Effect of mechanical force on domain switching in BiFeO3 ultrathin films

Ferroelectric polarization can be switched by an external applied electric field and may also be reversed by a mechanical force via flexoelectricity from the strain gradient. In this study, we report the mechanical writing of an epitaxial BiFeO3 (BFO) thin film and the combined action of an applied mechanical force and electric field on domain switching, where the mechanical force and electric field are applied using the tip of atomic force microscopy. When the applied force exceeds the threshold value, the upward polarization of the BFO thin film can be reversed by pure mechanical force via flexoelectricity; when an electric field is simultaneously applied, the mechanical force can reduce the coercive electric field because both the piezoelectricity from the homogeneous strain and the flexoelectricity from strain gradient contribute to the internal electric field in the film. The mechanically switched domains exhibit a slightly lower surface potential when compared with that exhibited by the electrically switched domains due to no charge injection in the mechanical method. Furthermore, both the mechanically and electrically switched domains exhibit a tunneling electroresistance in the BFO ferroelectric tunnel junction.