Study On The Microstructure And Physical Properties Of Yttrium-doped Lanthanum Cobaltite Thin Films

Lanthanum cobalt oxide?LaCoO₃?has stable crystal structure,unique electromagnetic properties and high electrocatalytic activity,which has an extensive application prospect in the fields such as information storage,environmental protection and industrial catalysis.Doping strontium?Sr?or introducing oxygen vacancy into the films can significantly change the physical properties of LaCoO₃ films.In this thesis,the effect of Sr-doping or oxygen vacancy on the microstructure of the La_1-xSr_xCoO₃?LSCO?films was investigated using high-resolution transmission electron microscopy?HRTEM?in details,and was further correlated with their electrical and magnetic properties.This thesis consists of four chapters.In chapter 1,the crystal structure,previous theoretical models and research development of perovsikte oxides were reviewed.The microstructure and physical properties of perovskite oxides were introduced;the recent research development of LSCO system was analyzed;and the future research direction was pointed out;the purpose and significance of this research were clarified.In chapter 2,the microstructure of annealed and unannealed La_0.33Sr_0.67CoO₃ thin films was systematically investigated using HRTEM,and their physical properties were correlated.It is found that the modulated structure in A sites is induced by cation ordering in the unannealed film.In the annealed film,the modulated structures in A and B sites are ascribed to cation ordering and oxygen vacancy ordering,respectively.After annealing the ferromagnetic-metallic film in the vacuum,it turned into a non-magnetic semiconductor,which is attributed to the oxygen vacancy ordering.In chapter 3,different oxygen-deficient La_0.33Sr_0.67CoO₃ thin films were prepared by adjusting the oxygen pressure during the deposition.The structure and microstructure of the films were revealed by x-ray diffraction analysis and HRTEM observations,and their magnetic properties were correlated.Oxygen vacancies lead to the elongation of out-of-plane lattice of LSCO films.Combining the magnetic properties test with first-principlecalculations,it is proved that the oxygen vacancies affect the exchange of cobalt ions by changing the valence state of cobalt ions,and thus greatly suppress the magnetic properties of the film.The working mechanism is similar to that of Sr-doping.In chapter 4,the effects of Sr-doping and oxygen vacancies on the electrical transport properties were investigated.As doped Sr increased from 0 to 0.3,the electrical transport mechanism transferred from variable range hopping to double-exchange,and the film simultaneously turned from an semiconductor into a conductor.From HRTEM observations,it is found that,only when the doping content reaches 0.3,the double exchange effect of LSCO film is the strongest and the electrical conductivity is the best.