| Recently, since the discovery of colossal magnetoresistance effect (CMR) in perovskite 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.Nowadays, it has been extensively accepted that the double exchange interaction plays a key role in perovskite manganites. Therefore, in this thesis the effect of the double exchange is studied by doping on Mn sites. To the universal behavior that there is not insulating-metallic transition in ferromagnetic background, we proposed a new view to explain it. Furthermore, this opinion had also been proved by some experiments. Therefore, the previous view about this phenomenon was overthrown.The whole thesis consists of four chapters.1.A brief overview of magnetoresistance effect. This chapter aims at a brief overview of the history, progress, and current status of all kinds of magnetoresis-tanc materials, such as magnetic multilayers, perovskite oxides, magnetite, and so on. By these illustration, we may acquire a basic sight on magnetorresistance 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 structural, magnetic, electronic transport, phase diagram, charge and orbital ordering. Some special physics concepts,suchas double exchange, Jahn-Teller effect, electron-phonon coupling, are interpreted. This part is helping to build up a background for the research on colossal magnetore-sistance.3.The theory of double exchange and mechanism of CMR effectexchange.In this chapter, The theory of double exchange will be analyzed in detail. In addition, two kinds of theory about CMR Mechanism are also dicussed.4.The effect on the perovskite CMR material Lao.erSro.asMnOs by doping Mg on Mn site. This chapter is focused on the change of CMR material Lao.eTSro.saMnOa by doping Mg on the Mn site. We observed that there exits insulating-metallic transition in the low doping region, but without that in the heavy doping zone, by the measurement of magnetization, resistivity, and electron spin resonance. We suggest that this behavior attributes to the difference between the Jahn-Teller distortion of the Mn3+ ion neighboring on Mg2+ and that of the Mn3+ion non-neighboring on Mg2+, which is confirmed by the IR and Raman spectra. The anomalous transport behavior is dominated by the different Jahn-Teller distortion. This opinion is also proved well by our experimental results.
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