Spin Dynamics with Inertia in Ferromagnetic Thin Films

Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient devices. On short time scales, the inertia term in the LLG equation becomes important, leading to the nutation of spins. However, experimental observation of nutation is still in its infancy. Here, we experimentally demonstrate a non-resonant excitation of inertial spin dynamics in ultrathin Ni<inf>80</inf>Fe<inf>20</inf> films leading to nutation at a frequency of ∼ 0.1 THz, measured by time-resolved magneto optical Kerr effect (TR-MOKE). The inertial regime has a lifetime of more than 300ps.