Perovskite Manganites The Charge Ordering State Stability And Phase Separation Of Interface Effects

The perovskite-type manganites have attracted considerable attention in recent years because they show rich physics as well as application potential. Since the manganites are spin, charge, orbit and lattice degree of freedom strongly correlated systems, the competition and coupling among all kinds of interactions lead to very complex electronic, magnetic and structural phase diagrams, and a lot of interesting phenomena such as the charge, spin and orbital orderings as well as phase separation, etc.. The intensive study is focused on the structure of all kinds of orderings and their effects on the electronic and magnetic properties, as well as the origin of phase separation and its relationship to the colossal magnetoresistance. However, the stability of the charge ordered (CO) state and the interfacial effects in phase separated manganites are left. The dissertation mainly focuses its attention on the stability of the CO state and the interfacial effects of different phases on the electronic and magnetic properties in phase separated manganites. The main contents in the dissertation are presented as follows:In chapter 1, we make a brief introduction to the research history, the crystal structure and lattice distortion, the essential physical mechanism such as the double exchange interaction, the Jahn-Teller (JT) effect and the superexchange interaction, the electronic and magnetic phase diagrams of La_1-xCa_xMnO₃ and other systems, the phase separation in all kinds of systems with different doped region, the charge ordering and its effect on the electronic and magnetic properties as well as its stabilization, Zener polaron ordering, manganite heteroepitaxial p-n junctions, and the latest evolution on application potential.In chapter 2, the effect of JT interactions on specific heat and magnetic propertieswas studied for the series of La_1-xCa_xMnO₃ (0.55≤x≤0.87) compounds by measurements of specific heat, ultrasonic velocity, and magnetization. Upon increasing x from 0.55 to 0.75, the enhancement of the JT interactions suppresses the ferromagnetic fluctuation above the CO transition temperature TCO, increases the change of entropy associated with the charge/orbital ordering transition, and therefore...