单相长周期铋层状多铁材料的交换偏置效应
[Abstract]:Bismuth layered oxides are a class of fluorite-like (Bi202)2+ and perovskite-like (Am-1BmO3m+1]2-(in which m is the number of layers) layered materials with periodic alignment along the C direction. Ti/Fe ions are not found in bismuth layered oxides due to their ferroelectric and ferromagnetic properties above room temperature. The perovskite-like B-site octahedron occupies the center of perovskite-like octahedron evenly and forms a strong interaction with the surrounding oxygen ions. This kind of non-central symmetric structure determines their special ferroelectricity and ferromagnetism by the strong interaction between ions, which provides a new way for people to study and explore new Polyferrous materials and is expected to be used. Studies have shown that the polyferric properties of bismuth layered oxides depend on the period length. Short-period oxides (such as 4-layer Bi5FeTi3O1 5-layer Bi6Fe2Ti3O18 and 6-layer Bi7Fe3Ti3O1) generally exhibit paramagnetism at room temperature, while long-period oxides (such as 7-layer BigFe) exhibit paramagnetism. 4Ti3O2 4 and 8-layer Bi9Fe5Ti3O2 7) usually exhibit antiferromagnetism at room temperature, sometimes even weak ferromagnetism. Long-period oxides, a unique coexistence of ferromagnetism and antiferromagnetism, and possible interactions, will make long-period oxides present glass state at a certain temperature and may even cause exchange bias. In order to further study and explore the novel multi-ferrous properties and mechanism of bismuth layered oxides, the following studies have been carried out: 1) preparation of Co/Y co-doped Bi_7Fe_3Ti_3O_2_1 and its performance relationship; 2) preparation, structure and properties of long-period Bi_ 10Fe_ 6Ti_ 3O_ 30 oxide. The main results of this paper are as follows: Chapter 1: Ferroelectric and magnetic materials are introduced separately, and the classification of various magnetism in magnetic materials is given; and the ferroelectric properties of Bi10Fe6Ti3O30 doped with Co are also introduced. In this paper, the structure and properties of bismuth layered oxides with ferroelectric and ferromagnetic properties at room temperature are analyzed, and the research direction of this paper is established. The preparation of chemically doped Aurivillius structural materials and the modification of their ferroelectric and ferromagnetic properties by doping content were investigated; the new exchange bias effect of long-period bismuth layered oxides and its relationship with doping elements were investigated. The traditional method of preparing bismuth layered oxides by solid-state reaction was improved. Oxide powders were prepared by improved combustion method, and ceramic samples were obtained by muffle furnace sintering or hot-pressing sintering. In this work, short period Bi7Ti3Fe3O2 1 was selected for exploratory preparation, and the effect of yttrium doping on the multi-ferromagnetic properties of the materials was discussed. The experimental results show that the ferromagnetic properties of the materials are greatly enhanced by the coupling of cobalt with iron through the surrounding oxygen ions. The addition of Yttrium with smaller radius and d-empty orbits also improves the ferroelectric and ferromagnetic properties of the materials. The results of magnetic weightlessness measurements show that in a certain range of Y-doping, the multi-ferroelectric behavior of the materials mainly comes from the intrinsic properties. Chapter 3: A coupling interaction between two kinds of magnetic materials is introduced. The exchange bias effect and the relationship between the exchange bias effect and the composition and structure of the material system are analyzed. A new idea for developing new single-phase long-period exchange bias materials is proposed. It appears as follows: 1) in systems containing both ferromagnetic and antiferromagnetic components; 2) at present, it is also extended to some multi-ferromagnetic heterojunction systems. 3) Hole-doped manganese oxides and cobalt oxides, due to the existence of structural phase separation and electronic phase separation, often form several different phase coexistence systems (including the coexistence of ferromagnetic and antiferromagnetic phases), the exchange bias effect produced by this internal phase separation for us to explore the exchange of single phase. Biased materials provide possibilities. Due to the complexity of the factors affecting the exchange bias field, the existing theoretical models are not clear enough to fully explain the phenomena observed in the experiments. The inhomogeneous distribution of magnetic iron ions in such long period oxides has been observed directly by means of high angle ring dark field phase (STEM-HAADF) and other experimental means of TEM, which confirms the existence of short period magnetic ordering, i.e. cluster glass state and spin-tilted antiferromagnetism. The exchange bias field of the sample is much higher than that of some materials in the phase separation system and the multiferrous heterojunction system. The discovery of a new single-phase layered multiferrous material with significant exchange bias effect is not only advantageous. The development of basic physics has promoted the application of exchange bias devices. Chapter 5: Trial preparation of B-site cobalt-doped Bi10Fe6Ti3O30, a single-phase long-period bismuth layered oxide polyferric material, by means of modification, and the study of its polyferricity and related properties have been carried out. When only Fe is present in the samples, the samples usually exhibit paramagnetism or antiferromagnetism at different temperatures. With the increase of Co content, the samples exhibit paramagnetism to ferromagnetism. This is mainly due to the fact that the positions of Fe and CO in perovskite-like units are more selective in oxides with long-period structure. For these materials, their unique noncentrosymmetric structure and the inhomogeneous distribution of Ti, Fe, Co at the center of the oxygen octahedron, as well as the strong interaction with the surrounding oxygen ions, are the main sources of their unique ferroelectric and ferromagnetic properties. Chapter: summary of the full text and prospects for future work.
【学位授予单位】:中国科学技术大学
【学位级别】:博士
【学位授予年份】:2015
【分类号】:O611
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