Abstract : The main subject of this thesis is the study of the unconventional superconductor Sr2Ru04. In this compound the Cooper pairing is not due to electron-phonon interaction but is more likely related to spin fluctuations, as in superfluid 3He. A p-wave symmetry is often invoked, induced by hypothetical ferromagnetic spin fluctuations. To address this problem, the first step was to grow and characterize high-purity single crystals. Some of the crystals I made are several cm long and 4.5 mm diameter. They were used for the study of the magnetic excitations in Sr2RuO4 by inelastic neutron scattering. These measurements confirmed that the spin fluctuations at the incommensurate wave vector Qo = ±2pi/3a, ±2pi/3a,0) are the main magnetic excitations in both the normal and superconducting states. Qo is the nesting vector of the Fermi surface. By studying these fluctuations along the c-axis, we showed that the magnetic correlations are two-dimensional. Additional measurements suggest that the dynamical spin susceptibility is isotropic in spin space. The fact that the incommensurate magnetic scattering is predominant and that the expected ferromagnetic spin fluctuations are not observed suggest that the simplistic model of p-wave superconductivity has to be reconsidered. Recent theories suggest an f-wave superconducting order parameter consistent with incommensurate spin fluctuations. However, our results do not show any connection between the most commonly spin-state of the superconducting order parameter (spins in the a-b plane and d-vector along the c-axis) and the incommensurate spin fluctuations.