Perovskite Manganites The Charge Order, Orbital Order, Spin Order And Their Interaction

The studies of transition-metal oxides with perovskite structure started in 1950's. In the recent ten years, the renaissance of these materials is due to the exciting discovery of high-T_c (high temperature superconductivity) in cuprates and CMR (colossal magnetoresistance ) effect in manganites. As we know, MR effects are great valuable in industrial demand such as the read-head of the magnetic memory, magnetic sensors, etc. Most importantly, as a strongly correlated electron system, these materials also exhibit intriguing physical properties such as insulator-metal and/or structure transition induced by applied field or photo radiation, charge ordering, orbital ordering and phase separation etc. Once the mechanism of the CMR effect is fully elucidated, the progress in many fields of condensed matter physics will be definitely stimulated. In this thesis, the author devoted his effort to the study of the fundamental physics in doped perovskite mangnites, by both experimental and theoretical methods.The whole thesis consists of six chapters.The chapter one: a brief introduction to the condensed matter physics. By these illustrations, we may acquire a basic sight of the correlated electron physics.The chapter two: a brief overview of the progress of the perovskite mangnites and related properties. This chapter devotes to a review of the magnetoresistance phenomena and related physical properties of perovskite mangnites, such as the structural, magnetic, electronic transport, etc. Some physics concept, such as double-exchange, Jahn-Teller effect, etc. are interpreted. This part is helping to build up a background for the research.The chapter three: a brief overview of the progress of ordering and unordering in perovskite mangnites. This is the base for the research of the author.The chapter four: by comparing the influence of Cr³⁺-doping and Ga³⁺-doping on the double-electron doped Ca_0.875Ce_0.125MnO₃ system, it is experimentally proved that there is a strong coupling interaction between CO and SO in the CO/C-type AFM system.The chapter five: by Ga³⁺-doping on the Nd_0.5Sr_0.5MnO₃ system, we proved that the CO phase coexists with the ferromagnetic phase below 240K in the Nd_0.5Sr_0.5MnO₃ system. The temperature (～240K) at which the CO phase is formed is much higher than the T_CO(～150K) which is reported by almost previous work.The chapter six: by comparing the influence of Cr³⁺-doping and Ga³⁺-doping on the Nd_0.5Sr_0.5MnO₃ system, it is experimentally proved that there is a strong coupling interaction between CO and SO in the CO/CE-type AFM system.