Comparative studies of pure, Ca-doped, Co-doped and co-doped BiFeO₃ nanoparticles

BiFeO₃ and Ca, Co doped BiFeO₃ nanoparticles have been synthesized by the sol–gel method. Rietveld refinement of X-ray diffraction data and Raman spectra reflect a structural phase transition from rhombohedral (pure BiFeO₃) to two phase coexistence (rhombohedral R3c and cubic PM-4M) in case of Bi_0.9Ca_0.1FeO₃ and Bi_0.9Ca_0.1Fe_0.95Co_0.05O₃, which indicate that Ca doping can induce the obvious change of crystal structure while Co doping cannot. An apparent blue-shift can be observed in all the doped samples along with a decrease of the direct optical band gap compared with pure BiFeO₃. XPS results reveal that Ca and Co doping increases the Fe³⁺ ions (unidoped samples show better results), which could be an evidence for decrease of oxygen vacancies and enhancement of magnetism. Ca and Co uni-doped into BiFeO₃ can enhance magnetism due to different mechanisms, more interestingly, the room temperature magnetic hysteresis (M–H) curve shows the largest saturation magnetization value in Ca, Co co-doped samples.