Structure And Electromagnetic Properties Of Multiferroic Bi_1-xLa_x (Mn_1-yFe_y)O₃ Samples

One particular current interest is multiferroic materials in which two or three ferroic properties, namely, ferroelectricity(FE), ferromagnetism(FM), and ferroelasticity coexist in the same phase. Particularly, the combination of ferromagnetism and ferroelectricity with coupling between the spontaneous polarization and the magnetization through the magnetoelectric(ME) effect, because they can support novel functionalities in electronic devices. Multiferroic BiMnO3 has been widely renewed interest because BiMnO3 exhibits ferroelectricity and ferromagnetism ordering simultaneously. However, because of the high volatility of Bi for numerous Bi-based compounds, it is difficulty in preparing single perovskite BiMnO3. On the other hand, BiMnO3 also can't support the applications because it's low Curie temperature(Tc) for the ferromagnetic property and high leakage current at roomtemperature(RT).In this thesis, Multiferroic Bi1-xLaxMnO3 and Bi0.8La0.2Mn1-xFexO3 ceramics samples were synthesized by the sol-gel method. The sintering temperature influence on the perovskite phase of Bi1-xLaxMnO3 samples was carefully studied, it is shown that there was a narrow range of sintering temperature(900～950℃) for the stable perovskite BLMO sample. On this basis, it was carefully investigated that the La doping effect on the structural and magnetic properties of the Bi1-xLaxMnO3 samples sintered at 950℃. And then, the Fe doping effect on the structural and electromagnetic properties of the Bi0.8La0.2Mn1-xFexO3 samples sintered at 950℃was also carefully studied.The whole thesis consists of four chapters, the main contents in each chapter are presented as follows:Chapter 1:The general review of the history and present research situation of the multiferroic materials. We make a brief introduction to the reasons for lack of single-multiferrioc materials and several mechanisms for the coexistence of the ferroelectricity and ferromagnetism. And then, we particularly introduced the structure and electromagnetic properties of BiMnO3. On this basis, we introduced several problems in research of BiMnO3, and then we brought out several methods to improve the electromagnetic properties of BiMnO3 by the relevant literature.Chapter 2:The Bi1-xLaxMnO3 and Bi0.8La0.2Mn1-xFexO3 ceramics preparation methods and the samples measurement methods are introduced, especially the details and instruments of each experimental are introduced.Chapter 3:The sintering temperature influence on the perovskite phase of Bi1-xLaxMnO3 samples was carefully studied by using X-ray diffraction (XRD), scanning electronic microscope (SEM), fourier transformation infrared spectroscopy(FT-IR), and the thermo gravimetric analysis-differential thermal analyses (TG-DTA). It was also carefully investigated that the La doping effect on the structural and magnetic properties of the BLMO samples sintered at 950℃. Our results indicated that the perovskite phase of samples was weakened as La doping reducing, the Curie temperature Tc decrease from 53K to 41K, corresponding to the saturation magnetization experienced a lowering and then an enhancing. It was ascribed to the increase of Bi vacancies in La doping reducing samples, which induced a decrease of Mn-O-Mn bond angle and an increase of Mn4+ ions. This resulted in an enhancing of the ferromagnetic superexchange between Mn3+ and Mn4+ ions and a disrupting of the dz2 orbital ordering around the Mn3+ matrix.Chapter 4:The structural, magnetization, and electrical properties of Bio.8Lao.2Mn1-xFexO3 samples were carefully investigated. It was found samples with x=0,0.1 shown a distorted orthorhombic crystal structure, and a distorted tetragonal crystal structure for x≥0.3, and we also found a clear correlation between the magnetization and the crystal structure. A magnetic transition from a ferromagnetic (FM) state (x=0,0.1) to a antiferromagneic (AFM) state (x≥0.3) with increasing Fe content, and the samples had a competition between FM and AFM at room temperature for x≥0.3 were observed by M-T and M-H curves. Compared the polarization hysteresis loops of the sample for x=0.9 and x=0.7, it is seen with decreasing x, the samples have a higher leakage current, which leading to the weakened ferroelectric(FE). It was ascribed to that excessive Mn-doped in Bi0.8La0.2MnxFe1-xO3 system can increase the ratio of Fe2+/Fe3+, which weaken the ferroelectric.