Multiferroic Properties Of Layered Perovskite-related A_nB_nO_3n+2

Along with the rapid development of science and technology,the demand for the electronic device multi-functionalization and miniaturization,as well as low energy consumption is growing rapidly,exploring novel multi-functionalized materials is the only way for the development of future thechnology.Multiferroics,wherein magnetism and ferroelectricity can coexist in one material and couple mutually between them,offer great possible application in spintronic devices,novel information storage devices and novel magnetic-electric sensor,has attracted much attentions.At present,in the search for single multiferroic compounds,the incompatibility between magnetism and ferroelectricity in one materiel is the biggest obstacle encountered.To solve the above problem,one effective approach which has been verified is to intercalate magnetic species into the central octahedron layer of ferroelectric hosts,while maintaining ferroelectric displacements in the outer octahedron layers.This thesis focus on layered perovskite-related oxide A_nB_nO_3n+2,by means of floating-zone melting technique,six-layered perovskite oxide R₆Ti₄Fe₂O₂0 was successfully synthesized through the intercalation of double RFeO₃ magnetic layer into rare earth titanium R₂Ti₂O₇ matrix.For the first time,we report the multiferroic properties of R₆Ti₄Fe₂O₂0,systematically investigate their crystal structure,magnetic,ferroelectric and photocatalytic properties,open a new window to developing novel multifunctional material in layered perovskite-related series A_nB_nO_3n+2.This thesis is consists of five chapters and the main contents of each chapter are as follows:In chapter one,we introduce the main progresses on single-phase multiferroic materials in layered perovskite-related oxide.First,the research background of multiferroics are summarized,and two different groups of multiferroics are introduced concisely.Second,the research progresses of bismuth-based layered-structured compounds Bi_m+1Ti₃Fe_m-3O_3m+3 are presented.Finally,the related research of layered perovskite-related oxide A_nB_nO_3n+2 are highlighted,in order to provide the background information for the study in subsequent chapters.In chapter two,six-layered perovskite oxide R₆Ti₄Fe₂O₂0 was successfully synthesized,and its multiferroic properties are reported firstly,this open a new window to developing novel single-phase multiferroic material in layered perovskite-related series A_nB_nO_3n+2.The "glassy" magnetic behavior of La₆Ti₄Fe₂O₂0 can be understood by the coexistence and competition between two different types of interaction,which originate from both antiferromagnetic interactions between Fe³⁺-O-Fe³⁺ in the central layers of slabs and ferromagnetic coupling which is induced by oxygen vacancies from the titanium ions enrichment zone at the borders,owing to the nonrandom distribution of magnetic Fe³⁺ ions.The frequency-dependent behavior of the dielectric loss peak in La₆Ti₄Fe₂O₂0 manifests itself a thermally activated relaxation process.In chapter three,layered perovskite-related Sm₆Ti₄Fe₂O₂0 compound was successfully synthesized through the intercalation of bilayer SmFeO₃ into the Sm₂Ti₂O₇ with pyrochlore structure by means of floating-zone melting technique.The microstructural properties were characterized using aberration-corrected scanning transmission electron microscopy and X-ray diffraction.Electron energy-loss spectroscopy investigation reveals that the Fe³⁺ ions prefer to occupy the inner sites within the perovskite-like layers.This compound exhibits clearly the spin glass-like behavior as demonstrated by the magnetic properties measurement.Such complex magnetic behavior could be attributed to the partial chemical order of Ti/Fe over the B sites and the interactions between magnetic ions including Sm³⁺ and Fe³⁺.In addition,the multiferroic behavior with the coexistence of the ferroelectricity and ferromagnetism was well established by magnetic and piezoresponse measurements..In chapter four,the layered perovskite-related oxide Nd₆Ti₄Fe₂O₂0 was prepared by incorporating NdFeO₃ in the host Nd₂Ti₂O₇ using floating-zone melting technique.XRD and HRTEM results suggested that the material has a layered structure of n = 6 type.Nd₆Ti₄Fe₂O₂0 exhibited spin glass-like behavior,and its magnetic behavior was affected by magnetic Nd³⁺ ions strongly at low temperature.The ferromagnetic and ferroelectric properties were observed by magnetic and PFM measurements at the room temperature.UV-Vis absorption spectroscopy revealed that the compound is a visible light absorbing photocatalyst with a direct band gap of 2.2 eV.In addition,the photocatalytic behaviors of bulk Nd₆Ti₄Fe₂O₂0 were evaluated by photodegradation of rhodamine B under visible light irradiationIn chapter five,the brief summary and prospect of in this thesis are given.