Multiferroic And Magntodielectric Studies Of BiFeO₃-based Solid Solutions

Among the different types of multiferroic compounds,bismuth ferrite(BiFeO3)is one of the few multiferroic materials with the simultitanious existence of magnetism and ferroelectricity at room temperature,and has attracted much attention in recent years.Ceramic bulk BiFeO3 and their solid solutions show excellent ferroelectric,piezoelectric and multiferroic properties and are thus promising candidates for lead-free ferroelectric,piezoelectric and magnetoelectric devices.However a high leakage current resulting from the presence of Fe2+ and oxygen vacancies due to the loss of Bi3+ during the sintering process in BiFeO3 ceramic bulk makes it difficult to obtain good ferroelectric properties.Therefore,it is necessary to improve the magnetic and ferroelectric properties of BiFeO3 by means of the doping and optimized preparation methods.It is found that the introduction of morphotropic phase boundary by the formation of solid solutions with BiFeO3 and other perovskite ferroelectric matertials can significantly improve the magnetic and ferroelctric properties.The main results are as follows:1.Chemical substitution is an effective method for tailoring the electrical and magnetic properties of functional materials.In this work,(1-x)Ba0.1Sr0.9TiO3-xBiFeO3(0.2? x?0.8)ceramics were prepared by the modified Pechini method.X-ray diffraction patterns manifest that the samples undergo a gradual structural transformation from rhombohedral phase to tetragonal phase with increasing the content of Ba0.1Sr0.9TiO3(BST).Dielectric properties show that TC can be modulated by the doping level of BST in a wide range from 885 K to 260 K accompanied by the ferroelectric transformation from non-relaxor type to relaxor type ferroelectricity.The room-temperature ferroelectricity,ferromagnetism and intrinsic magnetodielectric effect are observed in the BiFeO3-rich samples.Magnetization and magnetodielectric measurement both show that the spin cycloid is destroyed by the substitution of BST and the critical field for the transition from spiral to antiferromagnetic collinear magnetic structure decreases with increasing the amount of BST.The results manifest that these materials are quite promising for potential applications in BFO-based magnetoelectric devices and electrocaloric cooling technologies.2.(1-x)(1.94Bi0.5Na.5TiO3-0.06BaTiO3)-xBiFeO3(0 ? x ? 0.9)ternary ceramics were prepared by the modified Pechini method.X-ray diffraction patterns manifest that the samples undergo a gradual structural transformation from the rhombohedral-tetragonal morphotropic phase boundary to the rhombohedral phase with the increasing content of BiFeO3.Room-temperature ferromagnetism was observed in the x>0.6 samples,and this ferromagnetism can be ascribed to the suppression of the spiral spin structure with the canting of antiferromagnetically ordered spins.The BiFeO3-rich samples(x = 0.6-0.8)exhibit superior ferroelectric properties with the maximum remanent polarization of 44.7 ?C/cm2,as confirmed by the positive-up negative-down measurement to exclude the contribution of leakage current.3.The ternary(1-x)((1-y)BiFeO3-yBi0.5Na0.5TiO3)-xCaTiO3(0? x? 0.4,0? y?0.3)solid solutions were prepared by the modified Pechini method.X-ray diffraction patterns manifest that the samples undergo a gradual structural transformation from rhombohedral to orthorhombic phase with increasing the content of CaTiO3,and the phase boundary shifts to the side of lower CaTiO3 content with increasing the amount of Bi()5NaO 5TiO3.The phase diagram of the ternary system is obtained by connecting the straight lines of the respective MPBs of binary systems.Continuous variation in the magnetization across the structure transition testify that weak ferromagnetism in the rhombohedral and orthorhombic phases is both attributed to a destruction of the modulated structure by the substitution of the nonmagnetic Na and Ca/Ti ions in BiFeO3.The variation in remanent magnetization Mr with the Fe content and the temperature dependence of magnetization of the ceramics with different composition testify that long-range magnetic order is related to the Fe content on the B site and the increase in the Na and Ca/Ti ions concentration leads to a dilution of the magnetic sublattice.The percolation threshold of Fe content on the B sites for room-temperature long-range magnetic order is determined.The ferroelectricity of the samples near the morphotropic phase boundary is better than others due to more dense microstructure and better dielectric properties.