Room-temperature ferrimagnetic multiferroic BiFe0.5Co0.5O3 thin films with giant piezoelectric response

Single-phase multiferroic materials with large polarization and magnetization at room temperature are highly desirable but still rather rare. In this study, tetragonal-like $\mathrm{BiF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{o}}_{0.5}{\mathrm{O}}_{3}$ films have been synthesized and systematically characterized. For its normal state, both the highly polar structure and ferrimagnetic net magnetization are evidenced in experimental measurements at room temperature and further confirmed using first-principle calculations. Furthermore, an abnormal state driven by voltage with lattice compression along the $c$ axis (shorter for $\ensuremath{\sim}20%)$ is observed, rendering very rare giant piezoelectric response in multiferroics. The magnetoelectric coupling has been confirmed by the increasing saturated magnetization from 52.47 to $163.60\phantom{\rule{0.16em}{0ex}}\mathrm{emu}/\mathrm{c}{\mathrm{m}}^{3}$ after the application of electric field, which is due to the spin-state transition of $\mathrm{C}{\mathrm{o}}^{3+}$ ions.