Dynamical properties of non-interacting Co nanoparticles

The dynamical properties of the magnetic moment of non-interacting cobalt n anoparticles dispersed in a polymer were investigated by ac-susceptibility (10 Ha < f < 10(4) Hz). We thus estimated the relaxation time of the magnet isation (tau) associated to the superparamagnetic properties of the particl es, tau was found to follow the general Neel-Brown relaxation dynamics, wit h a pre-exponential relaxation time tau(0) of 4(+/-2).10(-12) s for tempera tures between 10 and 20 K. By combining the experimental results from ferro magnetic resonance, ac-susceptibility, and magnetisation, measurements givi ng the microscopic and macroscopic parameters, we demonstrate for the first time that the Coffey theoretical expressions were able to predict both tau and tau(0) values. Finally, the thermal dependence of tau(0) is discussed.