B-site cation mixed multiferroic perovskite materials

xiv, 120 leaves : ill. (some col.) ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577M AP 2010 SuenIn recent years, investigation in multiferroic materials has attracted considerable interest as these materials can simultaneously exhibit ferroelectric and magnetic properties. It is expected that the interaction between electric and magnetic orders will give rise to novel properties and hence attractive for practical applications. BiFeO₃ (BFO) is a promising multiferroic material because it possesses ferroelectric and antiferromagnetic behaviors at room temperature. However, the coupling between the ferroelectric and magnetic behaviors is very small and results in limited applications. Therefore, doping is sought after to improve the coupling. It has been reported that the solid solution between BFO and another multiferroic material BiMnO₃ (BMO) can have better electric and magnetic coupling. However, the underlying mechanism is not fully understood. Particularly, the relationship between the microstructure and the physical properties, which is important for complex perovskite oxides, is unclear. Therefore we propose to give a comprehensive study on the property-microstructure relationship of BFO-BMO solid solution. In this project, the solid solution BiFe₁₋ₓMnₓO₃(BFMO) has been prepared by several methods including the rapid liquid-phase sintering technique. The effects of Mn doping level on both the microstructures and properties have been examined. These include the doping induced variations in lattice parameters, microstructure, dielectric properties, ferroelectric properties, magnetic properties, and phase transition temperature, etc. From the results obtained, it is found that single phase ceramics could only be obtained in BFMO compound when x≤0.4. A structural phase transition took place from rhombohedral to tetragonal when the Mn doping level was increased to x=0.4. This result was consistence with the Raman analysis. The dielectric properties of BFMO were investigated in the frequency range of 10 Hz to 10 MHz over a broad temperature range. When Mn was doped into the solid solution, the dielectric constant was drastically increased and a relaxation peak corresponding to a step-decrease in dielectric constant was also observed. The relaxation peak was shifted to a higher frequency with increasing Mn concentration. The activation energy for the dielectric relaxation was found to decrease from 0.39 (x=0.1) to 0.29 eV (x=0.4), indicating a Mn-related polaron hopping motion Fe³⁺ and Mn³⁺ ions. The impedance of grain and grain boundary was resolved by an equivalent circuit model. The activation energies for grain and grain boundary conductions were similar and could be explained by a constriction model where the grain and grain boundary conductions have the same mechanism. Magnetic hysteresis loop measurements showed that the magnetization of the solid solution increased with increasing amount of Mn substitution. This result was consistent with the XPS analysis. Moreover, the microstructures of the ceramic samples were studied by electron diffraction. Superlattice structures were observed in Mn doped samples, where computer simulation was used to find out possible chemical ordering in the structure. Different synthesized techniques were used to synthesize BFO and comparisons were made with respect to their physical properties. Single phase BFO could only be synthesized by the rapid liquid-phase sintering and wet chemical reaction methods. BFO synthesized by the rapid liquid-phase sintering technique exhibited the least leakage current and therefore best ferroelectric properties. The wet chemical reaction method was effective to obtain stoichiometric and stable single phase BFO, which is a reliable method worthy of further investigation.Department of Applied PhysicsM.Phil., Dept. of Applied Physics, The Hong Kong Polytechnic University, 201