Cation Vacancies in NiFe2O4 During Heat Treatments at High Temperatures: Structural, Morphological and Magnetic Characterization

Nickel ferrite (NiFe2O4) was synthesized by mixing stoichiometric amounts of alpha-Fe2O3 and NiO using mechanical milling and heat treatments at high temperatures. The physical characterization of the samples was carried out using X-ray diffraction, infrared and Raman spectroscopies, Mossbauer spectrometry, magnetization measurements, scanning electron microscopy and energy dispersive X-ray spectroscopy. We found that NiFe2O4 production increases from 81 to 100 wt. % with increasing temperature. Additionally, the lattice parameter and the saturation magnetization increase with increasing temperature. On the other hand, Mossbauer spectrometry showed that there is a decrease in the subspectral areas ratio for Fe3+ cations at tetrahedral (A) and octahedral [B] sites, A(A)/A(B), with the increase of the temperature. In the SEM micrographs it was observed that the samples consisted of particles with irregular shapes and micrometric sizes. From IR spectra, the intensity of the 411 cm(-1) band (vibrations at octahedral sites) increases relative to the intensity of the 599 cm(-1) band (vibrations at tetrahedral sites) with increasing temperature. From the results obtained in the magnetization curves, it was possible to confirm the synthesis of NiFe2O4. As the heat treatment temperature increases, hysteresis loops with S-type geometric forms were obtained. All the results suggest that a defective spinel NiFe2O4 is formed at 1000 degrees C, and that as the temperature increases, the defects gradually disappear. Neither cation reordering phenomena nor possible evaporation of chemical elements were the dominant effects to account for the results. The results can be explained if it is assumed that [B]-sites cation vacancies are gradually filled with cations as the temperature of the heat treatment increases.