Optical pump-probe studies of the rise and damping of ferromagnetic resonance oscillations in a thin Fe film

Small-amplitude ferromagnetic resonance (FMR) oscillations have been studied in a thin Fe film by means of an optical pump-probe technique in which the sample magnetisation is pumped by an optically triggered magnetic field pulse and probed by means of the magneto-optical Kerr effect. The magnetic field pulse is generated by a current pulse in a coplanar transmission line on which the sample is overlaid. The sample was probed at different positions above the transmission line, to study the effect of pump fields applied at different angles to the sample plane, and the static field was applied in different directions relative to the plane of incidence. The Landau-Lifshitz-Gilbert equation has been used to model the response of the sample magnetisation and the experimental and theoretical curves are found to be in reasonable agreement. From the phase and decay of the FMR oscillations the rise time of the pump field inside the sample and the value of the Gilbert damping constant have been determined. The rise time of the pumping field within the sample is found to depend upon the orientation of the pumping field relative to the plane of the sample. The value of the damping constant is found to depend upon the orientation of the static field relative to the transmission line. We suggest that the observed behaviour results from the generation of eddy currents.