Magnetic and dielectric properties of nanostructured BiFeO ₃ prepared by sol-gel method

Polycrystalline BiFeO ₃ was synthesized at 400°C-700°C. Distinctive difference in the magnetic and dielectric properties was observed between the samples sintered at 400°C-500°C and those sintered at 600°C-700°C. The former showed ferromagnetic-like hysteresis loops with an increased magnetization of 0.54 emu/g, whereas the later showed linear loops with a small magnetization of 0.065 emu/g. Although X-ray did not identify any secondary phase, the suspected trace of some magnetic phase (Fe ₃ O ₄ ) in the samples was conceded by the occurrence of an exchange bias. The difference in dielectric response between the two groups of samples arose mainly from a different conductivity at the grain boundaries. Owing to Fe ₃ O ₄ coating at grain surface, the 400°C-500°C sintered samples behaved like a single parallel R-C circuit, whereas the dielectric response of the samples sintered at 600°C-700°C was represented by a series of two parallel R-C units for grains and grain boundaries, respectively. Two dielectric relaxation peaks observed at <700 Hz and 0.3~6 MHz in the high-temperature sintered samples were attributed to the Maxwell-Wagner relaxation and electron hopping, respectively.