Influence of High-Temperature Annealing on Structural and Magnetic Properties of Crystalline Cobalt Ferrite Nanoparticles in the Single-Domain Regime

We have carried out a systematic study on the effect of high-temperature annealing on the structural and magnetic properties of ultra-small crystalline cobalt ferrite nanoparticles prepared by the hydrothermal method. The structural and the magnetic characterizations of nanoparticles were investigated by XRD, TEM, FTIR, and VSM. The results of Rietveld refinement revealed that the nanoparticles have a cubic single-phase spinel structure. High-temperature annealing was found to increase the lattice parameter and average crystallite size of the cobalt ferrite nanoparticles. The TEM measurements showed that the nanoparticles were monodisperse and spherical in shape. The FTIR results confirmed the single-phase nature of the prepared nanoparticles. The magnetic measurements showed that the nanoparticles were ferromagnetic over a wide temperature range. We have observed that the high-temperature annealing increased both the saturation magnetization and magnetocrystalline anisotropy and decreased the coercivity. We demonstrated that magnetic field induced superparamagnetism can be induced by applying stronger magnetic fields that were able to shift the blocking temperature (T_B) down to below room temperature, resulting in a superparamagnetic behavior above T_B.