Preparation And Properties Of BiFeO₃-based Multiferroic Materials

Multiferroics,which capture great attention for their potential application in multifunctional devices,simultaneously possess ferroic properties,such as ferroelectricity,ferromagnetism and ferroelasticity.Among all multiferroic materials studied so far,BiFeO₃ is known to the only single-phase room temperature multiferroic materials,which has simultaneous ferroelectric?T_C¹103 K?and G-type antiferromagnetic?T_N⁶43 K?properties.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 co-doping with the alkaline earth and rare earth ions,co-doping with the rare earth and 3d transition metal ions.The following is the investigative content and main conclusions:1.The BiFeO₃ co-doped by the alkaline earth?Ca,Sr,Ba?and Eu ions were prepared by a sol-gel method.The influence of the co-doping on the strutural,ferromagnetic and ferroelectric properties were investigated.Firstly,we prepared the Ca-doped and?Eu,Ca?co-doped BiFeO₃.Analysis of magnetic hysteresis data reveals a further enhancement in magnetism in the?Eu,Ca?co-doped samples which is explained by the structural distortions and the magnetically active characteristic of Eu³⁺ions.The enhanced ferroelectric properties at room temperature can be observed in the co-doped samples by measuring the electric hysteresis loops due to Eu doping,which can suppress the formation of oxygen vacancies.Secondly,we prepared the Bi_0.95-xEu_0.05Sr_xFeO₃?x=0,0.05 and 0.10?samples.The maximum remnant magnetization of 0.287emu/g can be observed in the x=0.10 sample.The suppression of spin cycloid caused from the structural distortion can play an important role in the improvement of magnetic properties,which can be proved by the results of X-ray diffraction?XRD?and Fourier transform infrared spectroscopy.Thirdly,we prepared the Bi_0.90Eu_0.05Ba_0.05FeO₃ sample.The enhancement in ferromagnetic and ferroelectric properties are also observed.And the analysis of UV–visible absorption spectra shows the decrease of direct optical band gap by doping.Fourthly,the effect of three different alkaline earth ions?Ca,Sr,Ba?on ferromagnetic and ferroelectric properties of Bi_0.95Eu_0.05FeO₃ were investigated.The highest value of magnetization?¹.189 emu/g?at the maximum applied field of 70 kOe can be obtained from Bi_0.90Eu_0.05Ba_0.05FeO₃.The highest value of remnant magnetization?⁰.205 emu/g?can be obtained from Bi_0.90Eu_0.05Ca_0.05FeO₃,which may be induced by the structural distortion.2.The BiFe_0.95Co_0.05O₃ substituted with rare earth ions were prepared by a sol-gel method.The influence of the substitution on the strutural,ferromagnetic and ferroelectric properties were investigated.Firstly,we prepared the Bi_1-xEr_xFe_0.95Co_0.05O₃?x=0,0.05,0.10,0.15?samples.XRD data reveal a phase transition from rhombohedral to orthorhombic structure with 15%Er doping.This structural phase transition results in the significantly improved magnetic properties with the maximum magnetization at the maximum applied field of 70 kOe corresponds to 1.963 emu/g.The leakage current decreases with the increasing Er concentration.However,the ferroelectric properties dramatically decrease at x=0.15 due to the structural phase transition.Secondly,we prepared the Bi_0.95La_0.05Fe_0.95Co_0.05O₃ and Bi_0.95Pr_0.05Fe_0.95Co_0.05O₃ samples.Compared with Bi_0.95Pr_0.05Fe_0.95Co_0.05O₃,Bi_0.95La_0.05Fe_0.95Co_0.05O₃ can more effectively improve the ferromagnetic and ferroelectric properties.Thirdly,we prepared the Bi_0.9Eu_0.1Fe_0.95Co_0.05O₃ and Bi_0.9Eu_0.05La_0.05Fe_0.95Co_0.05O₃ samples.Magnetic hysteresis loops show a significant enhancement of magnetization with a remanent magnetization of about 0.22 emu/g for Bi_0.9Eu_0.1Fe_0.95Co_0.05O₃and that of about 0.35 emu/g for Bi_0.9Eu_0.05La_0.05Fe_0.95Co_0.05O₃.The local structure has been measured with X-ray absorption fine structure of the Fe K-edge,which reveal the decrease of Fe²⁺ions and the shift of Fe–O bond distance in codoped samples,compared with BiFeO₃ sample prepared under similar conditions.It is concluded that magnetic properties were greatly improved mainly due to the structural distortion rather than the existence of divalent Fe²⁺ions.3.Multiferroic BiFeO₃,Bi_0.95Dy_0.05FeO₃,and Bi_0.95Dy_0.05Fe_0.95Cr_0.05O₃ samples were successfully synthesized by a carbon microsphere-assisted sol-gel method.XRD data confirm the formation of single phase and a phase transition by doping Dy and Cr in BiFeO₃.The morphology of BiFeO₃ can be effectively controlled to form nanoparticles with homogeneous microstructures,due to the nucleation sites of carbon microspheres.The doping of Dy and Cr in BiFeO₃ has significant effect on improvement magnetic properties with the remnant magnetization of 0.557emu/g,due to the structural phase transition,size effects,and the strong ferromagnetic interaction between Fe³⁺-O-Cr³⁺ions by Cr substitution.Meanwhile,the doping of Dy into BiFeO₃ effectively reduce the leakage current and enhance the ferroelectric properties.4.The sintering temperature of solid state reaction to preparing BiFeO₃-based materials has been studied.And then we prepared Bi_0.95Dy_0.05Fe_0.95Cr_0.05O₃,Bi_0.95Dy_0.05Fe_0.95Mn_0.05O₃ and Bi_0.95Dy_0.05Fe_0.95Ni_0.05O₃ ceramics by solid state reaction.The influence of Dy and 3d-transition metal?Cr,Mn and Ni?ions co-doping on the strutural and multiferroic properties was investigated based on theoretical and experimental evidence.X-ray diffraction?XRD?results show that doping could not result in a transition of crystal structure,and all the samples are indexed to rhombohedral struture?R3c?,which is consistent with the results of the first principle calculation.The magnetism of Bi_0.95Dy_0.05Fe_0.95Mn_0.05O₃ is weakest among the three samples.There is an obvious enhanced magnetic properties for Bi_0.95Dy_0.05Fe_0.95Cr_0.05O₃ and Bi_0.95Dy_0.05Fe_0.95Ni_0.05O₃ ceramics.The remanent magnetization and coercivity in the Bi_0.95Dy_0.05Fe_0.95Cr_0.05O₃ are 0.159 emu/g and 0.931 kOe,respectively.The highest value of magnetization?¹.941 emu/g?at the maximum applied field of 30 kOe can be obtained from Bi_0.95Dy_0.05Fe_0.95Ni_0.05O₃.By the comparison of the experimental and calculated results,the enhanced magnetic properties of Bi_0.95Dy_0.05Fe_0.95Cr_0.05O₃ and Bi_0.95Dy_0.05Fe_0.95Ni_0.05O₃ can be ascribed to the Fe³⁺-O-Cr³⁺(Fe³⁺-O-Ni³⁺)magnetic interactions.Meanwhile,the doping of Dy into BiFeO₃ can effectively reduce the leakage current and enhance the ferroelectric properties.Furthermore,the ferroelectric properties can be related to the preparation of single phase and the density of ceramics.