Multiferroic Properties Of Bismuth Oxides With Layered Pervoskite Structure

Multiferroics refer to a big class of materials which simultaneously exhibit two or more primary ferroic orders,such as magnetic,ferroelecrtic,....,in a single phase.In 21th century,multiferroic materials experience a booming development,which is spurred by the following two aspects.On the one hand,viewed from the basic research point,the coexistence of spin,charge,orbit,lattice orderings in a single system make multiferroic materials can response to a several of external stimulations,such as magnetic field,electric field,optical field,stress and temperature.This complex,interdisciplinary system is an ideal object for research,and drive people to lucubrate.On the other hand,viewed from the application point,multiferroic materials have two or more functional features in a single system,and can be used to design new devices with multi-functionalization and miniaturization.During the first ten years in 21th century,the research interest of multiferroic materials was mainly on ABO3 simple perovskite structures,among which a great success has been achieved.However,room-temperature strong coupling multiferroic materials still have not been discovered.Thus in the second decade of 21th century,researchers turn to more complex system,such as layered perovskite,to hunt for new multeferric materials.There are four series of structures,Dion-Jacobson,double perovskite,Ruddlesden-Popper and Aurivillius,being proposed to have potential multiferroic properties.Among them,Aurivillius series structure,as a natural ferroelectric material,is demonstrated to exhibit multiferroic properties by doping magnetic ions.But the magnetic property of Aurivillius structure is disputable.Besides,it is still unknown that the effects of magnetic ions doping on ferroelectric,magnetic and multiferroic properties are still unknown in such an intergrowth superlattice system.Focusing on these problems,this dissertation proceeds a systemic study.The main contents of the thesis are divided into five chapters.In chapter 1,starting from a brief introduction of history,definition as well as classification of multiferroic materials,we respectively reviewed the research results of multiferroic properties of Dion-Jacobson,double perovskite,Ruddlesden-Popper and Aurivillius series structures.In chapter 2,we prepared the polycrystalline samples Bi4NdTi3FeO15(BNTF)by a conventional solid-state reaction method,and systematically studied the vacuum annealing effect on the structure,ferroelectric,dielectric and magnetic properties of Bi4NdTi3FeO15.After vacuum annealing at 800?,the unit cell of the sample has a significant shrinkage,which will suppress the ferroelectricity and dielectric constant.Besides,a ferromagnetic coupling is induced into the system due to the increase of oxygen vacancy concentration.These findings would be popularized to explain the variations of ferroelectricity and magnetism for other Aurivillius compounds synthesized in different conditions.In chapter 3,through the substitution of Fe by Co in Bi4NdTi3FeO15,we found that the as the cobalt doping concentration increases from 0 to 1,the samples suffer a structure evolution from 4-layers to 3-layers,implying that a 4-3 intergrowth Aurivillius system Bi4NdTi3Fe1-xCoxO15-Bi3NdTi2Fe1-xCoxO12-?(BNTFC-x,x = 0,0.1,0.3,0.5,0.7,0.9 and 1.0)was synthesized.Then we studied the structure,ferroelectric,magnetic and magnetoelectric coupling properties of BNTFC-x,and found that the remanent magnetization and polarization show opposite variation tendencies against the doping level,and the sample with x = 0.3 has the largest remanent magnetization and the smallest polarization.Besides,a magnetoelectric coupling coefficient of 1.24 mV/cm·Oe was observed at x = 0.5.In chapter 4,we improved a methodology for the detection,localization and quantification of the magnetic inclusions Fe3-yCoyO4(0?y?3)in BNTFC-x via energy selective backscatter image and energy dispersive X-ray analysis.By calculating the magnetization of the inclusions,we found that for the samples with ferromagnetic signals,namely,x = 0.1,0.3,0.5,and 0.7,the magnetic contributions to the corresponding main phases of the inclusions are conservatively estimated to be about or smaller than 3.9%,1.9%,3.8%,and 1.5%,respectively.These results indicates that the magnetic results do represent intrinsic information of the main phase.In addition,based on the X-ray diffraction refinements in Chapter 2&3 and energy dispersive X-ray analysis results,we also determined the actual stoichiometries of the samples.In chapter 5,using pulsed laser deposition technique,we grown SrRuO3 film with thickness of 22 nm as the bottom electrode on(001)-oriented SrTiO3(STO),(La0.18Sr0.82)(Al0.59T0.41)O3(LSAT)and LaAlO3(LAO)substrates,and then in situ grow 3-layered Aurivillius Bi3NdTi2Fe0.5Co0.5O11.5 film with thickness of 187 nm.The X-ray diffraction and scanning transmission electron microscopy images indicate that the films mainly grow along(001)direction,where a lattice imperfection was observed inside the film and small non-c oriented grains was found at the surface of the films.At room temperature,the film grown on LAO substrate exhibits the largest ferroelectric polarization(10 ?C/cm2)and magnetization(9.8 emu/cm3),together with magnetodielectric effect(15%)and magnetoelectric coupling effect(8 V/cm·Oe).