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Atomistic simulations of polarization switching in ferroelectric materials
Nobarani, Hamed
Nobarani, Hamed
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2021
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Domain polarization switching in ferroelectric materials plays an essential role in determining their properties and performance for applications in capacitors, transducers, sensors, actuators, and memory devices. Understanding the intrinsic behavior of ferroelectrics during domain polarization switching caused by an electric field is challenging due to the very fast switching process of domains and the nano/micro scale size of domains. In this Ph.D. research, an atomistic-scale computational framework is developed to study the domain switching process of bismuth ferrite (BiFeO3) samples in nanoscale. An in-house second nearest-neighbor modified embedded atom method plus charge equilibration interatomic potential for BiFeO3 is used for the atomistic simulations. We performed a series of molecular dynamics simulations to study the effects of magnitude and duration of an applied electric field, domain size and specimen thickness on the domain polarization switching.
The switching map with respect to the magnitude and duration of the electric field was obtained for BiFeO3 sample possessing 180° domain walls, showing that 180° switching occurred when a relatively small electric field was applied for a long enough time; on the other hand, a relatively large electric field was needed when the duration of the applied electric field was short. Also, we studied the sample size effect, and we did not observe a significant change in the switching process when the surface size increased, however, by increasing the sample thickness, the time to reach the complete 180° switching increased. In addition, we studied the effects of defects such as vacancies and dislocations, as well as applied tensile strain on the switching process. Oxygen vacancies and Schottky defects led to a localized reduction in the polarization. Overall, dislocations deteriorated the expected increase in the polarization magnitude with the electric field. Unlike the monodomain with almost the same polarization, the polarization calculated for a structure with dislocations was nonhomogeneous. Large localized polarization was observed when the stress field between the dislocation cores exceeded the MD-calculated threshold of 4 GPa. Moreover, the hysteresis loop for BiFeO3 sample was obtained, containing essential information about the ferroelectric material, such as remanent polarization and saturation points. This work demonstrated the capabilities of atomistic simulations for better understanding the effects of different factors on the switching behaviors of ferroelectric materials, which can contribute to the design of ferroelectrics with enhanced or controllable switching performance.
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