SOL-GEL Preparation And Characterization Of Multiferroic BiFeO₃ Ceramics

Multiferroic materials, which are a class of materials that yield simultaneous effects of ferroelectricity, ferromagnetism, or ferroelasticity in the same material, offer a wide opportunity for potential applications in information storage, such as spintronic devices and sensors, especially in new four-state memory device. They have exhibited interesting physical properties. Among single-phase multiferric materials, BiFeO₃ has attracted much attention recently due to its relatively high Curie (TC ⁸30℃), Néél temperature (TN³70℃) and high polarization value (¹00μC·cm^-2), which is advantageous for research and various applications. BiFeO₃ is well known to be an antiferromagnetic and ferroelectric material, which exhibits most possibility of practical applications at room temperature. Although structure and properties of BiFeO₃ have been studied extensively since first discovery in 1960s, several obstacles are still need to be overcomed for practical applications. Low synthesis temperature of BiFeO₃ material necessary for the current integrated circuit. Beacause BiFeO₃ is matastable at high temperature, it is challenge to obtain single phase BiFeO₃ and to avoid the formation of the secondary ternary oxides Bi₂₅FeO₃₉ and Bi₂Fe₄O₉. Two of the main obstacles for BiFeO₃ before its application are large leakage current and a superimposed incommensurate cycloid spin structure. This structure cancels the macroscopic magnetization and inhibits observation of the linear ME effect. In addition, it is especially important to investigate properties of multiferroic materials in GHz frequency region in order to meet higher operating speed of the future memory device.This dissertation aims at overcoming obstacles mentioned above. Main contents and innovations in the dissertation are listed in the following:The synthesis temperature of BiFeO₃ prepared by a simple sol-gel method is as low as 450℃, further lowing to 400℃by adding acetylacetone as a stabilizer. The chemical reaction and crystallization process were investigated in detail, showing that the formation of BiFeO₃ follows the chemical equation: Bi₂O₂CO₃+Fe₂O₃ 2BiFeO₃+CO₂ . BiFe_1-xTa_xO₃ nanopowders were synthesized by a simply sol-gel method. A stable sol was obtained by controlling hydrolysis and polymeric reaction.BiFeO₃ nanoparticles show strong size dependent magnetic properties, which increase with reduce in the particle size. This ferromagnetic property, which is different from the linear M-H relationship in the bulk, is attributed to the increased suppression of the kown spiral spin structure due to uncompensated spins and strain anisotropies at the surface. Ta substitution increased the magnetic property by 1 orders of magnitude and dramanticly decreased the coercive field and exchange bias. The enhancement of magnetism was possibly associated with the distortion of the oxygen octahedral by the Ta substitution or/and the statistical distribution of Fe3+ and Fe₂+. Dielectric anomaly at round 330℃near the magnetic transition point corresponds to the anti-ferromagnetic to paramagnetic phase transition, indicating the coupling between polarization and magnetization in BiFeO₃ particles. Microwave dielectric investigation present that BiFeO₃ exhibit a relaxation-like response with a characteristic frequency of 15GHz, which could be associated with an overdamped process. Ta-substituted BiFeO₃ samples, however, show resonant behaviour with resonant frequencies of 12.5 and 14.6GHz. The intensity of the resonant peak near 14.6GHz decreased with increasing Ta addition. This behavior was associated with a damped resonance process. BiFeO₃ shows a reversible ferroelectric phase transition, with a Curie temperature at 827℃.(100)-preferential orientation BiFeO₃ thin film and SrBi₂Ta₂O₉/BiFeO₃/SrBi₂Ta₂O₉ sandwich structural film were deposited on Pt/Ti/SiO₂/Si substrate by sol-gel method. The preferred orientation was found to be controlled by three factors: Thermal-treatment of Pt substrate at 600℃for 10min; heating rapidly for thin film crystallization; lowering sol concentration; Controlling film thickness within a certain range. SrBi₂Ta₂O₉/BiFeO₃/SrBi₂Ta₂O₉ sandwich structural film was well crystallized and no impure phases were detected. SrBi₂Ta₂O₉ barrier layer didn't change the (100)-preferentail orientation of BiFeO₃ thin film. The film was uniform, smooth, dense and crack-free, which was favorable for a good performance.The ceramic may be formed by sintering the calcined (ethylene alcohol-based and citrate acid-based) sol-gel green bulk. The green bulk was prepared by a cold isostatic pressing. The high temperature thermaldynamic stability and attempts to obtain single-phase BiFeO₃ ceramic were investigatied. The grain growth and defects in the ceramic were observed. In the citrate acid-based sol-gel route, because of the dimeric nature of the complex, the possiblity of formation of Bi-Fe heteronuclear arrangement has not become predominant. The inhomogeneous distribution of Bi and Fe elements leads to the formation of impurity phases in BiFeO₃ ceramic, such as Bi₂₅FeO₄₀ and Bi₂Fe₄O₉. TEM observation reveals that Bi₂₅FeO₄₀ grain in a spheroidal shape and almost cubic Bi₂Fe₄O₉ grain (nanometer scale) were located in the grain boundary among BiFeO₃ grains (micrometer scale). A possible domain structure and defects were also observed.