Dependence of structural phase transition and lattice strain of Fe₃O₄ nanoparticles on calcination temperature

Fe₃O₄ nanoparticles were synthesized, using a simple co-precipitation method and then calcined at various temperatures in the range of 50–850 °C for 1 h in air. After calcination, the nanoparticles were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and vibrating sample magnetometer. The X-ray diffraction results indicated that Fe₃O₄ nanoparticles were converted to γ-Fe₂O₃ by calcining at 250 °C for 1 h and then to α-Fe₂O₃ on calcining in the range of 550–650 °C. The average crystallite size of the nanoparticles was calculated by using the Scherrer and Williamson-Hall methods. The average crystallite size of the iron oxides NPs increased from 7.2 to 35.8 nm by increasing calcination temperature from 50 to 850 °C. A small strain existed, which were affected on the physical and structural properties of Fe₃O4. The vibrating sample magnetometer results indicated that, the as-synthesized nanoparticles converted from superparamagnetic to ferromagnetic phase with calcinations up to 650 °C, due to increasing size of nanoparticles from a single domain to multidomain as indicated in the X-ray diffraction results. (author)