Dielectric properties and high temperature magnetic behavior on multiferroics Bi1-xCaxFeO3 ceramics

Multiferroic Bi1-xCaxFeO3 (x=0, 0.05, 0.1, 0.15, 0.2) ceramics are prepared by sol-gel method. The effects of Ca doping on the structure, delectrical, ferromagnetism properties and high temperature magnetic behavior of BiFeO3 ceramics are studied. The structures of BiFeO3 ceramics are characterized by X-ray diffraction (XRD). The results show that all the peaks for Bi1-xFexO3 samples can be indexed according to the crystal structure of pure BiFeO3. The characteristic diffraction peaks of Bi1-xCaxFeO3 samples become gradually wider and the (104) and (110) peaks of BiFeO3 merge partially into a broadened peak (110) with Ca2+ doping increasing. XRD analysis reveals a phase transition in Ca-doped BiFeO3 from rhombohedral to orthorhombic when x is larger than 0.15. The scan electron microscope images indicate that Ca2+ doping significantly increases the grain sizes of BiFeO3 ceramic. The average grain sizes of Bi1-xCaxFeO3 samples range from 0.5 to 2 μm.#br#The dielectric behaviors of Bi1-xCaxFeO3 ceramics change with content x and frequency. The dielectric constant measured at 1 kHz reaches a maximum value of εr=4603.9 when x=0.1, seven times as big as that of pure BiFeO3. With further increasing the Ca content (x=0.15, 0.2), the value of the dielectric constant is back to the level of pure BiFeO3. The dielectric constant of Bi0.8Ca0.2FeO3 (εr=57) is less than one-tenth that of BiFeO3 (εr=629.9). The dielectric losses of Bi1-xCaxFeO3 samples become smaller than that of BiFeO3 ceramic. This dramatic change in the dielectric properties of Bi1-xCaxFeO3 samples can be understood in terms of orientational relaxation of dipoles and the space charge limited conduction associated with crystal defects at low frequency.#br#The magnetic measurements show that all samples possess strong ferromagnetism at room temperature expect BiFeO3 which is weakly ferromagnetic. The X-ray photoelectron spectroscopy spectrum indicates the coexistence of Fe2+ and Fe3+ in Bi1-xCaxFeO3 samples. The ratios of Fe2+/Fe3+ are 21/79, 23/77, 27/73, 32/68, and 32/68, respectively. The ratio of Fe2+/Fe3+ increases with doping Ca content increasing, and the magnetic preparation of BiFeO3 is enhanced.#br#It is evidenced that the ferromagnetic phase transitions of Bi1-xCaxFeO3samples occur at 878 K from M-T curve and the phase transition of BiFeO3 happens at 878 K by DSC measurement. The change in TN of Bi1-xCaxFeO3 depends mainly on the Fe-O-Fe super-exchange strength and magnetic structure of relative stability.