| BiFeO3,one of very few multiferroics with a simultaneous coexistence of ferroelectric and antiferromagnetic at room temperature,Multiferroics are materials possessing a spontaneous polarization,magnetization,and piezoelectricity that can be switched on by an applied electric field and magnetic field,respectively.These compounds present opportunities for potential applications in information storage, spintronics,and sensors.Of particular interest is BiFeO3,which exhibits the coexistence of fcrroelcctric and antiferromagnetic orders up to quite high temperature.Recently,it has been reported that both epitaxial and polycrystalline BiFeO3 thin films with reasonably high resistivity show large polarizations.For example,(001)oriented thin films BiFeO3 display a polarization of-90μC/cm2, comparable to the very popular ferroelectric system,(Pb(ZrxTil-x) O3),which has been studied for decades for applications in memory,transducers,and micro-electro-mechanical system(MEMS).An aspect of concern with the PZT system is its relative toxicity accruing from lead.BiFeO3 provides an alternative choice of a Pb-free ferro/piezoelectric material,which is environmentally preferable. There are intense researches on the enhancement of the ferroelectric property for BiFeO3;however,the magnetization is still small for its applications in information storage,spintronics,and sensors.The main results and conclusions in the dissertation can be summarized as following:1.BiFeO3 films were prepared on SiO2/Si substrates by sol-gel process and using Bi(NO3)3 ? 5H2O and Fe(NO3)3 ? 9H2O as starting materials. Ferroelectric/ferromagnetic composite BFO/PZT was also prepared on SiO2/Si substrates by sol-gel process.BiFeO3 nanopowder obtained by wet chemical route (combustion technique) at low temperature.2.The samples are characterized by using various techniques:X-ray diffraction (XRD) study is carried out for phase determination and lattice parameter calculations; scanning electron microscopy(SEM) and TEM to find out grain size and morphology.XRD results indicate that the BiFeO3 films can be well crystallized above a annealing temperature 500℃,The XRD patterns of the BFO films grown on the SiO2/Si substrates show that single BFO phase is crystallized in (110)-preferential orientation.With the increasing of annealing temperature the intensity of diffraction peaks increased and the half peak width decreased.However, impurity phase has appeared in the BFO films which annealed at 600℃.Obviously the annealing temperature has important influence to the structure of the BFO films. The intensity of diffraction peaks and the half peak width of BFO/PZT films increased with the LaNiO3 bottom electrode from 5 layers to 10 layers.The SEM results show that the BFO/PZT films have smoother surface and relatively dense structure with finer crystallites,the size of the crystal grain is approximately100nm. The diffraction peaks of BFO nanopowder prepared at 180℃has appeared,which indicate that we can synthesize BFO nanopowder at low temperature.We also found that reunion phenomenon has appeared when the nanopowder annealed at 500℃.3.Differential thermal analysis(DTA) to determine transformations of the starting materials.X-ray photoelectron Spectroscopy(XPS) is carried out to determine the element of BiFeO3 nanopowder.A large weight loss at 180℃of TG curve of the xerogel is likely attributed to decomposition of water and organic compound.DSC analysis showed that phase transition temperature and decomposition temperature were 830℃and 909℃respectively.Two symmetrical peaks appeared in the XPS curve of Bi4f.The peaks are at 165.0V and 158eV, respectively,corresponding to Bi4f5/2 and Bi4f7/2.Two symmetrical peaks appeared in the XPS curve of Fe2p corresponding to Fe2p3/2 and Fe2p1/2.There was a wide dissymmetry peak in the XPS curve of Ols.4.Ferroelectric measurements include polarization hysteresis loops and dielectric loss and C-F curves of the BFO films also have carried out.The remnant polarizations of the BFO films annealed at 500℃,550℃,600℃are 2.49μC/cm2, 4.51μC/cm2 and 9.74μC/cm2,respectively.With the increasing of temperature the ferroelectri city of BFO films increased.The crystallization of BFO films annealed at low temperature was bad.Along with the increasing of annealing temperature the crystallinity is better,however high annealing temperature would lead large leakage current. 5.Magnetism tests include magnetic hysteresis loop and Curie temperature of the BFO nanopoweder have carried out.The results indicated that the Curie temperature was 380℃which consist with other reports.The annealing temperature has small influence on magnetic property at low temperature(180℃-400℃).The influence is big when the nanopowder annealed at high temperature(500℃-700℃).
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