  Bi1-xLaxFe1-xMnxO3 (x = 0.000 - 0.300) ceramics prepared by the tartaric acid
modified sol-gel technique have been studied to analyze the effect of
composition driven structural transition on the magnetic properties of bismuth
ferrite (BiFeO3). It was found that the co-substitution of La & Mn at Bi & Fe
sites leads to suppression of impurity phases (Bi2Fe4O9 and Bi25FeO40) which
generally appear in BiFeO3. 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 (space group R3c) to the orthorhombic (space
group Pbnm) with the increase in substitution concentration. The 5%
co-substituted sample exhibit high remnant magnetization i.e. about 15 times
that of BiFeO3 which is due to the suppression of cycloid spin structure and
enhanced canting angle of antiferromagnetically ordered spins caused by the
crystal lattice distortion. However, further increase in substitution results
in the reduction of remnant magnetization and coercivity 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). The frequency
independent higher dielectric constant and lower dielectric loss were observed
for 5% co-substitution. Hence, this particular composition may be interesting
for device applications.
