Coherent magnetization dynamics in strongly quenched Ni thin films

The remagnetization process after ultrafast demagnetization can be described by relaxation mechanisms between the spin, electron, and lattice reservoirs. Thereby, collective spin excitations in form of spin waves and their angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the strength of demagnetization affects the coherency and the phase of the excited spin waves. We present a study of coherent magnetization dynamics in thin nickel films after ultrafast demagnetization using the all-optical, time-resolved magneto-optical Kerr-effect (tr-MOKE) technique. The largest coherent oscillation amplitude was observed for strongly quenched systems, showing the conservation of coherency for demagnetizations of up to 90%. Moreover, the phase of the excited spin-waves increases with pump power, indicating a delayed start of the precession during the remagnetization.