The Preparation Of BiFeO₃ Based Solid Sol Utions And The Studies Of Their Ferroelectric And Magnetic Properties

Multiferroics are such materials which simultaneously exhibit two or more"ferroic" properties in one phase.The "ferroic" including ferromagnetic,ferroelectric or ferroelastic might yield coupling effects between these ordering parameters.The coupling effect of electrical and magnetic properties offers a wide range of novel applications,such as magnetic sensors,transformers,multiple state memories and microwave devices.BiFeO3 is the most promising multiferroic material which has attracted much attention because BiFeO3 simultaneously exhibits ferroelectric ordering and magnetic ordering at room temperature.However,it is difficult to prepare single-phase BiFeO3 ceramic,and the ceramic sample has low resistance,large leakage current making it difficult to obtain good ferroelectric properties.Moreover,the magnetism of BiFeO3 at room temperature is weak.These problems are severely restrict the development and application of BiFeO3.It is found that the element doping and the formation of solid solutions with other perovskite ferroelectrics doping can effectively improve ferroelectric and magnetic properties of BiFeO3.It this thesis,we use element doping at Bi and Fe sites in 0.6BiFeO3-0.4(Bi0.5K0.5)TiO3 to tune its electric and magnetic properties.The main contents are as follows:1.Co-doped 0.6BiFe1-xCoxO3-0.4(Bi0.5K0.5)TiO3(0 ?x ? 0.2)solid solutions were prepared by modified Pechini method and the structural,dielectric,ferroelectric,and magnetic properties were studied.The Co-doped samples undergo a structural transition from pseudocubic to rhombohedral phase atx = 0.1.The FE-SEM results manifest that the substitution of Co3+ ions can significantly promote the grain growth resulting in a dense morphology.The x = 0.15 sample exhibits a maximum remnant magnetization of 0.107 emu/g and a superior remnant polarization as large as 57.8 ?C/cm2.The ferromagnetism is proposed to originate from the suppression of spiral spin structure with the canting of the anti-ferromagnetically ordered spins caused by structural distortion due to the substitution of Co for Fe ions.Dielectric studies reveal the relaxor nature of the Co-doped samples resulting from mixed valence cation-site occupation.In addition,the MD effect(0.03%at RT)can be observed in the x = 0.15 sample.2.La-doped 0.6Bi1-xLaxFeO3-0.4(Bi0.5K0.4)TiO3(0?x?0.2)solid solutions were prepared by modified Pechini method and the structural,dielectric,ferroelectric,and magnetic properties were studied.The structure of the La-doped sample exhibits a coexistence of R3c and P4mm.The increase in the content of La decrease the ferroelctric properties of solid solutions.Whereas,doping La gives rise to the distortion of structure,and destroy the spin spiral magnetic structure of BiFeO3 resulting in an increase in the magnetic properties.The x = 0.2 sample exhibits the remnant magnetization of 0.03 emu/g and magnetoelectric coupling.3.2BiFeO3-Bi4Ti3O12(BFTO)particles were prepared through a magnetic-field assisted hydrothermal method.High magnetic field can greatly affect the growth behavior of BFTO particles.The magnetic fields promote the BFTO nanoplates to grow along(001)direction and form a high-index facet bipyramid particles.And the possible mechanisms of crystal growth and the magnetic field-induced acceleration of the Ostwald ripening process of bipyramid particles were discussed.The magnetization of the as-prepared BFTO particles are slightly enhanced with the increase in magnetic fields.