Preparation And Investigation Of The Bismuth Iron Oxide Based Multiferroics

As one of the most promising multiferroic materials, BiFeO₃ is known to be the only material that is both ferroelectric and antiferromagnetic at room temperature, which appears to have important potential application in information storage, transducer, spin electronics and driver system. However, the difficulty in preparing single phase BiFeO₃, high leakage current and weak magnetism have been obstacles for its further applications. In this thesis, to overcome these obstacles, we demonstrated the substitution effect on the multiferroic properties of BiFeO₃, including A-site substitution by the rare earth element or alkaline earth element, B-site substitution by the 3d transition metal element and the solid solution with a perovskite ferroelectric. We detailedly investigate the structure, morphology, ferroelectric properties, magnetic properties, and magnetoelectric properties of these oxides. From the study, we have an in-depth understanding of the correlations between the crystalline structure and multiferroic properties of these systems, and successfully improve multiferroic performance of BiFeO₃. The main results are listed as following:Firstly, we prepared series Bi_1-xA_xFeO₃ (A=Ca, Sr, Pb, Ba) ceramics by rapid liquid phase sintering method and systemically investigated the alkaline earth-substitution effect on the structure and multiferroic properties of BiFeO₃. It has been shown that heterovalent A²⁺ substitution results in the formation of oxygen vacancies in the host lattices of both antiferromagnetic and weak ferromagnetic Bi_1-xA_xFeO₃ compounds. We found that adding of lead atom in the Bi subsystem of BiFeO₃ destroys the long-range ferroelectric order induced by Bi lone pairs on contrary, and progressively breaks the ferroelectric ordering up to the point where the structure becomes cubic on average. A correlation between the ionic radius of the substituting element and the value of the spontaneous magnetization of the corresponding solid solution has also been studied. The experimental results suggest that A-site substitution with the biggest ionic radius ions effectively suppresses the spiral spin configuration of antiferromagnetic BiFeO₃.Secondly, we prepared the Bi_1-xEu_xFeO₃ (0≤x≤0.3) ceramics by rapid liquid phase sintering method and systemically investigated the Eu-substitution effect on the structure and multiferroic properties of BiFeO₃. The measured XRD patterns of Bi_1-xEu_xFeO₃ samples were analyzed with Rietveld refinement program based on different space group. We can conclude that Eu substitution induce a polar-to-polar R3c→Pn2₁a structural phase transition. We systemically studied the correlation between the structure and magnetic properties of Rare earth doped BiFeO₃ and quantitatively the magnetization computed the anisotropy density K. It was found that the enhanced magnetization in Rare earth doped BiFeO₃ can be attributed to the enhanced the anisotropy density. We prepared the Bi_0.9La_0.1FeO₃ and Bi_0.85La₍0.1_Ho_0.05FeO₃ ceramics by rapid liquid phase sintering method and systemically investigated the influence of radius and the effective magnetic moment of doped ions on the structure and multiferroic properties of BiFeO₃. The La³⁺ ions with larger ion radius stabilize the perovskite of BiFeO₃ and suppress the high leakage current of BiFeO₃, while magnetic Ho³⁺ ions with smaller ion radius can induce the distortion increase, suppress the antiferromagnetic spiral structure of BiFeO₃, and thus the ferromagnetism is more distinct at room temperature.Thirdly, we studied the effect of Co doping on the magnetic structures, electronic structures and band structures based on the first principle calculation. It was found that the magnetization of BiFeO₃ can be significantly improved since the doping changes the G-type antiferromagnetic order into the ferromagnetic one. We prepared the BiFe_1-xCo_xO₃ (x=0, 0.1, 0.2) ceramics by high temperature and high pressure synthesis, which can enhance the solubility limit of Co in BiFeO₃. The analysis of the Raman patterns reveals that the Co³⁺ ions enter the Fe³⁺ sublattices on B-site. By the comparison of the experimental and calculated results, we discussed the mechanism of Co-substitution effects on the magnetic properties of BiFeO₃, which can be ascribed to the Fe³⁺-O-Co³⁺ magnetic interactions in the present compounds.Fourthly, in order to develop multiferroics with large magnetization and polarization, we have prepared the (l-x)BiFeO_3-xGdCrO₃ (x=0, 0.1, 0.2) solid solution by high temperature and high pressure synthesis, and systemically investigated the structure, dielectric, ferroelectric and magnetic properties and analyzed the relative mechanism of this solid solution system. With the increase of GdCrO₃ content, a morphotropic phase transition from rhombohedral (R3c) perovskite to rthorhombic orthoferrite (Pbnm) was found at x around 0.1. The substitution of GdCrO₃ effectively improve the room temperature ferromagnetism of the solid solution, which is ascribed to the change of the degree of the title Fe³⁺ ions due to the structural distortion and the interactions between Cr³⁺ and Fe³⁺ ions; the magnetic Gd³⁺ ions also contribute to the magnetization of the solid solution at low temperature. Meanwhile, the evident magnetoelectric effect in 0.9BiFeO₃-0.1GdCrO₃ suggests the coupling between ferromagnetic ordering and ferroelectric ordering.In summary, we successfully enhanced the multiferroic properties of BiFeO₃-based materials based on the mechanism influencing the magnetic and ferroelectric properties of the studied systems. The results are in favor of the improvement of the multiferroic properties of BiFeO₃-based materials at room temperature in practical applications.