Magnetic behaviour of sol-gel driven BiFeO₃ thin films with different grain size distribution

BiFeO₃ (BFO) thin films with uniform thickness of ~200 nm were prepared by the sol-gel assisted spin coating method. Different grain size distributions in the as-grown BFO films were then induced by varying the annealing temperature between 525 and 600 °C. It is found that the grain size distribution become wider as the annealing temperature increases. All the films showed a well-saturated magnetization (M) versus magnetic field (H) hysteresis loops at 300 K. A strong dependence of M on the grain size distribution is observed. An optimal grain size distribution with average grain size ~90 nm is responsible for high M in the BFO films. The non-saturated M-H loops obtained at 10 K suggest the spin glass behaviour of BFO films. The zero field cooled (ZFC) and field cooled (FC) magnetization curves shows split at 300 K and a cusp at ∼50 K in the ZFC curve, which further confirms the spin glass state of polycrystalline BFO thin films.