The Magnetoresistance Effects In Rare Earth Manganites Of La_2/3(Ca_1-xMg_x)_1/3 MnO₃ And La_1-xSr_xMnO₃/CuO Granular System

This thesis is based on the purpose of studying magnetoresistance (MR) effect in La2/3(Ca1-xMgx)1/3MnO3 system and Cu-dependent material coated-La1-xSrxMnO3 granular system. In the first part of this thesis, we have studied the colossal magnetoresistance (CMR) effect in La2/3(Ca1-xMgx)1/3MnO3 system, explored the mechanism of the CMR ,and discussed the several important factors to this effect. In the second part, we have studied the magnetoresistance (MR) effect in Cu-dependent material coated-La1-ySryMnO3 granular system, and considering the contrast function of other several similar system, we systemically study the behavior of these composite materials, and the main research results are as follows: In the first chapter, the research history and the fact application of the colossal magnetoresistance materials are introduced. the crystal structure, electron structure, magnetic structure and electric transport properties of perovskite manganites are summarized, and then several theoretical work about these materials are described, especially, some work about the main factors, which affect the electric transport properties and MR effect of RE1-xAExMnO3 compound. Then some experiment and theory work to low field MR effect for perovskite manganites are introduced. Lastly, the purpose of research work in this thesis is giaven. In the second chapter, we focused attention on the experiment methods used in our research work. We list the detailed operating process of Sol-Gel preparation method, and analyzed the crystal structure by X-ray and our SEM results of other similar samples. The transport and magnetic properties were measured in liquid nitrogen measure system and commercial Physical Property Measurement System (Quantum Design PPMS). Finally, we explained the MR with the A-site cation size effect. In the third chapter, a series of composite samples of La2/3Sr1/3MnO3/CuO was studied. Firstly, the sample preparation method was given in details, and then the transport properties and MR were discussed by difference heat treatment temperature. Results show that Cu content and heat treatment both can bring a large effect to the transport and magnetic properties for our composite sample. At last, depending on the results of high temperature heat treatment sample, we concluded the combination course of the LSMO grains. In the fourth chapter, the electric transport properties and MR effect of La0.8Sr0.2MnO3/CuO, La0.8Sr0.2Mn1-xCuxO3 and La2/3(Sr1-yCay)1/3MnO3/CuO samples were discussed. The resistance of La0.8Sr0.2Mn0.9Cu0.1O3 sample exhibits a minimum near 40K, that results well fit thermal activation form of lnρ=T-1/2. Finally, after comparing the electric transport properties of La2/3(Sr1-yCay)1/3MnO3/CuO samples and La2/3Ca1/3MnO3/CuO samples cited from literatures, we concluded that the Ca2+content can largely affect the electric transport properties of La2/3(Sr1-yCay)1/3MnO3/CuO system.