strong heating of the sample, and a loss of the coherent interaction between M(t) and the THz laser field. In this regime, incoherent electronic excitations with subsequent spin-flip scatterings induce a dephasing of M(t) without precessional motion. This is similar to the processes observed after optical pulses. Finally, an additional challenge at large field comes in the form of per manent modifications of film properties, and possible damage observable with scanning electron microscopy. Here, we approach the ultrafast magnetisation dynamics from the photon energy perspective, where we compare excitation dynamics from high (optical) to low (THz) photon energies. We experimentally study the THz-induced ultrafast electronic and structural dynamics in nickel thin films. In the THz frequency range, partial demagnetization in Ni has been previously reported, followed by permanent demagnetization [2]. Here, we carefully design the experiment to accurately map the THz-induced demagnetization. We report that sufficiently intense THz pulses lead to full demagnetization on the ultrafast time scale without sample damage. The process is combined with strong electronic excitation and the generation of acoustic phonons.