Thermal, structural and magnetic studies on chromite spinel synthesized using citrate precursor method and annealed at 450 and 650 °C

Chromite Spinel materials were synthesized in this study by the citrate precursor method using four divalent cations (Ni2+, Co2+, Zn2+, and Cu2+). Citrate precursors consisting of mixed chromium citrates were first subjected to a thermogravimetric (TG) analysis for determining optimum temperatures for annealing. TG of coprecipitated chromium(III) citrate–zinc citrate gel has been carried out separately in N2 and O2 atmospheres. In both the cases, dehydration is followed by a four-step decomposition. The TG data were subjected to kinetic/mechanistic analysis, and the values of activation energy and Arrhenius factor were approximated. TG curves of various powders which were obtained on annealing at the two temperatures did exhibit thermal instability when carried out in N2 atmosphere. A large coercivity of 2701.01 Oe was observed for NiCr2O4 at 650 °C. On the basis of the results, 450 °C has been chosen for annealing treatment of the four gels. The samples were accordingly annealed at two different temperatures (450 and 650 °C) in a muffle furnace for 1 h in each case. The annealed powders were characterized using X-ray diffraction (XRD), SEM, and vibrating sample magnetometer (VSM). The XRD patterns show that annealing of CuCr2O4, NiCr2O4, and CoCr2O4 at 450 °C yields very small crystallites with poor Bragg reflections, although ZnCr2O4 samples show better peaks in XRD data. Annealing at 650 °C resulted in particle size range of 8–89 nm in the four cases. In the case of ZnCr2O4, the particle size was 8 nm.