Lattice expansion and saturation magnetization of nickel-zinc ferrite nanoparticles prepared by aqueous precipitation

Nanoparticles of nickel-zinc ferrite prepared across the solid solution compositional range by aqueous precipitation at 100 degrees C are all found to have both a larger lattice parameter and a lower saturation magnetization than bulk forms of the same compositions. The lower saturation magnetization is attributed to a combination of the larger lattice parameter, decreasing the super-exchange interactions between the Ni2+ and Fe3+ ions, and incomplete ordering of the cations between the octahedral and tetrahedral sites in the spinel structure. Annealing experiments suggest that the lattice expansion is probably due to the incorporation of OH(-)groups during synthesis. Characterization of the nanoparticles indicates that the saturation magnetization can be restored while preserving their super-paramagnetic behavior by heating to temperatures below which they coarsen, typically 900 degrees C.