A Study Of Preparation And Properties Of La_0.7Ca_0.3MnO₃ Composites Coated By Cu Using Electroless Process

Since the observation of colossal magnetoresistance (CMR) in perovskite manganites, these compounds had brought considerable attention in the study of these compounds because of their scientific and practical application. It is generally thought that the CMR effect should be closely related to the field-induced alignment of Mn spins, which is belong to the intrinsic MR effect. However, MR magnitude of these materials, which are commonly put in the very large applied magnetic field, reached a maximum in a narrow temperature range. At the same time the larger magnetoresistance in this oxide was only observed in a low temperature and decreased rapidly with the increase of temperature, which severely limit their practical application. Towards this end, another property of perovskite manganites, extrinsic MR effect, can be utilized in the form of the"MM/ML/MM"structure,which just like sandwich structure, there MM means magnetic material and ML means middle layer. In our work, we take La0.7Ca0.3MnO3, which were prepared by solid state method, as MM, and Cu, which were made into the interface of La0.7Ca0.3MnO3 of grains by special chemical process, as ML. We observed that the compound have special features in transport properties. Especially, we have observed the special low-field magnetoresistance (LFMR) and anisotropic magnetoresistance (AMR) effect of the Cu-coated system. In this thesis, the main investigation including:(1) A summary of the formation and its physical problems of the structure, properties and magnetoresistance in the manganese oxides have been given. We summarize the research work related to the FM/ML/FM material, and the fundamental of theory and experiment of the thesis has put forward on the basis of it.(2) The influences of different preparation procedures are compared and optimal one is chosen. We prepared the manganese oxides using the sol-gel method and investigated the electric properties of La0.7Ca0.3MnO3.(3) A summary of electroless process was given, the LCMO powders coated by Cu were obtained by the electroless plating technique. The microstructure of these powders was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). (4) For the electroless process of composite La0.7Ca0.3MnO3-Cu, we observed that sintering temperature evidently affects the results, because the sintering temperature results in the formation of Cu among the La0.7Ca0.3MnO3 particle of boundaries. We optimized the preparation procedure for the composites by changing the sintering temperature.(5) For a low applied magnetic field (0.3T), the composite which were sintered at 500℃displays a familiar electric properties with matrix La0.7Ca0.3MnO3,which derive from the reduction of electron scattering among boundaries. A behavior of anisotropic magnetoresistance effect (AMR) which is different from matrix La0.7Ca0.3MnO3 was observed, the AMR is different from the intrinsic CMR of LCMO. Our observation may reveal the special features of MR due to the introduction of Cu into the grain boundary of LCMO, which is the extrinsic CMR.