| Multiferroic materials, simultaneously possessing the properties of ferroelectricity, ferromagnetism and even ferroelasticity, have aroused much attention in recent years. As a prospective candidate for potential applications in many aspects, multiferroic materials have been one of the most focused research fields at present.BiFeO3(BFO) is a single-phase multiferroic material with a high Curie temperature(Tc～1100K) and a high Neel temperature (TN～643K). It is one of the fewmultiferroic materials exhibiting both ferroelectric and ferromagnetic properties at room temperature. Theoretical calculations indicated that large polarization could be achieved in BFO. Moreover, interaction between the ferroelectric and ferromagnetic orders will lead to the effect of magnetoelectric (ME) coupling. The research on BFO material has not only the theoretical importance for understanding the physical mechanism of the coexistence of the ferroelectric and ferromagnetic properties, but also the importance for potential applications in novel devices of information storages,spintronics, ME sensor etc.However, much work has to be done to understand the physical mechanisms of the coexistence of the multiferroic properties of the BFO, and to solve the key problems emerged in the preparation of pure BFO ceramic and thin film with satisfied high quality for various novel device applications. Therefore, this dissertation focuses on the study of the preparations and properties of BFO based ceramics and thin films in the following aspects and some original novel results had been obtained as follows:1. Ti substitution for Fe has been proven effective to reduce the leakage current in BFO. Undoped BFO, Ti doped BiFe0.9Ti0.05O3and BiFe0.9Ti0.1O3ceramics were prepared by a rapid liquid phase sintering technique. The introduction of Ti not only raises the sintering temperature but also suppresses the appearance of liquid phases. The ceramic sintering shrinkage and density of the samples are also improved. XRD analysis results show that Ti-doping induces some subtle changes in BFO lattice.2. The nominal nonstoichiometric Ti doped ceramics BiFe0.9Ti0.05O3were prepared in order to introduce some Fe vacancy and probe the conduction mechanism of BFO. The leakage current of BiFeo.9Tio.05O3is decreased by nine orders of magnitude from that of BiFeO3. With an ultrahigh electrical resistivity, over1014Ωcm, BiFe0.9Ti0.05O3ceramic displays an especially low dielectric loss,0.015at100Hz, a remanent polarization Pr of0.23μC/cm2and a remanent magnetization Mr of0.13emu/g at room temperature. It is proposed that the Fe-deficiency in BiFeo.9Ti0.05O3decreases the amount of Fe2+and leads to the ultrahigh electrical resistivity. However the result of ferromagnctism presents that the introduction of Fe vacancy can not effectively cause the change of spatially modulated spin structure.3. Bi0.9Dy0.1Fe0.95O3and Bi0.9Dy0.1Fe0.9Ti0.05O3ceramics of high rhombohedral perovskite phase content were prepared and compared, which shows that Ti-doping leads to not only a great increase in electrical resistivity but also a further dramatic magnetization enhancement in addition to that induced by Dy-doping. With a high electrical resistivity around1×10Ωcm and a remanent polarization Pr of0.25μC/cm2, Bio.9Dyo.1Fco.9Tio.05O3ceramics sintered at960°C display a saturation magnetization Ms of lemu/g, which is three times as much as that of Bi0.9Dy0.1Fe0.95O3sintered at900°C. It is proposed that there exist some synergistic action between Ti and Dy in Bi0.9Dy0.1Fe0.9Ti0.05O3which gives rise to additional magnetization enhancement in BiFeO3.4. In order to improve the fcrromagnetism of BFO and study the effect of eodoping of Ti and rare earth elements on the properties of BFO, Bio.9Hoo.1Fco.95O3and Bi0.9Ho0.1Fe0.9Ti0.05O3ceramics were prepared through conventional solid-state-reaction. X-ray diffraction indicated that both ceramics were of a high rhombohedral perovskite phase content and contained minor impurities phases such as Bi12(Bi0.5Fe0.5)O19.5and Bi46Fe2O12. Microstructural analyses showed that the grains of Bi0.9Ho0.1Fe0.9Ti0.05O3ceramics were much smaller than those of Bi0.9Ho0.1Fe0.95O3. Bi0.9Ho0.1Fe0.9Ti0.05O3ceramics showed a electrical resistivity over1×1014Qcm at room temperature, a very low. dielectric loss of0.02at100Hz, magnetic hysteresis loop with a remnant magnetization2Mrof～0.485emu/g, both much higher than those of Bi0.9Ho0.1Fe0.95O3. It was proposed that the defect subsystem of BiFeO3is responsible for the improvements and attention should be paid to tailor the defect subsystem through adopting nonstoichiometric compositions and codoping of multiple elements. |
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