Polarization switching and fatigue anisotropy in relaxor-lead titanate ferroelectric single crystals

by 1973- Ozgul, Metin

The hysteretic behavior of ferroelectrics relating polarization and electric field, is utilized for nonvolatile memory applications. Nonvolatile memories are attractive due to the ability to hold information without requiring an external field; i.e., a back-up battery. One of the critical features of nonvolatile memory devices is typically destructive reading, requiring the ferroelectric to endure large number of electrical cycles during operation. However, decrease of switching charge due to repeated polarization reversal, known as fatigue, is a common problem in bulk and thin film ferroelectrics. Fundamental studies of ferroelectric materials are essential in understanding the origin of fatigue mechanisms. There have been a number of advances to overcome fatigue in ferroelectric films, such as the utilization of oxide electrodes, or using layered type ferroelectrics that exhibit fatigue-free behavior. There still exist certain issues which need to be solved when employing these strategies. In this study, fatigue anisotropy was discovered in rhombohedral Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) single crystals. When the electric field is applied along [001]C or [110]C orientations, single crystals do not fatigue, while normal fatigue occurs along [111]C orientations. Further studies focused on the study of the origin of fatigue anisotropy in PZN-PT and other ferroelectric systems. Experiments consistently showed that if a ferroelectric to ferroelectric phase transition occurs (i.e, rhombohedral --- > tetragonal) through composition, temperature, and field strength, fatigue reappears in otherwise “fatigue-free” orientations. These results indicated that the fatigue rates depend on both the ferroelectric phase and crystallographic orientation. Fatigue iv anisotropy was investigated also in several other ferroelectric systems including both relaxor based and normal ferroelectrics. Normal ferroelectric BaTiO3 and its solid solutions with BaZrO3 did not exhibit fatigue anisotropy in the rhombohedral phase in [001]C orientations. From these studies it seems a combination of engineered domain states (orientation) and relaxor nature is required for fatigue free orientations. Given the relaxor ferroelectric nature of PZN-PT single crystals, the field and frequency dependence of switching and relaxation of sub-coercive field dc field excited polarization were studied as a function of fatigue history. A power law fit gives less field and frequency dependence for [001]C. The behavior remains constant throughout cycling. However, strong field and frequency dependence was noted in [111]C as a function of fatigue. Polarization relaxation data was analyzed by a stretched exponential function. Fitting parameters indicate a broader time constant distribution for relaxation along [001]C, meaning more diverse contribution to the switching process. These parameters also remained constant with cycling along [001]C. On the other hand, a narrower time constant distribution with a higher stretched exponent was determined along [111]C direction. With fatigue evolution, changes occur along [111]C. These observations are consistent with the progressive loss of the slower elements from the switched polarization signal, demonstrated in conventional P-E loops. Finally, optical microscopy was performed in PZN-4.5PT single crystals along [001]C and [111]C orientations as a function of fatigue (cycling) history. With fatigue evolution, domains became more fractal (discontinuous) along [111]C, whereas the presence of finer domains with a wide size range was noted along [001]C. If these observations can be confirmed by a high magnification microscopy, the microscopic v domain pictures are consistent with the macroscopic electrical characterization findings. This would suggest that the presence of domain structure at a very fine scale in [001]C rhombohedral crystals may make it extremely difficult to effectively pin the polarization out on cycling. The result is considerable fatigue resistance. vi