The relaxation processes of electrons and spins systems following the absorption of femtosecond optical pulses in ferromagnetic nickel have been studied using optical and magneto-optical pump-probe techniques. The magnetization of the film drops rapidly during the first picosecond, but different electron and spin dynamics are observed for delays in the range 0-5 ps. The experimental results are adequately described by a model including three interacting reservoirs (electron, spin, and lattice).[S0031-9007(96)00167-6] PACS numbers: 75.40. Gb, 78.20.Ls, 78.47.+p Ultrafast optical spectroscopy is an ideal technique to investigate the electronic relaxation processes in metallic materials [1][2][3]. A femtosecond optical pulse can induce a nascent nonequilibrium electron gas which subsequently thermalizes to a Fermi distribution. This thermalization, which takes place within about 500 fs as measured for instance in noble metals, is due to electron-electron interactions [4][5][6]. In the next step, the hot electron gas relaxes its energy to the lattice due to electron-phonon interactions, a process which occurs within 1-10 ps depending on the incident laser intensity. During this time, the temperature exchange between the electron bath (temperature T e ) and the lattice (temperature T l ) can be investigated. From such measurements, one can deduce the characteristic times of the microscopic interactions which govern the basic metallic properties like electron transport and superconductivity [7].In spite of the large amount of work performed in this field, little attention has been paid to magnetic effects occurring in the femtosecond time scale. An important challenge is to know whether the initial hot electron distribution can induce a spin dynamics associated with a spin temperature T s different from T e and T l . This would lead to an ultrafast demagnetization, a relevant effect in regards to applications in magneto-optic devices. From a conceptual point of view, such a fast magnetization change is not trivial since the optical transitions preserve the electronic spin. However, one can expect that, during the transient hot electron regime, spin populations are modified due to spin dependent electron scattering. The characteristic time of this process has not been investigated with a femtosecond temporal resolution. Previous experiments performed with picosecond pulses on Ni [8] or Fe [9] have shown no demagnetization effect up to the melting point of the samples. The authors concluded that the spin-lattice relaxation times of these metals are larger than 30 ps. A more sophisticated pump-probe experiment, using 10 ns pump and 60 ps probe pulses, made on Gd films could deduce a spinlattice relaxation time of 100 6 80 ps [10,11]. All these experiments were performed on a time scale where electrons and lattice temperatures are in equilibrium and therefore could not resolve separately the effects of electron-spin and spin-lattice relaxation mechanisms on the demagnetization process.The aim of this paper is to study b...

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