Effect of isochronal annealing on phase transformation studies of iron oxide nanoparticles

The effect of isochronal annealing on the phase transformation in iron oxide nanoparticles is reported in this work. Iron oxide nanoparticles were successfully synthesized using an ash supported technique followed by annealing for 2 h at various temperatures between 300 and 700°C. It was observed using X-ray diffraction (XRD) and transmission electron microscopy (TEM) that as-grown samples have mixed phases of crystalline haematite (α-Fe2O3) and a minor phase of either maghemite (γ-Fe2O3) or magnetite (Fe3O4). On annealing, the minor phase transforms gradually to haematite. The phase transformation is complete at annealing temperature of 442°C as confirmed by differential scanning calorimetric (DSC) analysis. The unresolved phases in XRD were further analysed and confirmed to be maghemite from the X-ray absorption near edge structure (XANES) studies. The magnetic measurements showed that at room temperature nano- α-Fe2O3 is weak ferromagnetic, and its magnetization is larger than the bulk value. The mixed phase sample shows higher value of magnetization because of the presence of ferromagnetic γ-Fe2O3 phase.