The Giant Magnetoresistance Effect In Perovskite Mn Oxides

Nowadays, computer tends miniaturization quickly, which requires high-density recording media as well as computer reading and writing head materials. The computer reading heads fabricated by using the anisotropic magnetoresistance (AMR) and metal multilayer film giant magnetoresistance (GMR) materials have come into commercialization. Recently, colossal magenetoresistance (CMR) effect has been observed in the perovskite-like manganese oxide bulk materials. In this kind of material, magnetism, conductivity and crystal structure interplay and abundant physical phenomenon appear. Perovskite manganese oxide has aroused much interest due to its potential application and theoretical research value.This thesis investigated systematically the influence of doping at A, B site on structure, magnetism, conductivity and MR of ABO3 perovskite manganese oxide, it also investigated the influence of process condition on the structure and CMR of self-doping La1-|?Mn1-|?O3 system.Samples were prepared by the Sol-Gel technique and sintered in air at different temperatures. The structure, magnetism and conductivity (H=0, 1T) of these samples were studied in detail. The main results are as follows.(1) The structure of self-doping La1-|?Mn1-|?O3 vary in the order of cubic-rhombohedral-orthorhombic with the increase of sintering temperature. The Mn4+ contents in the sample were determined by means of redox titration. We found that the samples sintered at 800-1200oC shows ferromagnetism (FM) and CMR above liquid nitrogen temperature. Two peaks in the R-T curve of the samples sintered at 1200??C for 24 hours was observed. In the MR-T curve, asharp peak was observed near the Curie temperature. The MR value at the peak amounts to 37%, which is much higher than that for other samples at the same temperature and applied magnetic field.(2) The La2/3Ba1/3MnO3 samples sintered at various temperatures are of single phase with a cubic structure. As the sintering temperature increase, the grain size of the samples increases, the resistance decrease, and the M-I transition shift toward high temperature. The sample with nominal composition La2/3Yb1/3MnO3 is not a single phase. This mixing-phase sample exhibits a large CMR effect with MR=30%. The sample with nominal composition La2/3Eu1/3MnO3 sintered at 1200 oC is of single phase with orthorhombic structure and is a antiferromagnetic isolator without MR effect above 77 K.(3) The effect of substitution of various transition metals (Cr, Fe, Co, Ni, and Cu) for Mn in (La,A)MnO3 (A=Sr, Ba) on the magnetic property and conductivity was investigated. It was found that all substitution suppresses the Mn3+-Mn4+ double exchange interaction, resulting in a decrease in Tc and an increase in resistance.