Structure And Multiferroic Properties Of BiFeO₃-BaTiO₃-based Pseudo-ternary System Ceramics

Multiferroics refer to materials that simultaneously possess two or more kinds primary ferroic orders（ferroelectricity,ferromagnetism,ferroelasticity,et al.）in the same phase.Among them,the magnetoelectric multiferroics with the coexistence of ferroelectricity and magnetism not only have great scientific research value in the field of condensed matter physics due to its magnetoelectric coupling effect,but also have potential application prospects in the field of new storage.Bi Fe O₃ is the most typical single-phase multiferroic material which possesses higher ferroelectric Curie temperature and antiferromagnetic Néel temperature.However,the poor electric properties and weak magnetoelectric coupling effect limit its practical application in bulk Bi Fe O₃.In view of the above problems,this thesis focuses on Bi Fe O₃-Ba Ti O₃-based solid solution ceramics,and systematically investigates its structure evolution and multiferroic properties through ion replacement.The main conclusions have been obtained.Firstly,the effect of A/B sites substitution on the structure and properties of Bi Fe O₃ ceramics was studied.The（1-x-y）Bi Fe O₃-x Sm Fe O₃-y Ba Ti O₃ ceramics were synthesized by a standard solid-state reaction sintering method.According to the XRD and Raman spectrum analysis,the structure transforms gradually from rhombohedral R3c to pseudocubic with the substitution of Ba Ti O₃.Meanwhile,the orthorhombic Pbnm phase content gradually increases with the addition of Sm Fe O₃.The DSC results indicate that the magnetic transition temperature T_N decreases linearly with increasing B-site substitution content.The ferroelectric properties are significantly enhanced at BF008-01 sample,and maximum piezoelectric coefficient d₃₃ obtained is 17.3 p C/N.The best magnetic properties are achieved at BF017-008 sample,where the maximum magnetization～M_max=0.99 emu/g,and remnant magnetization～M_r=0.44 emu/g.Then,the magnetic ions（Co,Mn）were chosen to substitute Bi Fe O₃-Ba Ti O₃-based solid solution ceramics,and the structure and properties of ceramics were systematically studied.The（0.75-x）Bi Fe O₃-0.25Ba Ti O₃-x Bi AO₃（A=Co,Mn）ceramics were synthesized via a solid-state reaction method.The XRD and SAED analysis confirms the coexistence of tetragonal（T）and rhombohedral（R）phases of all samples.The best ferroelectric properties are obtained at BF-BT-003BC sample,where the remnant polarization P_r is 6.23μC/cm²,and the piezoelectric coefficient d₃₃ is as high as 79.3 p C/N.The M-H loops and XRD patterns before and after poling indicate that the electric field can induce the magnetic transition,which is mainly attributed to the phase transformation from rhombohedral phase R3c to the tetragonal phase P4mm during poling.The maximum linear magnetoelectric coefficient is obtained at BF-BT-005BC sample,whereα_ME=0.66 m V/cm Oe.The enhancement of magnetic and magnetoelectric properties are resorted to the distortion of the spiral spin structure by transition metal ions.Finally,on the basis of study for Bi Fe O₃-Ba Ti O₃ system,the B site is replaced by the composite perovskite compounds Bi(Mg_1/2Hf_1/2)O₃and Bi(Mg_2/3Ta_1/3)O₃.Through theoretical analysis and experimental characterization,the influence of different ions content on the structure and properties was explored.XRD results suggest the coexistence of tetragonal and rhombohedral phases of all samples,and a small amount of secondary phases is observed with increasing substitution content.The dielectric results indicate that the dielectric constants of two systems have obvious frequency dispersion,and there are two dielectric relaxation processes.The high temperature dielectric relaxation is related to the diffusion of oxygen vacancies.The leakage current density of composite ions substitution ceramics is significantly decreased.Meanwhile,the M-H loops confirm that the magnetic properties of ceramics are significantly improved by adding(Mg_1/2Hf_1/2)³⁺and(Mg_2/3Ta_1/3)³⁺.The best magnetic properties are obtained at 0.025MH sample,where the remnant magnetization M_r=0.83emu/g,the saturated magnetization M_s=1.88emu/g.The enhanced magnetism is ascribed to the suppression of the spiral spin structure by B site ions substitution and DM interaction.