Abstract
Inelastic neutron scattering has been used to study the dynamics of a dispersion of fine Fe particles (diameter ∼ 20 Å) on the time scale of 10-10-10-11 s. It gives the first experimental evidence of a dual dynamics in zero field corresponding to a relaxation time (longitudinal) and to a fluctuation one (semi-transverse) at low temperature, merging in an isotropic regime at high temperature. In this isotropic regime, the fluctuation time appears limited to a temperature-independent value which could be related to the anisotropy energy of the particle.