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Using a "pump and probe" technique, the time scale of spin relaxation effects in ferromagnetic thin films is investigated. Samples are excited by a 3 eV "pump" laser pulse ͑t ഠ 170 fs͒, and electrons are photoemitted by a 6 eV "probe" pulse, after delays ranging up to 1 ns. The spin polarization of the photoelectrons is measured using a Mott detector. The time dependence of the spin polarization of thin Ni films on Ag is explained in a local magnetic moment picture with two excitation processes. Stoner excitations are responsible for the reduction of the observed spin polarization on a time scale of 1 ps and phonon-magnon scattering leads to the loss of long range magnetic order on a time scale of 500 ps. [S0031-9007(97)04890-4] The interactions between quasiparticles like electronelectron, electron-phonon, and electron-magnon scattering determine the macroscopic properties of matter at finite temperatures, and the dynamical response to external excitations; also properties like superconductivity and giant magnetoresistance are governed by them. Experimentally these interactions are reflected by the lifetime broadening of spectroscopic features. If these interactions are specifically included in the interpretation of experimental results, then they are usually treated as instantaneous, meaning that the system under consideration is always in thermodynamical equilibrium, whereby the true dynamics of these processes is neglected.The magnetism of transition metals at finite temperatures is one of the most challenging topics in modern physics and much work has been done to calculate thermodynamic properties like the magnetization M, the susceptibility x, and the Curie temperature T C from first principles. An itinerant ferromagnet shows two types of magnetic excitations. Magnons which follow an approximately quadratic dispersion law are responsible for the low and medium temperature properties, whereas single particle excitations, the Stoner excitations, cost much more energy. Early models either neglected spin fluctuations (Stoner model) and thus predicted much too high Curie temperatures, or neglected the itinerant character leading to a Heisenberg-like description. Modern theories combine both pictures. A Stoner-type intinerant part leads to the formation of local moments and a Heisenberg-like part describes their interaction [1,2]. We will show in this Letter that both kinds of magnetic excitations are important to understand the dynamics of transition metal ferromagnets.An experimental approach to investigate these interactions within a real-time experiment is very difficult, because a time resolution on the time scale of the electronelectron scattering (ϳfs) has to be achieved [3][4][5][6][7]. In the last few years these time scales became accessible by the development of commercial laser systems generating ultrashort pulses. These laser systems are most prominently applied in investigating electron scattering dynamics in semiconductors due to the industrial interest in high frequency devices [8][9][10][11]....

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