The Screening Mechanisms Of The Depolarization Field In Ferroelectric Nanostructures：a Multi-scale Investigation

Over decades, ferroelectric materials have been widely used in many important application fields, e.g., functional devices, smart structures, etc. With the demand of the devices miniaturization, ferroelectric nanostructures have attracted tremendous amount of research interests both academic and industrial. Among all the topics, investigations on the screening mechanism of the depolarization field, which determines the size effect (e.g., the domain morphology at nanoscale) and the critical size phenomenon (under which the ferroelectricity will disappear), are of great significance, since it can not only improve the fundamental understanding in physics and material science, but also instruct the potential application designs. In this thesis, we perform ab initio simulations within the framework of Density Functional Theory (DFT) to investigate the screening mechanism in different ferroelectric nanostructures under different mechanical and electrical boundary conditions. Additionally, phase-field simulations are conducted for larger size models which go far beyond the capacity of standard DFT.(1) The first screening mechanism discussed in this thesis is the "polydomain formation". With the formation of polydomain, the polarization discontinuity can be significantly reduced near the free surface, which consequently decreases the depolarization field, and stabilizes the ferroelectricity in sub-domains. In the simulation of free-standing PbTiO3 ultrathin films, the flux-closure domain patterns are discussed. By comparing with BaTiO3 cases, we explain the vanishing critical thickness in PbTiO3 thin films, where the strain gradient plays an important role. In order to check this mechanism, we design the model of BaTiO3 nanotubes, and confirm the flexoelectricity as predicted.(2) Still in the polydomain configuration, we use phase-field simulation in free-standing ferroelectric thin films of larger sizes. Within the extended range of thin film thickness, Kittel law and generalized scaling law are studied. Phase-field simulation is a continuum method highly depending on the energy formulism and the parameters. However, in previous literatures, the parameters of the gradient energy are not accurately determined but only fall within a range. Combined the two points mentioned above, we consider different gradient energy coefficients in the 180° stripe domains to reveal their effect on the Kittel law. Furthermore, once the domain wall thickness is incorporated, the generalized scaling law is confirmed.(3) The second screening mechanism discussed in this thesis is the "two-dimensional electron gas (2DEG)". Based on the discovery of 2DEG at the interface of LaAlO3/SrTiO3, we propose a novel screening mechanism within the Landau theory, where the ferroelectric polarization is used to replace the "built-in" polarization in the polar materials (like LaAl03). According to the Landau model, when the thickness of the ferroelectric layer is larger than a critical value, a 2DEG can be stably formed at the ferroelectric interface, which largely compensates the depolarization field and thus stabilizes the ferroelectric monodomain structure. Based on this model, we carry out DFT calculations on PbTiC3/SrTiO3 heterostructures, with the results confirming this novel mechanism.(4) The third screening mechanism discussed in this thesis is the "metallic electrode". Previous literatures have continuously demonstrated two different interfacial structures in Fe/BaTiO2/Fe multiferroic tunnel junctions. Based on this results, we perform ab initio simulations of the interface effect on the ferroelectricity, magnetism, and interfacial magnetoelectric coupling. Our results reveal that due to the unstable chemical bond, the ferroelectricity of the whole junction and the interfacial magnetoelectric coupling are enhanced.