Preparation Of BiFeO₃Materials Under Different Sizes And Investigation Of Multiferroic Properties And Transport Behavior

Multiferroic material is defined as material at least has two characteristics in ferroelectric, ferromagnetic, and ferro-elasticity in one phase. Bismuth Ferrite (BiFeO3, BFO), as a typical ferroelectric material, has become a research focus because of its potential applications in devices of spintronics and information storage due to high Curie temperature (Tc-1103K) and high Neel temperature (TN～643K) The ferroelectric ordering is originated from the distortion of lone pair6s electron in Bi3+ions, while G-type antiferromagnetic ordering is due to the62nm spin spiral cycloid modulation of Fe sub-lattice. Currently, bulk BFO has been proven that not suitable for device application because of internal defects, such as the presence of impurity phase, high leakage current or different ferroelectric transition temperature. Further, bulk BFO seldom shows macroscopic magnetization due to a long-range cycloidal spin arrangement of wavelength62nm incommensurate with the lattice. To overcome these obstacles, in this thesis we have taken some attempts to improve the physical properties of the material, such as doping with small amount of iso-valence ions into bulk material, or preparing a nano-scale BFO material.We use a modified solid-state reaction method, prepared by the Ba2+and Ta5+co-doped BFO ceramic material. X-ray diffraction analysis confirmed the structure of the sample having the rhombohedral perovskite structure. With the increase of Ta5+dopant the dielectric constant and leakage current strength were reduced to some extent. Furthermore, for all sample, clear threshold switching effect appeared in leakage current measurement. We also carried out an impedance analysis and found that the resistance of the sample depends on the grain effect.Furthermore, alkaline-earth element ion Ca2+were employed as dopant to modify the BFO nanoparticles by sol-gel method. TEM analysis revealed that particle size is about140-25nm. By UV-Vis absorbance analysis, it is found that increases the amount of Ca dopant optical band-gap of BFO nanoparticles samples decreased from2.16eV to2.02eV. The main reason can be attributed to the emergence and accumulation of oxygen vacancies. Also for this reason, the leakage current density increased with Ca concentration. Moreover, higher conductivity leads to a significant threshold switching effect at room temperature. In addition, the measurement of dielectric constant, dielectric loss and magnetic properties showed that decreasing grain size and the introduction of Ca2+have a significant impact on the dielectric and magnetic properties of bismuth ferrite material.