Study On Magnetic Properties Of New Five Layered Sillen-Aurivillius Oxides

Perovskite-like layered oxides refer to a kind of oxide materials containing simple perovskite slabs as the main constituent units.Because of their special and rich crystal structures and electronic structures,a series of interesting physical properties have been born,such as multiferroic,giant magnetoresistance,high temperature superconductivity,photocatalytic activity,etc.As electronic devices enter the post-Moore era,people are paying more attention to find new types of material systems to meet increasingly high and complicated performance requirements.With the increasing understanding of electron spins,it was found that transmission of electron spins has shorter magnetic interaction distance and lower energy input than those of charges,which can effectively avoid the limitations caused by the size effect In recent years,what exciting us is the study of spin quantum devices composed of ferromagnetic insulators and other materials,which makes it possible to substitute the spin wave for the charge as the transmission method to achieve low or no loss in energy and information transmission.Such spin related devices are also to be expected to play importance role in quantum computing,energy transmission,information storage and quantum information technology in the future.However,current perovskite-like ferromagnetic insulators generally have the disadvantages of low Curie temperature(below the room temperature)and low compatibility between insulation and ferromagnetic properties,which severely limits the application of such materials.Sillen-Aurivillius oxides are a type of special perovskite-like layered oxide materials,of which the structure is mainly formed by stacking simple perovskite slabs and Sillen layers alternatively.Due to the existence of Sillen layer,Sillen-aurivillius oxides not only have the rich structures and the coupling of various quantum order parameters,but also have higher magnetic ion concentration and/or better insulation performance than other perovskite-like layered oxides,which makes it a potential carrier for transmitting quantum spins in the future.Abundant physical mechanisms and functions may be realized on these materials,which should contribute to the development of multi-functional quantum spin devices.In this thesis,based on the summary and combination of the multifunctional quantum material systems constructed by perovskite-like layered oxide materials,anew type of perovskite-like layered oxides,five layered Sillen-Aurivillius oxides,is proposed and prepared using structural and compositional modulation methods.And the effect of these modulations on magnetic performance and exchange bias effect were studied.Four parts(four chapters)are included in this thesis,shown as follows.The first chapter gives the review part of this thesis,which mainly introduces the following aspects:(1)the crystal and electronic structures of perovskite-like oxides and the common methods of structure modulation along with the types of magnetic interactions involved;(2)the concept of exchange bias effect,and the significance of ferromagnetic insulators and the limitations of the current material systems;(3)the research purpose and contents of this thesis.The second chapter mainly describes the design ideas of the new five layered Sillen-Aurivillius oxides,and the methods of preparation,characterization,performance testing,and magnetic impurity analysis used in this thesis.Firstly,the preparation method of ceramic precursors and the sintering methods of ceramic used in this thesis are introduced.Secondly,the characterization methods of crystal structure and electron configuration of materials,including X-ray diffraction,X-ray absorption,X-ray magnetic circular dichroism chromatography,Raman spectroscopy and high-angle annular dark field scanning transmission electron microscope,are introduced.Thirdly,the test methods of samples' magnetic properties and electrical transport properties are introduced.Finally,the analysis methods of magnetic impurities in perovskite-like layered oxides is introduced.The third chapter is the main part of the thesis,and can be divided into two subsections:(1)Based on the structure modulation of Sillen-Aurivillius compound via perovskite layer control,a new five layered Sillen-Aurivillius oxide Bi8Fe3Ti2O20Cl(B8FTOC1)was prepared,and the effect of perovskite layer number on the exchange bias performance was studied.It was found that with the increase of the layer number of perovskite slab from 4 to 5,the magnetic ions(Fe3+)have increased from 50%of the total number of B-site ions to 60%.This increasement has successfully improved the exchange bias performance of the sample,and the exchange bias field reached 639 Oe at 50 K,about 64%higher than that of the four-layered one.(2)On the basis of above research,a new compound Bi8Fe2.8Co0.2Ti2O20Cl(B8FC0.2TOC1)with Co-doped in B-site was prepared,focusing on the effect of transition metal dopant on the magnetic properties of the material.It was suggested that Co doping could reinforce the deformation of FeO6 octahedral and make the formation the Dzyaloshinskii-Moriya(DM)interaction of Fe-O-Co,which jointly enhanced the magnetic properties of the system.Compared with Aurivillius compound[Bi2O2]2-[An-1BnO3n+1]2+,B8FC0.2TOCl has better magnetic properties under the same cobalt doping amount because of its higher magnetic ion concentration due to the existence of Sillen layer[(Bi2O2)2+Cl-(Bi2O2)2+]3+,which effectively reduce the total valence of cation in B-sites.More importantly,an excellent high-temperature ferromagnetic insulating material with high symmetry was successfully prepared.At room temperature(300 K),the residual magnetization(2Mr)is 0.77 emu/g,and the resistivity exceeds 1010?*cm.Meanwhile,it has a high Curie temperature of 804 K and a uniform and ordered layer structure.The fourth chapter summarizes the whole work,and gives the future study direction.