The Preparation And Characterization Of Multiferroic BiFeO₃

Multiferroic materials are a class of materials that yield simultaneous ferroelectricity and ferromagnetism.It has a potential applications in information storage,spintronic devices and sensors.Among all multiferroics,BiFeO₃ that exhibits coexistence of ferroelectric and antiferromagnetic has been interested due to its relatively high Neel temperature(T_N～673 K)and Curie temperature(T_C～1103 K), which makes it to be one of the prime candidates for room-temperature magnetoelectronic applications.However,there are still several obstacles to be solved in the modern research of BiFeO₃.They are the problems of the larger leakage and no magnetism at room temperature because of the periodic antiferromagnetic and spiral spin structure.The magnetism of BiFeO₃ bulk and nanostructures has been specially investigated in this paper.The magnetism can be enhanced by the ironic substitution through suppressing the spiral spin structure of BiFeO₃,The novel magnetism of BiFeO₃ nanoparticles also has been studied in detail.The single-phase BiFeO₃, Bi_1-xCa_xFeO₃ and Bi_1-xLa_xFeO₃ are prepared by sol-gel method.The properties of thermal stabilities,structures,surface morphologies,micromagnetism and macromagnetism are investigated by TG,XRD,SEM,VSM and impedance analyzer respectively.The main results and conclusions are as following:1.The BiFeO₃ are synthesized by sol-gel process using PVA as dispersing medium.The single-phase BiFeO₃ without formation of other impurities is attained by rapid annealing the dry gel.Compare with the traditional solid-state sintering process, The method is more simple and convenient for preparation of pure BiFeO₃ The magnetic hysteresis loops of BiFeO₃ indicate that it has no magnetism at room temperature.The results are consistent with the literatures.2.The polycrystalline BiFeO₃ nanoparticles are also prepared successfully by sol-gel method using EDTA as the chelating agent.The SEM image shows that the BiFeO₃ is composited of nanoparticles with the size of 110-160 nm.By the VSM analysis,the BiFeO₃ shows a weak ferromagnetic at room temperature,which is quite different from the linear M-H relationship of bulk BiFeO₃.The weak magnetic property of BiFeO₃ nanoparticle sample should be attributed to the increase of the suppression of the spiral spin structure(period length-62 nm)with the decreasing of the nanoparticle size,and the increase of the uncompensated spins and strain anisotropies at the surface.3.The Bi_1-xCa_xFeO₃ and Bi_1-xLa_xFeO₃ have been successfully prepared using a sol-gel method.The structures and properties of BiFeO₃ with Ca and La substitution also have been studied.The structure of Bi_1-xCa_xFeO₃ transits from rhombohedral to triclinic with 10%Ca doping.The Ca and La substitution of Bi eliminates the impurity phase of BiFeO₃,the substitution of Bi with Ca and La suppresses the spin cycloid,releases a locked magnetization,which results in significantly improved ferroelectric and magnetic properties.The dielectric properties are increased by Ca or La doping compared to that of the pure BiFeO₃.