We find three unanticipated features in the magnetic response of dilute systems of highly monodisperse Fe nanoparticles. Above a spin freezing temperature ${(T}_f)$ the remanent magnetization relaxes smoothly to zero, but below $T_f$ the relaxation halts abruptly at a nonzero value. The distribution of relaxation rates changes at a percolation temperature ${(T}_p),$ consistent with chainlike structures above $T_p$ and three-dimensional clusters below $T_p.$ The blocking temperature ${(T}_b)$ varies inversely proportional to particle diameter, opposite to the behavior of the Néel-Brown model for individual domains, but consistent with a type of Casimir-Polder interaction expected between dilute nanometer-scale particles.

• Received 5 March 2002

DOI:https://doi.org/10.1103/PhysRevB.66.172403