| 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, charge ordering, and phase separation etc. The full understanding of these properties will definitely stimulate the progress of condensed matter physics and material physics. However, in previous work, large CMR effect is achieved only in a strong magnetic field of several Teslas and in a narrow temperature range around Curie temperature, severely limiting their practical applications. Thus, reducing the field scale and increasing the operation temperature are the key points to be studied widely. One way to promote CMR effect at room temperature is to choose the parent materials with high Curie temperature. By tuning the composition of these compounds, e.g.,doping ions into the A or B sites with diferent ionic sizes as well as adjusting the appropriate dopant amount, the CMR efect can be improved a lot. Introducing second phase particles, such as insulating oxides, ferrite materials, aliens or kindreds with diferent grain sizes, into granular manganite matrix can also cause considerable MR effect. Many researches revealed that MR effect in such composites can be obtained in a wide temperature range, which indicates a promising way to practical applications. In this thesis, we use the optimal hole-doping manganites of La2/3Ca1/3MnO3 as the mother material, and compound it with the heterogeneous structure oxide of Cr2O3 and Bi2O3 separately. we try to bring Cr2O3 and Bi2O3 into the grain-boundaries of La2/3Ca1/3MnO3, changing its electric and magnetic properties, to enhance the MR. The main investigation of my thesis as the following: 1,A brief overview of magnetoresistance,structure, electric and magnetic properties in the manganese oxides has been given. We summarize the previous research work related to the LFMR effect, on the basis of which we has put forward the fundamental of theory and experiment of the thesis.2,This part aims at the preparation methods of the samples, including the solid state reaction, the sol-gel method and the composite method of the sample. The electric, magnetic properties and magnetoresistance of La2/3Ca1/3MnO3 are investigated. Results show that sintering temperature evidently affectes the results by changing the grain size of La2/3Ca1/3MnO3.3,We have experimentally investigated the effect of high resistivity Cr2O3 content on the electric,magnetic properties and magnetoresistance of (1-x)La2/3Ca1/3MnO3/xCr2O3 composites . It was found that with the increase of Cr2O3 content, the insulator-metal (I-M) transition temperature Tp decreases sharply, and the transition is completely absent for Cr2O3 content x>0.1. Meanwhile, the low-temperature resistivity increases dramatically due to the Cr2O3 content. Specially, an enhanced magnetoresistance with a broadened temperature window is observed in the composites.4,The electrical transport properties in the zero magnetic field of the (1-x)La2/3Ca1/3MnO3 / x Bi2O3 system and the La2/3Ca1/3(Mn1-xCrx)O3 system have been given. We compare these results in three systems and simply give the qualitative indicant.5,The conclusion.
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