Abstract: The field of ultrafast magnetism is at the frontier of current physics research, with fundamental questions that are still unanswered but which have the potential of impacting the data storage technology upon which our digitalized world relies on. Ultrafast lasers (with energies in the eV range) have been widely used to study ultrafast magnetization dynamics, but due to the relatively large photon energy, they create highly non equilibrium states which tend to mask the fundamental coupling processes leading to ultrafast magnetization. However, recently the use of intense coherent terahertz (THz) radiation (with photon energies in the meV range) is receiving much attention in the research community as it offers a new way to understand and manipulate the magnetic order.

In this talk, I will illustrate the use of THz radiation to study spin dynamics in metallic thin film ferromagnets. First, I will illustrate a direct experimental observation of inertial spin dynamics (in the form of spin nutation in THz frequency regime) in ferromagnetic thin films, which we could describe with a recent modification of the Landau-Lifshitz-Gilbert equation (LLG). Second, by performing simulations with this modified LLG equation, I will discuss the role of inertia in magnetization switching using picosecond magnetic field pulses. It is found that magnetization switching with inertia will be more robust to the details of applied magnetic field. And finally, I will talk about the role of crystalline order on the charge and spin transport in ferromagnetic films. It is found that while the charge scattering follows the development of crystalline order, the spin scattering is rather enhanced at the intermediate crystalline phase, where the magnetic anisotropy is the largest.