Influence of the stoichiometry and grain morphology on the magnetic properties of Co substituted Ni–Zn nanoferrites

A set of Co substituted Ni–Zn nanoferrites with a nominal composition Co0.2Ni0.3Zn0.5Fe2O4 was prepared by the polyol method. The influence of a number of synthetic parameters on the structure, microstructure, and the magnetic properties was investigated. X-ray diffraction, infrared, energy dispersive X-ray, transmission electron microscopy, and vibrating sample magnetometry were employed for this purpose. The X-ray diffraction results confirmed the formation of a single phase nanocrystalline spinel-type ferrite powders. In addition, the cell parameter and the integrated intensity ratio I220/I422 was found to vary with the synthesis conditions suggesting deviation from the nominal chemical composition and/or a probable deviation from the preferential (that of the bulk) cations occupancy of the tetrahedral (A) and the octahedral (B) spinel sites. For the so-called bulk ferrite obtained by moderate sintering nanoparticles of the stoichiometric ferrite, a cation distribution similar to that of the bulk was suggested. Transmission electron microscopy analysis of the as-produced ferrites revealed the formation of nanoparticles with mean particle size in the range ~4–12 nm. The magnetic properties of both as-prepared nanoparticles and the so-called bulk ferrite were studied. All the as-produced particles exhibited superparamagnetic behavior at room temperature with a gradual decrease of the blocking temperature with the decrease of crystallite size. Additionally, the saturation magnetization, the coercivity, and the Curie temperature were found to be clearly dependent on the stoichiometry, the cations occupancy, and/or the grains morphology. For the stoichiometric ferrites the relatively higher Curie temperature values measured for the smaller particles was interpreted on the basis on the cations distribution change; for the as-produced nanoparticles a fraction of Zn2+ ions is expected to migrate from A to B sites accompanied with a reverse transfer of an equal amount of the paramagnetic cations from B to A sites.