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

Bismuth iron oxide (BiFeO₃) 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 BiFeO₃ is extremely difficult due to the volatile nature of Bi₂O₃ that results in formation of bismuth deficient and/or rich phases. Most of the previous research is based on obtaining pure BiFeO₃ 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 BiFeO₃ 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 BiFeO₃ 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 BiFeO₃. 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 BiFeO₃ (62 nm).