2, Sun Yat-sen University, Zhuhai, , China
Ferroelectrics, as phase change materials, have long been studied for non-volatile phase change memories due to spontaneous and electrically switchable polarization. However, for electrical switching in ferroelectrics, an intractable problem is fatigue on account of the happening of electric-field-driven processes such as defect generation and movement. Electrical switching is also known to cause complex memristive dynamics, leakage and even dielectric break-down problem. For these reasons, it is desirable to seek out non-electrically switching of ferroelectric domains to meet the needs of high-performance storage. In fact, novel information storage concept has been put forward to store data in the mechanical state of a ferroelectric thin film. Recently, mechanical switching of ferroelectric polarization has been demonstrated in experiments. The strain gradient, generated by the tip of an atomic force microscope, affects the polarization bi-stability through the so-called flexoelectric effect. Nevertheless, it is still not clear if this mechanical switching behavior realized in experiments could be solely attributed to flexoelectricity. In fact, epitaxial strain has influences on the polarization bistability of ferroelectric tunnel junctions (FTJs). For asymmetric FTJs, the surface asymmetric effect, caused by the different bonding environment at the two surface of the ferroelectric nanofilm, can lead to broken degeneracy of the two polarization states by declining the double-well free energy, making one polarization state metastable. In this paper, we performed ab initio calculations to demonstrate the effects of epitaxial strain on polarization bi-stability and pinpoint its role in the mechanical switching. The results reveal that the metastable polarization state in ferroelectric thin film maintains stability at a relative compressive epitaxial strain; however, it would become unstable at a relative tensile strain due to an increase in the critical thickness of polarization bi-stability. To verify such strain-dependent polarization bistability and include the surface asymmetric effect, we constructed a phase-field model for Pt/(TiO2-BaO)m/SrRuO3, with the surface parameters fitted from the ab initio calculations. The fitting results are in good coincidence with that of the ab initio calculations. Phase field simulation result shows that local switching of the metastable polarization state can occur if the strain state of the FTJ can be controlled. In practice, a relatively tensile strain might be exerted to the FTJs by a local tip force. Depending on the FTJ layer stacking sequence, the epitaxial strain effect on polarization bi-stability can either promote or offset the flexoelectric switching in asymmetric FTJ systems. These results provide us a deeper understanding of mechanical control of phase change memories.