  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).
