Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films

Understanding spin dynamics on femto- and picosecond timescales offers new opportunities for faster and more efficient spintronic devices. Here, we experimentally investigate the coherent spin dynamics after ultrashort laser excitation by performing time-resolved magneto-optical Kerr effect experiments with thin ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ films. We provide a detailed study of the magnetic field and pump fluence dependence of the coherent precessional dynamics after optical excitation with femtosecond laser pulses. We show that the coherent precession lifetime increases with the strength of the applied magnetic field, which cannot be understood in terms of viscous Gilbert damping of the coherent magnons. Instead, it can be explained by nonlinear magnon interactions and by the change in the fraction of incoherent magnons. This interpretation is in agreement with the observed trends of the coherent magnon amplitude and lifetime as a function of the applied exciting laser fluence. Our results provide insight into the magnetization relaxation processes in ferromagnetic thin films, which are of great importance for further spintronic applications.