Self-Poling of BiFeO₃ Thick Films

Bismuth ferrite (BiFeO₃) is difficult to pole because of the combination of its high coercive field and high electrical conductivity. This problem is particularly pronounced in thick films. The poling, however, must be performed to achieve a large macroscopic piezoelectric response. This study presents evidence of a prominent and reproducible self-poling effect in few-tens-of-micrometer-thick BiFeO₃ films. Direct and converse piezoelectric measurements confirmed that the as-sintered BiFeO₃ thick films yield <i>d</i>₃₃ values of up to ∼20 pC/N. It was observed that a significant self-poling effect only appears in cases when the films are heated and cooled through the ferroelectric-paraelectric phase transition (Curie temperature <i>T</i>_C ∼ 820 °C). These self-poled films exhibit a microstructure with randomly oriented columnar grains. The presence of a compressive strain gradient across the film thickness cooled from above the <i>T</i>_C was experimentally confirmed and is suggested to be responsible for the self-poling effect. Finally, the macroscopic <i>d</i>₃₃ response of the self-poled BiFeO₃ film was characterized as a function of the driving-field frequency and amplitude