Nonlinearity in the high-electric-field piezoelectricity of epitaxial BiFeO₃ on SrTiO₃

The piezoelectricity of a multiferroic BiFeO₃ thin film deviates from its low-field linear response in electric fields higher than 150 MV/m. Time-resolved synchrotron x-ray microdiffraction reveals a low-field piezoelectric coefficient of 55 pm/V and a steeper increase in strain at higher fields, with an effective piezoelectric coefficient of 86 pm/V. The strain reaches 2% at 281 MV/m, a factor of 1.3 higher than expected based on an extrapolation from low fields. The peak intensity of the BiFeO₃ (002) Bragg reflection decreases throughout the high-electric-field regime, accompanied by increased diffuse scattering, consistent with lattice softening lattice near a field-induced phase transition.