Magnetization dynamics of Ni and Co films on Cu(001) and of bulk nickel surfaces

The ultrafast magnetization dynamics of thin Ni and Co films on Cu(001) and of polycrystalline Ni surfaces was studied by pump-probe reflection second-harmonic generation, utilizing 150 fs/800 nm laser pulses. In all cases no delay between electron excitation and magnetization breakdown was observed within the experimental time resolution. An upper limit of such delay is 50 fs in case of bulk Ni surfaces. The recovery of magnetization follows the electron temperature relaxation during the first few picoseconds and thereafter cooling by regular thermal diffusion.