Inductive measurement of ultrafast magnetization dynamics in thin-film Permalloy

An inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described. The technique is demonstrated using 50 nm films of Permalloy (Ni81Fe19). Data are presented for impulse- and step -response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis. Rotation times as short as 200 ps and free oscillations of the magnetization after excitation are cle arly observed. The oscillation frequency increases as the dc bias field par allel to the anisotropy axis increases as predicted by classical gyromagnet ic theory. The data are fitted to the Landau-Lifshitz equation, and damping parameters are determined as a function of dc bias field. Damping for both impulse and step excitations exhibits a strong dependence on bias field. D amping for step excitations is characterized by an anomalous transient damp ing which rapidly increases at low dc bias field. Transformation of the dat a to the frequency domain reveals a higher order precessional mode which is also preferentially excited at low dc bias fields. A possible source for b oth phenomena is precessional mode saturation for large peak rotations. The technique has the potential for 20 ps resolution, although only 120 ps res olution is demonstrated due to the limited bandwidth of the waveguides used . (C) 1999 American Institute of Physics. [S0021-8979(99)06111-3].