Influence of parasitic phases on the magnetic properties of BiFeO₃ powders

Single phase BiFeO₃ nanoparticles were successfully prepared by sol-gel method. The influence of annealing temperature and precursor drying process on phase purity, microstructures and magnetic properties of BiFeO₃ powders were explored. The precursor was pre-dried at 70 °C for 6 hours and then heated at 100 °C for 12 hours. The single-phase BiFeO₃ powders were successfully obtained by annealing the dried samples at 500°C for 1 hour. However, powder X-ray diffraction reveals the presence of parasitic phase Bi₂Fe₄O₉ or iron oxide under other preparation conditions. With the increase of annealing temperature, the content of parasitic phase Bi₂Fe₄O₉ increases while the grain size, particle size, morphology and distribution uniformity of BiFeO₃ also change. When the precursor solution was dried directly at 100°C and then annealed at 500 °C for 1 hour, the parasitic phase of Fe_1.966O_2.963 appeared. The particles exhibit sphere-like morphology covered with small BiFeO₃ grains. With the increase of drying time, the surface grains changed and the content of parasitic phase Fe_1.966O_2.963 increased. FTIR and EDS confirm the existence of BiFeO₃. UV-VIS measurements show that the optical band gap of BiFeO3 nano-powders decrease with the increase of the content of these two parasitic phases. In this paper, effects of these parasitic phases on the magnetic properties of BiFeO3 have been studied in detail. Results show that the ferromagnetic behavior of the powders could be attributed to the Fe_1.966O_2.963 parasitic phases. The parasitic phase of Bi₂Fe₄O₉ in the powders does not change the room temperature magnetic properties of BiFeO₃. Both the single phase BiFeO₃ and the Bi₂Fe₄O₉ incorporated powders exhibit bulk antiferromagnetic behavior. (paper)