Percolation, relaxation halt, and retarded van der Waals interaction in dilute systems of iron nanoparticles

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