Preparation Of Sr, Co Doped Bismuth Layer Structured Oxides And Its Multiferroic Properties

Multiferroics are a kind of materials which simultaneously possess polarization ordering and magnetic ordering. The conflicting requirements for J-obital electron configurations of ferroelectricity and ferromagnetism result in the scarcity of multiferroic materials. For a very long time, there is only one "star" multiferroic material, BiFeO3(BFO), which has the ferroelectric and magnetic transition temperature above room temperature. Unfortunately, BFO is antiferromangnetic with weak spontaneous magnetization, and usually has large current leakage, which makes it unsuitable for practical application in devices. Bismuth layered perovskite structured Aurivillius oxide of Bi5Fe0.5Co0.5Ti3O15was firstly found to have the coexistence of ferroelectricity and ferromagnetism above room temperature in2009, which provides another way to explore the potential high temperature multiferroics to be used in information storage, sensor and other spintronic devices.This thesis mainly focused on the enhancement of multiferroic properties of Aurivillius structured oxides, including:1) Studying the effects of Sr doping on the structure and the multiferroic properties of Bi7Fe1.5Co1.5Ti3O21;2) Applying the magnetic layer insertion method to prepare a novel multiferroic material with SrBi4Ti4O15as the ferroelectric host, and investigating the magneto-dielectric and magneto-electric coupling effect of this new material at room and high temperatures (100℃);3) Fabricating Sr, Co co-doped Aurivillius poly crystalline films, and exploring the impact of Pt substrate on the phase structure, the magnetic and the leakage properties of Aurivillius oxides. The main results are listed as follows:Chapter1:A brief introduction of the multiferroics and the multiferrroic materials, as well as the the detail description of Aurivlillius bismuth layered perovskite oxide materials and its research history. Major challenges for bismuth layered perovskite oxide materials as a good multiferroics are also emphasized. Based on the previous investigation, the main topics of this thesis are issued.Chapter2:General synthesis methods for multiferroic ceramic materials were introduced, including the preparation of powders and the calcination of ceramics.0.66Pb(Ni1/3Nb2/3)O3-0.34PbTiO3(0.66PNN-0.34PT) ferroelectric relaxor was taken as example to disscuss the different synthesize methods. In addition, some techniques and instruments to characterize the multiferroic properties, such as ferroelectric and ferromagnetic performance, are also issued. The magneto-dielectric and magneto-electric coupling characterization methods were adddressed in details.Chapter3:To further improve the multiferroics of Bi7Fe1.5Co1.5Ti3O21, Sr2+is applied to subtitute for Bi3+ions. The effects of Sr2+substitution for Bi3+ion on the phase structure, ferroelectric and ferromagnetic properties of compound are investigated. It was found that the ferroelectric and ferromagnetic properties were enhanced at limited amout of Sr doping. The Sr-0.25ceramic sample has the best performance with the remnant polarization (2Pr) of～2.89μC/cm2at the electric field of100kV/cm, and the remnant magnetization (2Mr) of～2.27emu/g. Unfortunately, when the Sr doping level exceeded a certain value (x>0.25), the Aurivillius structure would collapse accompanying with the formation of the secondary phase Sr1-mBimFe1-iCoiO3-γ, which deteriorated the multiferroic performance of samples.Chapter4:Applying the magnetic layer insertion method to synthesis a new multiferric material SrBi5Fe0.5Co0.5Ti4O18. The remnant polarization and the remnant magnetization of SrBi5Fe0.5Co0.5Ti4O18was52.4μC/cm2and2.24emu/g, respectively, measured at room temperature. Especially, the high temperature magneto-electric coupling effects of SrBi5Fe0.5Co0.5Ti4O18were reached～350μV·cm-1·Oe-1at100℃, which was so far the best result of all available high temperature multiferroic ceramics reported. The magnetic field prototype senor we manufactured using SrBi5Fe0.5Co0.5Ti4O18, works very well under low magnetic field at room temperature. This makes multiferroic materials with the intrinsic magneto-electric coupling effect attainable, which is essential to both foundamental research and device application such as senor, information storage and qantum control.Chapter5:The impacts of Pt bottom electrode on the structural evolution, ferromagnetic and ferroelectric properties of SrBi5Fe0.5Co0.5Ti4O18(SBFCT-x) poly crystalline films were investigated in this chapter. The results suggest that the secondary phase of PtCoO2, which is highly conductive and weak antiferromagnetic, formed at the interface of the prepared film and the Pt substrate. The formation of PtCoO2partially consumes the Co source, and leads to the Co deficiency in SBFCT-x, and finally structure collapse from5perovskite layers down to4. It should be also noted that the antiferromagnetic property of PtCoO2indirectly proves the intrinsic ferromagnetism of the main Aurivillius compound observed at room temperature.Chapter6:Deliver the summary of this thesis and an outlook for the related future work.