| The discovery of colossal magnetoresistance (CMR) in manganese oxides with perovskite structure have attracted a great deal interest for the potential applications in magnetic storage and magnetic sensors. The manganites exhibit a variety of electronic transport and magnetic properties because of the interaction among spin, charge and lattice. In this dissertation, we studied the structure, transport and magnetic properties of mananites. Some important results were obtained and the detailed experimental results are shown as follows:In chapter 1, the structure, transport and magnetic properties of manganese oxides were introduced, and the basic models to explain the properties, such as double interaction, were described in detail. In addition, the recent research of manganites were reviewed.In chapter 2, we have probed the magnetic behaviors of polarons by ESR measurements of polycrystalline samples La0.75(CaxSr1?x)0.25MnO3 (x = 0, 0.45, 1). The temperature dependence of the ESR intensity for the samples in the paramagnetic regime is described by a thermally activated model. It is found that the activation energy of La0.75Sr0.25MnO3 is higher than that of La0.75Ca0.25MnO3, which is attributed to the stronger exchange interaction in La0.75Sr0.25MnO3. For La0.75(Ca0.45Sr0.55)0.25MnO3, it is found that the activation energy in the orthorhombic phase is higher than that in the rhombohedral phase, which is suggested that the correlated polarons only exist in the orthorhombic phase and a higher energy is required to destruct the correlated polarons. This proposition is confirmed by the analysis of the ESR linewidth data, which can be well fitted by the model of adiabatic hopping motion of small polarons. At a fixed temperature the linewidth decreases with increasing Sr doping, which reveals that the tolerance factor has a significant effect on the linewidth.In chapter 3, the structure, magnetic and electrical transport properties of La0.5Sr0.5MnO3 annealed in air, nitrogen, and oxygen atmospheres, respectively, have been investigated. It is found that there is no evident change of crystal structure and the Curie temperature for the samples, but the magnetization is different, which may arise from the competition between FM and AFM phase affected by the annealing atmosphere. The resistivity at zero magnetic field of the samples annealed in air and nitrogen exhibits a metal-insulator transition, while no metal-insulator transition is observed for the sample annealed in oxygen, which decreases monotonously with increasing temperature. It can also be found that the resistivity for the sample annealed in nitrogen is smaller than that of the sample annealed in air, it is suggested that annealing in nitrogen will increase the FM regions which will contribute to the conductivity. Surprisingly, when an external magnetic field is applied, a metal-insulator transition appears for the sample annealed in oxygen. It is suggested that the different reports on the properties of La0.5Sr0.5MnO3 may attribute to the annealing atmosphere, which will have a great influence on the competition between FM and AFM phase and the electrical transport properties of the samples.In chapter 4, The influence of sintering temperature and annealing atmosphere on transport properties of La0.5Sr0.5MnO3 was investigated. It is found that sintering temperature and annealing atmosphere have a great influence on the transport properties. From SEM study, it is found that the grain size of the sample sintered at 1450℃is larger than that of the sample sintered at 1400℃, and the grain boundaries of the sample sintered at 1450℃are more compact. The resistivity of the sample sintered at 1450℃is smaller than that of the sample sintered at 1400℃. From the study of the electrical transport, it can also be found that annealing atmosphere have a great influence on the transport properties and the metal-insulator transition temperature. |
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