Fabrication of Zinc Ferrite Nanocrystals by Sonochemical Emulsification and Evaporation:  Observation of Magnetization and Its Relaxation at Low Temperature

A new ultrasound assisted emulsion (consisting of rapeseed oil and aqueous solution of Zn²⁺ and Fe²⁺ acetates) and evaporation protocol has been developed for the synthesis of zinc ferrite (ZnFe₂O₄) nanoparticles with narrow size distribution. The as-synthesized sample consisted of crystalline zinc ferrite particles with an average diameter of ∼4 nm, whereas the average size of the heat-treated ferrite particles increases to ∼12 nm. To remove the small amount of oil present on the surface of the as-synthesized ferrite sample, heat treatment was carried out at 350 °C for 3 h. The as-synthesized and heat-treated ferrites were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), TGA/DTA, transmission electron microscopy (TEM), and energy dispersion X-ray spectroscopy (EDS) techniques. Magnetic measurements show that the nanocrystalline ZnFe₂O₄, prepared through this technique, is either at par with those obtained in other cases or even more improved. Both the as-synthesized and heat-treated samples reveal relaxation of magnetization. Our study also shows that one can tailor the magnetization and relaxation pattern by suitably controlling the particle size of the nanocrystalline ZnFe₂O₄. The key features of this method are avoiding (a) the cumbersome conditions that exist in the conventional methods, (b) the usage of necessary additive components (stabilizers or surfactants, precipitants), and (c) calcination requirements. In addition, rapeseed oil has replaced organic nonpolar solvents used in earlier studies. As a whole, this simple straightforward sonochemical approach results in a better pure phase system of nanoferrite with improved magnetic properties.