Effect of Structural Transition on Magnetic Properties of Ca and Mn co-substituted BiFeO3 Ceramics

Composition-driven structural transitions in Bi1-xCaxFe1-xMnxO3 ceramics prepared by the tartaric acid modified sol-gel technique have been studied to analyze its effect on the magnetic properties of bismuth ferrite (BiFeO3). It was observed that the co-substitution of Ca & Mn at Bi & Fe sites in BiFeO3 (BFO) significantly suppress the impurity phases. The quantitative crystallographic phase analysis has been carried out by double phase Rietveld analysis of all the XRD patterns which indicates the existence of compositional driven crystal structure transformation from rhombohederal (R3c space group, lower crystal symmetry) to the orthorhombic (Pbnm space group, higher crystal symmetry) with the increase in substitution concentration due to excess chemical pressure (lattice strain). Magnetic measurements reveal that co-substituted BFO nanoparticles for x = 0.15 have enhanced remnant magnetization about 14 times that of pure one due to the suppression of cycloid spin structure which could be explained in terms of field induced spin reorientation and weak ferromagnetism. However, at the morphological phase boundary (x = 0.15), the remnant and maximum magnetization at 8 T reaches a maximum which indicates almost broken spin cycloid structure and further increase in substitution results in the reduction of both magnetizations due to the appearance of complete antiferromagnetic ordering in the orthorhombic structure because of the significant contribution from the crystallographic phase of Pbnm space group (as obtained from double phase Rietveld analysis).