Dynamical properties of non-interacting Co nanoparticles

The dynamical properties of the magnetic moment of non-interacting cobalt nanoparticles dispersed in a polymer were investigated by ac-susceptibility (10 Ha < f < 10(4) Hz). We thus estimated the relaxation time of the magnetisation (tau) associated to the superparamagnetic properties of the particles, tau was found to follow the general Neel-Brown relaxation dynamics, with a pre-exponential relaxation time tau(0) of 4(+/-2).10(-12) s for temperatures between 10 and 20 K. By combining the experimental results from ferromagnetic resonance, ac-susceptibility, and magnetisation, measurements giving 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.