| Single-phase multiferroic materials not only have rich physical connotation,but also have potential application prospects in spintronic devices,sensors,memory and so on.Recently,such layered Aurivillius oxides,with formula Bin+1Fen-3Ti3O3n+3 have been widely investigated as one of the most promising single phase multiferroics.In such compounds,the perovskite-like layers(Bin-1Fen-3Ti3O3n+1)2-are sandwiched with(Bi2O2)2+layers along c-axis in half an unit cell,and n refers to the number of pseudo-perovskite layers.However,due to the volatilization of Bi and the valence change of Fe in high temperature synthesis,Bin+iFen-3Ti3O3n+3 still has some problems and deficiencies in multiferroic properties,such as small residual polarization,large leakage current,low magnetic transition temperature(TCM),weak magnetoelectric coupling effect and so on.The aim of this thesis is to improve the multiferroic properties of layer-structured Aurivillius compounds,and mainly concentrated on n=4,5 bismuth layer-structured Aurivillius multiferroic materials.In order to improve the room temperature(RT)multiferroic property of the material,the effect and mechanism of doping on the properties of the material were investigated by doping at A and B site.The work provides the experimental basis for the research and development of single-phase multiferroic materials based on layer-structured Aurivillius compounds above room temperature.The essay consists of six chapters,as follows:In Chapter 1:The research background of multiferroic materials,the research status of Bin+1Fen-3Ti3O3n+3 bismuth layered Aurivillius oxides.In Chapter 2:The preparation methods of layered perovskite ceramics are described.And the improved sol-gel auto-combustion process,as well as the structure and properties of single-phase multiferroic materials are described.In Chapter 3:Multiferroic Bi5Ti3Fe1-xN1xO15(BFNT-x:x=0,0.1,0.2,0.3,0.4,0.5)ceramics were prepared by the sol-gel auto-combustion method,and their microstructures,ferroelectric(FE),magnetic and dielectric properties were investigated systemically.A small amount of Ni doping can enhance magnetic and FE property.In particular,the BFNT-0.2 sample shows the highest remnant magnetization(2Mr)0.24 emu/g and the largest remnant polarization(2Pr)～11.6μC/cm2.The ferroelectric transition temperature(TC)and TCM of the BFNT-0.2 sample is 1016.5 K and 796 K respectively.In addition,dielectric anomalies were found from 400 K to 600 K,which can be attributed to the transition of oxygen vacancies.The effects of Ni doping on ferroelectric,magnetic and dielectric properties were discussed.In Chapter 4:Multiferroic Sr0.5Bi5.5Fe1.5-xCoxTi3.5O18(SBFCT-x:x=0,0.1,0.2,0.3,0.4,0.5)ceramics were successfully prepared by the sol-gel auto-combustion method.Co substitution can enhance magnetic and FE property obviously.In particular,the SBFCT-0.4 sample shows the highest remnant magnetization(2Mr)～1.82 emu/g and the largest remnant polarization(2Pr)～24.4 μC/cm2.Furthermore,dielectric anomalies(x=0.2-0.5)have been found around 400 K,which can be attributed to the long-range migration of oxygen vacancies.The effects of Co doping on ferroelectric,magnetic and dielectric properties were discussed.The SBFCT-0.4 sample was found to exhibit the magnetoelectric effect detectable under a low response magnetic field at RT with the largest ME coefficient～250μV·cm-1·Oe-1 at RT.And the obvious magnetocapacitance performance was also observed at and above RT(373 K),which is important to potential applications in sensing,memory,etc.In Chapter 5:Multiferroic SrxBi6-xFe1-x/2CO1-x/2Ti3+xO18(SBFCT-x:x=0,0.25,0.5,0.75,1)ceramics were prepared by the sol-gel auto-combustion method,and their microstructures,ferroelectric,magnetic and dielectric properties were investigated systemically.The highest remnant polarization(2Pr～17.37μC/cm2)has been observed in SBFCT-1 ceramic while the SBFCT-0.5 ceramic shows the highest remnant magnetization(2Mr～0.74 emu/g).To explore the effect of valance states of Fe and Co ions,we analysed the content variation of Fe2+,Co2+,and oxygen vacancies by the X-ray photoelectron spectroscopy results.Furthermore,dielectric anomalies have been found around 400 K,which can be ascribed to the hopping process of oxygen vacancies.In Chapter 6:The summary of the essay,and the vista for future work were presented. |
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