Observation of the propagation and interference of spin waves in ferromagnetic thin films

We have observed the propagation of spin waves and spin-wave packets in 20-nm-thick Permalloy films upon microwave excitation by means of time-resolved scanning Kerr microscopy. Spin waves can be detected up to 80 mu m away from the driving coplanar strip line. In doing so, we were able to directly determine the phase velocities, dispersion relations and group velocities for different geometries of an externally applied in-plane magnetic field. In addition, we report on the direct optical observation of spin-wave interference with lateral and time (phase) resolution.