Effect of Bi/Fe Ratio on the Structural and Magnetic Properties of BiFeO3 Thin Films by Sol-Gel

Bismuth iron oxide (BiFeO
₃
) is amongst the class of multiferroic materials that has attracted world's attraction because of its high antiferromagnetic Neel temperature and high ferroelectric Curie temperature of 643 and 1103 K, respectively. However, synthesis of phase pure BiFeO
₃
is extremely difficult due to the volatile nature of Bi
₂
O
₃
that results in formation of bismuth deficient and/or rich phases. Most of the previous research is based on obtaining pure BiFeO
₃
phase with variation in annealing/calcination temperature in the range 400 °C-700 °C. However, very little consideration is given to change Bi/Fe ratio during synthesis. Here, we report preparation of phase pure BiFeO
₃
thin films using cost effective sol-gel and spin coating method. Bismuth nitrate and iron nitrate are used as precursors whereas ethylene glycol is used as solvent. Molar ratio of Bi/Fe is varied as 0.9, 0.95, 1.0, 1.05, 1.1, and 1.20. Films are annealed at 300 °C for 60 mins in the presence of vacuum under 500 Oe applied magnetic field. X-ray diffraction results indicate formation of phase pure BiFeO
₃
thin films for Bi/Fe ratio of 1.0-1.1. However, both high (1.2) and low (0.9-0.95) Bi/Fe molar ratios result in the appearance of impurity phases. In addition, bismuth iron oxide shows ferromagnetic behavior as opposed to the antiferromagnetic nature of bulk BiFeO
₃
. Strong ferromagnetic behavior, for Bi/Fe ratios of 1.0 and 1.05, can be explained on the basis of suppression of helical spin structure as the crystallite size reduces to ~30 nm, which is well below the cycloidal spin arrangement of BiFeO
₃
(62 nm).