Strain tunability of the downward effective polarization of mechanically written domains in ferroelectric nanofilms

Nano 180 degrees domains written by local mechanical force via the flexoelectric effect have recently attracted great attention since they may enable applications in which memory bits are written mechanically. Downward effective polarization (DEP), i.e., the net downward-pointing polarization in the zone underneath the pressure-surface contact area, is expected to be large for clear storage and readout of the memory bits. Here, the strain tunability of the DEP of mechanically written domains is investigated by using a multi-field coupling model of the flexoelectric effect. The DEP of mechanically written domains in PbTiO3 films grown on five different substrates is examined. It is found that, when the mechanical force is not large enough for the mechanical writing process with the largest tensile misfit strain to reach a steady state, the DEP of the mechanically written domain increases as the misfit strain changes from being compressive to tensile. In contrast, when the mechanical force applied is sufficiently large for the writing process with the largest compressive misfit strain to reach a steady state, the effect of misfit strain on the DEP is opposite. In the latter case, the reduction of the DEP is mainly ascribed to the decrease of the flexoelectric field, which is reflected by the asymmetry of the polarization-electric field hysteresis loops under local mechanical force. Our work suggests that misfit compressive strains and sufficiently large mechanical force are useful for improving the DEP.