| Since the discovery of colossal magnetoresistance effect (CMR) in per-ovskite manganites, it has sparked considerable renewed interests in these long-known materials with an eye towards both an understanding of the CMR and related properties and potential applications in magnetic information store and low-field magnetic sensors. Beside the CMR effect, these materials also exhibit intriguing physical properties such as insulator-metal and/or structure transition induced by applied magnetic-field or photo radiation, charge ordering, orbital ordering, and phase separation etc. The full understanding of these properties will definitely stimulate the progress of condensed matter physics. In this thesis, the author devoted his effort to the study of the magnetic and electronic properties in CMR oxides by experimental methods. The whole thesis consists of six chapters.1. A brief overview of magnetoresistance effectThis chaper aims at a brief overview of the history, progress and current status of all kinds of magnetoresistance materials, such as OMR, AMR, GMR and CMR and so on. By these illustration, we may acquire a basic sight on magnetoresistance effect as well as magnetoelectronics.2. An introduction to the physical properties of perovskite manganites.This chapter deals with the influent physics properties and some spectacular phenomenon observed in perovskite manganites, including the structure, magnetic and electronic transport, phase diagram, charge/orbital ordering, and insulator-metal transition induced by applied magnetic field or photo radiation etc. This part is helpful to build up a background for the research on colossal magnetoresistance.3. A review of the CMR mechanism in perovskites.This chapter gives review of CMR mechanism, earlier research works on GMR mechanism are listed. Some special physics concepts, such as double exchange(DE), Jahn-Teller(J-T) effect, electron-phonon coupling, are interpreted.4. Magnetism study onWe presented a detailed study of Dy-doping effect on magnetism. It is found the magnetism changes with substituting Dy3+ for La3+. The difference between the magnetic moments for the samples with respective doping level can be ascribed to the variation of the competition between thermal effect and the magnetic coupling. Based on the spin orientation rotation of Dy sublattice as well as the antiferomagnetic coupling between Dy sublattice and Mn sublattice, we successfully elucidate the changes of magnetic structure in perovskite compoundsS.Electron spin resonance(ESR) study on perovskite compoundsOn the basis of chapter 4, we give further study on micromagneticism of Dy-doping perovskite compounds La0.67-xDyxSr0.33MnO3. The influence on the magnetism and the electron spin resonance (ESR) over a wide range of composition and temperature have been studied systematically. An obvious symmetric ESR signal with a Lorentzian lineshape in the paramagnetic regime is observed. Below the curie temperature Tc the resonance lines spread and shift to lower field. Upon high doping, FM state separates from PM state above Tc, which is called phase separation. Comparing the intensity of ESR line with the magnetization curve, we gain that all spins contributing to the magnetization are really observed by ESR.6. Electronic transport and CMR effect in perovskite compoundsThe electronic transport properties of the Dy-doped perovskite compounds La0.67-xDyxSr0.33MnO3 have been investigated. The electrical resistivity curve was replotted with p-T2 and three models of conductivity. The minimum of resistivity at low temperature range can be explainedby magnetic scattering which corresponds to the peak of magnetization. All of the experimental data can be well explained on the basis of VRH model, which comes from not the clusters, but the inhomogenous magnetic background. Based on the mechanism of transport, CMR effect is elucidated.Conclusion:The influences of Dy doping at A site on the magnetism and transport properties in La0.67-xDyxSr0.33MnO3 system...
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