Ma L L, Chen W J, Liu Y L, Wang Biao, Zheng Yue
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, 510275 Guangzhou, People's Republic of China. Micro and Nano Physics and Mechanics Research Laboratory, School of Physics, Sun Yat-sen University, 510275 Guangzhou, People's Republic of China. School of Engineering, Sun Yat-sen University, 510275 Guangzhou, People's Republic of China.
J Phys Condens Matter. 2019 Apr 10;31(14):145701. doi: 10.1088/1361-648X/aafdf2. Epub 2019 Jan 11.
The recent observation of mechanical switching of ferroelectric polarization has placed the mechanical manipulation of ferroelectrics on an equal footing with the conventional electrical manipulation. However, discussions on the exact switching mechanisms due to mechanical loads are ongoing for the complexity in experimental situations. In this work, based on continuum mechanical and thermodynamic modeling and simulation, we analyze the mechanisms of tip-force induced switching in ferroelectric thin films. The roles of depolarization, shear strain and flexoelectricity in mechanical switching, both in normal and sliding loading modes, are separated out and the switching characteristics are analyzed. The depolarization field in the film is demonstrated to enable bidirectional switching. The coupling between shear strain and polarization components is shown to be important in the sliding loading mode. A great influence of flexoelectricity-modified polarization boundary condition on the switching process is revealed. The previous speculation that the switching process experiences an intermediate paraelectric phase is proved. The regulation of loading force, misfit strain, temperature and film thickness on the switching are further given for each mechanism. Taking all of the three mechanisms into account, we present the phase diagrams of mechanical switching for films in an initial upward or downward polarization state. The revealed characteristics of various switching mechanisms should provide useful guidelines for their verification in experiments, and the tunability of the switching by various influencing factors is instructive for the design and optimization of ferroelectric devices via mechanical engineering.
最近对铁电极化机械切换的观察使得铁电体的机械操纵与传统的电操纵处于同等地位。然而，由于实验情况的复杂性，关于机械载荷引起的确切切换机制的讨论仍在进行中。在这项工作中，基于连续介质力学和热力学建模与模拟，我们分析了铁电薄膜中尖端力诱导切换的机制。分离出了退极化、剪切应变和挠曲电在正常和滑动加载模式下机械切换中的作用，并分析了切换特性。证明了薄膜中的退极化场能够实现双向切换。结果表明，剪切应变与极化分量之间的耦合在滑动加载模式中很重要。揭示了挠曲电修正的极化边界条件对切换过程的重大影响。证明了之前关于切换过程经历中间顺电相的推测。进一步给出了每种机制下加载力、失配应变、温度和薄膜厚度对切换的调控。综合考虑所有三种机制，我们给出了处于初始向上或向下极化状态的薄膜的机械切换相图。所揭示的各种切换机制的特性应为其实验验证提供有用的指导，并且各种影响因素对切换的可调性对于通过机械工程设计和优化铁电器件具有指导意义。
