The Structure,Spectra And Multiferroics Of Modified Bismuth Ferrite Thin Films

Materials possessing two or more than two ferric phase,including ferroelectrics,ferromagnetism and ferroelastics in one system are call multiferroics.Multiferroics was first proposed by French scientist Pierre Curie for the magnetoelectric effect in19^th century.However,it was first observed in the hydrogen materials in last 30^thh century.Till then,the investigations were able to be available in experiments.Due to the unique properties,mutiferroics are recognized as the most possible materials to be applied in the future information technology,sensors,spin electronics,etc.Among them,bismuth ferrite?BiFeO₃?was favored not only for its coexistence of ferroelectrics and ferromagnetism,but also for its high ferroelctric Curie temperature?T_C?and its high antiferromagnetism Néel temperature?T_N?which are both greatly higher than room temperature.These properties make BFO a powerful competitor for the magnetoelectric material under room temperature.BFO have received a lot of concentration by researchers,but the work of high quality epitaxial film obtained by Ramesh in 2003 rekindled the interests to its outstanding properties and the novel problems inside it.In the last decades,there are quite some works have been done on BFO,which can be separated in two directions.One is to replace the A site or B site of the pure BFO to make the solid solutions.The other one is to fabricate thin films through the strain constrained by the substrate,which is also call strain engineering.Besides,to fabricate the films with the components with solid solutions is adopt as a common method.These works emphasized on the improvement of the crystal structure,the electrical properties,including the giant ferroelectric polarization and the electro-resistive effect,etc.and the magnetic properties such as the spin spiral structure.Particularly,due to the demanding needs for the devices of high density,low energy cost and fast read/write speed,BFO has a bright application future in the field of magnetoelelctrci effect in room temperature.We focuses on the fabrication of the?110?,?111?and?100?-oriented epitaxial BFO with pure rhombohedral phase deposited by pulse laser deposition system?PLD?.We also did a series of characterizations on the crystal structure,topography,electrical property and magnetic property.The coexistence of the ferroelectrics and ferromagnetism was demonstrated in our films.In this case,we tried to carry out the design and test the magnetoelelctric effect.This thesis is present in the following order,1.The introduction of the multiferroics are given to clarify the definition and typical mutiferroic materials,especially for the bismuth ferrite.We also give basic ideas about the symmetry of the materials which are essential for the single-phase materials and the thermodynamics involved in our target in the film fabrication.2.The introduction of the pulse laser deposition system and the monitoring accessory,reflecting high energy electron diffraction?RHEED?.The advantages and disadvantages of the system are mentioned and give a relationship with our materials.The principles of the equipment used for topography and structure characterization.The crystal structure was identified by the X-ray diffraction.Scanning electron microscope was used to confirm the thickness as well as the growth pattern of the films.Atomic force microscope and piezoelectric force microscope was used for the topography and primary ferroelectricity characterization.X-ray photoelectron spectroscopy was used to analyze the valence of the element in the film.3.The electrical properties of the film were measured in capacitor structure,including the dielectric constant and low dielectric loss?⁰.05?,Arrhenius plot to exclude the existence of the oxide vacancy within the measuring range,and P-E hystereis?P_r⁴6.2?C/cm²?.The?110?-oriented film grown on the non-conductive substrate was used to the magnetic measurement,M_r^<sup>4.6emu/cm³.Magnetoelectric test was carried out by the available devices by combining the electrical sources,preamplifiers and the designed Helmholtz coils.