[64] Imprint control of nonvolatile shape memory with asymmetric ferroelectric multilayers

Ferroelectric materials exhibit multifunctional properties such as piezoelectricity, nonvolatile charge, and electro-optic functions. Accordingly, ferroelectric materials have been an interesting subject of intensive scientific investigation, especially for ferroelectric nonvolatile random access memories (FeRAM) and actuators. Basically, ferroelectric memories utilize the remanent polarization originating from the employed ferroelectric materials themselves, enabling suitable operation by charge detection. Once electric polarization is changed by an external electric field, piezoelectric strain is generated under the biased states. Even though the applied electric field is switched off, two different polarized states still remain to be observed and thereby can be utilized for nonvolatile digital information. In contrast, piezoelectric strains cannot possess these nonvolatile characteristics with the two observable different states because the generated piezoelectric strains are returned to the same positions without an applied electric field. In other words, a symmetric piezoelectric butterfly curve is expected with regard to the piezoelectric strain axis, indicating that no shape memory window is available.