Modified crystal structure, dielectric properties, and magnetic phase transition temperature of Ca doped BiFeO3 ceramic

Abstract

This paper deals with the preparation of multiferroic Bi1−x Ca x FeO3(x = 0–0.2) ceramics with the sol–gel method and a study on the influence of Ca2+ doping on the structure, dielectric, and ferromagnetic properties of BiFeO3 ceramics. The result shows that the XRD analysis reveals a phase transition in Ca-doped BiFeO3 from rhombohedral to orthorhombic when x is greater than 0.1. The dielectric constant (e r) of Bi0.9Ca0.1FeO3 measured at 1 kHz is about seven times greater than that of BiFeO3, and Bi0.8Ca0.2FeO3 is less than one-tenth of BiFeO3. It might be understood in terms of the dipole polarization, oxygen vacancy and lattice phase transition. Magnetic measurements show that the M-H of Bi1 − x Ca x FeO3 samples exhibit unsaturated and symmetric magnetic hysteresis loops with the increase of Ca2+, indicating the weakly ferromagnetic behavior. It indicates that there is coexistence of Fe2+ and Fe3+ in Bi1 − x Ca x FeO3 samples according to the XPS spectrum. The ratio of Fe2+/Fe3+ increases with doping Ca2+ content and the magnetic properties of BiFeO3 are enhanced. It is evident that the ferromagnetic phase transition of BiFeO3 samples occurs at 878 K by measuring the M–T and DSC curves. The T N of BiFeO3 will be reduced from 644 to 638 K and the T M does not change slightly at 878 K with increasing Ca2+ content. T N and T M of Bi1 − x Ca x FeO3 change depends mainly on the magnetic structure of relative stability and Fe–O–Fe super-exchange strength.