Reversible Control over the Polar-to-Antipolar Phase Transition in Multiferroic Thin Films

Inversion-symmetry breaking and the emergence of a polar state is a ubiquitous concept in condensed matter science. It is a prerequisite for technologically relevant effects such as ferroelectricity, nonlinear optical properties, and spin-transport phenomena. Hence, the ability to reversibly control the onset of such symmetry breaking may be instrumental in the establishment of energy-efficient devices and emergent computing schemes. For instance, the continuous control of a ferroelectric to antipolar state may enable application-relevant, beyond-binary responses.
In this work, we present a novel approach for reversible control over the polar-to-antipolar phase transition in epitaxial multiferroic La-substituted BiFeO₃ (LBFO) thin films using local pressure and electric field. By means of a local stress application via scanning probe microscopy (SPM) tip, we stabilize an anti-polar phase. An electric field restores the original ferroelectric phase. Leveraging these insights, we tune the polar/anti-polar phase coexistence to set the net polarization of LBFO to any desired value between its saturation limits. Finally, using optical second harmonic generation as a non-invasive probe, we control the net polarization of our films in device-compliant capacitor heterostructures.