Inertial terms to magnetization dynamics in ferromagnetic thin films

Inertial magnetization dynamics have been predicted at ultrahigh speeds, or frequencies approaching the inverse Drude scattering time, in ferromagnetic metals. Here, we identify inertial terms to magnetization dynamics in thin Ni79Fe21 and Co films near room temperature. Effective magnetic fields measured in high-frequency ferromagnetic resonance (115-345 GHz) show an additional stiffening term which is quadratic in frequency and similar to 80 mT at the high-frequency limit of our experiment. Our results extend understanding of magnetization dynamics at picosecond time scales.