Magnetic relaxation of Co nanoclusters in a bias magnetic field

We report ac magnetic susceptibility data measured on two samples of Co nano-particles, with average diameters and 3.1 nm, respectively. The temperatures T max of the maxima of the real and imaginary components of the susceptibility shift in opposite directions when an increasing bias magnetic field H is applied. We show that the increase of T max of with H is caused by the non-linear field dependence of the equilibrium magnetization and that only provides reliable information about the magnetic relaxation mechanisms in a bias field. Our data show that the magnetization reversal takes place via a classical thermally activated relaxation process. The influence that inter-cluster dipolar interactions have on the relaxation time depends on H . At zero field, the magnetic relaxation is slower than in the limit of no interactions. By contrast, as the magnetic moments of the particles become gradually polarized by H , the relaxation approaches the theoretical predictions for non-interacting particles.