| As a typical single phase multiferroic material, Bismuth ferrite (BFO) has recently attracted extensive attention. It has a rhombohedrally distorted perovskite structure with space group R3c. BFO shows both ferroelectricity (Tc=810℃) and antiferromagnetism (TN=380℃) at room temperature, and therefore, have the potential applications in information storage. Researchers have found this coexistence of ferroelectricity and antiferromagnetism through experiment long before, however, well saturated polarization hysteresis loops have not been obtained because of low electrical resistivity, which caused by oxygen vacancies origin from the valence fluctuation of Fe3+ to Fe2+. Additional, the weak magnetization at room temperature need to be improved for the further application. To solve these problems, this paper has concentrated on five aspects and the main results and conclusions can be summarized as following:1. Undopped and substitution on A and B site BiFeO3 films were prepared by sol-gel method and with Bi(NO3)3·5H2O and Fe(NO3)3·9H2O as starting materials. Pure BiFeO3 thin films without any secondary phase have been obtained through improving the process. XRD spectra showed that the thin films annealed at 600℃have already well crystallized, secondary phase have been found in the sample annealed at 700℃. SEM and AFM spectra also indicated that a dense films have been formed after annealed at 600℃.2. Undopped and substitution on A and B site BiFeO3 films were prepared by sol-gel method and with Bi(NO3)3·5H2O and Fe(NO3)3·9H2O as starting materials. Pure BiFeO3 powders without any secondary phase have been obtained through high temperature annealed in muffle furnace for 2 hours. XRD spectra showed that powders without annealing gave lots of peaks for impure phase, while after annealed at 600℃,700℃and 800℃for 2 hours, the impure phase have almost disappeared and the BiFeO3 powders crystallized better.3. Electric measurements of the BFO films have been carried out. The dielectric measurements results have showed that the dielectric constant and dielectric loss have been increased through the ion substitution on A and B site. The P-E measurements results indicated that the undopped BFO films gave well saturated loops with the remnant polarization is 1.88μC/cm2. However, the BFO films with 10% La and 10% Ni substitution showed better saturated loops with the remnant polarization are 5.03μC/cm2 and 7.57μC/cm2, much larger than that of the undopped films. The results indicated that the large leakage current in BFO films have been restrained through substitution on A and B site, thus to improve the electric properties.4. Magnetic measurements of the BFO powders have been carried out. The magnetic tests results indicated that the annealing temperature as well as the ion substitution on A and B site had a great effect on the magnetization at room temperature. The particle size increased with the annealing temperature raised, thus the magnetization decreased because of the size strain effect. Ion substitution on A and B site can caused the suppression of the spiral modulate spin structure (SMSS), thus to increase the magnetization at room temperature.5. Optical measurements of the BFO thin films have been carried out. The effects of different annealing temperatures and different substrates on the Raman spectra have been investigated. The film on LNO/Si substrate annealing at 750℃gave almost all the E modes and the peaks appeared at 268,320,365,425,479,546,639 and 725 cm-1. Furthermore, the E modes positions of the film deposited on LNO-coated Si substrate have moved to higher frequency, compared to the film deposited on Si substrate. This movement can be ascribed to the tensile stress exits at the interface between BiFeO3 and LaNiO3 while compress stress at the interface between BiFeO3 and Si.
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