Magnetic interactions in silica coated nanoporous assemblies of CoFe₂O₄ nanoparticles with cubic magnetic anisotropy

Magnetic interactions in silica coated spherical nanoporous assemblies of CoFe₂O₄ nanoparticles have been investigated by low temperature field dependent remanent magnetization (M^DCD and M^IRM) and magnetization relaxation measurements. The synthesis procedure leads to the formation of spherical aggregates of about 50–60 nm in diameter composed of hexagonal shaped nanocrystals with shared edges. The negative deviation from the non-interacting case in the Henkel plot indicates the predominance of dipole–dipole interactions favouring the demagnetized state, although the presence of exchange interactions in the porous system cannot be excluded. The activation volume, derived from time dependent magnetization measurements, turns out to be comparable with the particle physical volume, thus indicating, in agreement with static and dynamic irreversible magnetization measurements, that the magnetization reversal actually involves individual crystals.