Reversal of Ferroelectric Polarization by Mechanical Means

Ferroelectric materials are characterized by the presence of an electric dipole that can be reversed by application of an external electric field, a feature that is exploited in ferroelectric memories. All ferroelectrics are piezoelectric, and therefore exhibit a strong intrinsic coupling between polarization and elastic deformation - a feature widely used in piezoelectric transducers and high-displacement actuators. A less explored and exploited property is flexoelectricity, i.e. the coupling between polarization and a strain gradient. Though flexoelectricity is an old concept (it was discovered in the Soviet Union almost 50 years ago), it is only with the advent of nanotechnology that its full potential is beginning to be realized, as gradients at the nanoscale can be much larger than at the macroscopic scale. Here, we demonstrate that the stress gradient generated by the tip of an atomic force microscope can be used to mechanically switch the polarization in the nanoscale volume of a ferroelectric film. This observation shows that pure mechanical force can be used as a dynamic tool for polarization control, enabling information processing in a new type of multiferroic high-density data storage devices where the memory bits are written mechanically and read electrically.