Landau Mean-field Theory:Application In The Nanoscale Ferroelectric Thin Films And Superlattices

This thesis is mainly based on the Landau mean-field theory and using this theory to inter-pret the size effect, strain effect, space charge effect, electrode effect and electrocaloric effect in nanoscale ferroelectric thin films and superlattices.We firstly make a brief introduction about Landau theory in Chapter Two, including second order transition, first transition and the validity of Landau theory.In Chapter Three, we study the structural characteristics of phase transition in single-domain epitaxial BiFeO3films by Landau-Devonshire theory. It is predicted that remanent polarization shows strong strain dependence for different temperatures while spontaneous po-larization is almost independent of strain over a wide temperature (0℃-500℃). We also obtain thickness dependence of the c-axis lattice parameter and Curie temperature, and make compar-ison between the polarization rotation angle and the angle attributed to the structural evolution in epitaxial (001)p BiFeO3films grown on SrTiO3substrate. The theoretical results are in agreement with the recent experimental and theoretical data. Our calculations show that the clamping effect should also be taken into account to depict the mechanism of the polarization rotation completely.In Chapter Four, we analyze and introduce systematically that the current version of ther-modynamic models of space charge effect in ferroelectric multilayers and superlattices are self-contradictory. On one hand, the author examines the validity of nonlinear thermodynamic mod-els which incorporate the contribution of electrostatic coupling and interfacial space charge in ferroelectric-paraelectric bilayers. Inconsistency of the directions between build-in polarization and field is found. Evolution of ferroelectricity does not follow the variation in coupling strength between different layers. In addition, we observe that the field-induced polarization in para-electric layer deviates from the polarization derived by thermodynamic theory. These findings indicate that the current version of thermodynamic models needs to be modified. On the other hand, based on a thermodynamic model, we study the interfacial free charge effect on the polar-ization, spatial inversion symmetry, and dielectric response in ferroelectric-paraelectric bilayer. The broken spatial inversion symmetry and imprint due to free charge are not found. Effect of free charge on the dielectric constant is not monotonic. Contradiction between the equilibrium polarization and non-linear field-induced dielectric polarization in paraelectric layer is found. Our study reveals that treating coupling strength as an "input" parameter taken dependent of the polarization is inappropriate to describe electrostatic coupling with interfacial free charge.In Chapter Five, in order to give better description of electrode effect and space charge effect on ferroelectricity in ferroelectric multilayers and superlattices, we develop a multiscale thermodynamic model which combines phenomenological theory and first-principles calcula-tions to solve the problems in Chapter Four. In nanoscale ferroelectric thin films, the interface between film and electrode determines the film’s properties. However, current research in fer-roelectric multilayers and superlattices has partially or completely ignored the electrode effects, which is definitely inappropriate. We use this multiscale thermodynamic model to study the electrode effects on the ferroelectricity in epitaxial BaTiO3/SrTiO3superlattices. It is predicted that the long-range electrostatic screening by the incomplete charge compensation in the real electrodes and the surface field originated from the short-range coupling of the atoms at super-lattice/electrode interface can significantly enhance the critical thickness where ferroelectric-paraelectric phase transition occurs. The second order short-range contribution behaves like misfit strain effect. Our results indicate that superlattice/elelctrode interface plays a key role in the design of devices of ferroelectric superlattices. The authors develop a multiscale thermo-dynamic model to study the effect of interfacial space charge on the ferroelectricity in epitaxial ultrathin BaTiO3/SrTiO3superlattices, incorporating the contribution of electrode-superlattice coupling. It is found that the ferroelectricity of superlattices behaves as a result of the elec-trostatic interactions between space charge, polarization mismatch effect, and finite screening by the real electrodes. We find that localized space charge is independent of the recovery of ferroelectricity observed by experiments. The critical thickness and total average polarization of superlattices in the presence of space charges are enhanced by the short-range electrode- superlattice coupling.In Chapter Six, electrocaloric effect in ultrathin SrRuO3/BaTiO3/SrRuO3capacitors is cal-culated by a multiscale thermodynamic model. It is found that the electrocaloric coefficient vs working temperature in the ferroelectric phase is shifted to higher temperatures with increasing the BaTiO3layer thickness. The electrocaloric effect in the ferroelectric phase is much stronger than that in the size-driven paraelectric phase. Furthermore, it is demonstrated that a giant elec-trocaloric effect (3.5K under0.24V) in such capacitors (six BaTiO3unit cells) at300K can be achieved, which suggests that ultrathin ferroelectric capacitors may be promising candidates for room temperature solid-state refrigeration.In Chapter Seven, we summarize the main results of the thesis and make a brief introduc-tion to the latest research progress and outlook in ferroelectric materials especially ferroelectric thin films, ferroelectric superlattices and multiferroics.