| In recent years, many semiconductor devices which are designed by magnetoresistance effect have been widely accepted because of its outstanding performance. Perovskite manganese oxides which has the complex charge, spin, orbitals and lattice degrees of freedom that are strongiy coupled to each other have aroused much attention because of its huge magnetoresistance effect and abundant physical phenomenon. Studying the magnetoresistance effect of perovskite manganese oxides provides us some new approaches to research strongly correlated electron systems and will certainly improve the development of magnetoresistance materials.In this dissertation, we chose the manganese oxide La0.67Ba0.33Mn1-xZnxO3(0≤x≤0.2) and La0.5Sr0.5Mn1-xCrx03(0≤x≤0.3) as the study objects, and investigate the effect of doping on Mn sites, their crystal structure, microstructure, magnetic, transport properties, and magnetoresistance have been studied. We find that Zn-doping can not only increase the samples’magnetoresistance but also widen the temperature zone of MR, Cr-doping can disturb the exchange effect between Mn3+and Mn4+, and introduce a new double exchange. In this way the macroscopic magnetism of the samples will be changed.This paper includes below four parts:The first chapter elaborates research background of this paper firstly. Then we give a brief introduction of the knowledge and theoretical model of magnetoresistance effect.In chapter two, we describe the preparation methods and testing methods for polycrystalline samples.In chapter three. Zn-doping La0.67Ba0.33Mn1-xZnxO3(0≤x≤0.2) samples have been prepared by conventional solid-state reaction method. We have systematacially investigated the influence of Zn-doping on the structure, transport properties and magnetoresistance. The X-ray diffraction results revealed that all the samples exhibits simple cubic perovskite structure at room temperature. The M-T curves for zero filed-cooled (ZFC) condition in a magnetic filed of1000Oe suggests that with the increase of the temperature all the samples exhibit a ferromagnetic to paramagnetic phase transition. The Curie temperature and the intensity of magnetization are found to decrease upon Zn doping. Compared with un-doping samples,when x=0.1the Curie temperature and intensity of magnetization of all the samples have no obvious change but when x=0.2the Curie temperature and intensity of magnetization experience a remarkable decrease. The temperature dependence of resistance curves are measured under H=OT and H=2T, which reveals a metal-insulator transition, with the substitution of Zn the Tim move to low temperature zone, but the resistance of all the samples increase gradually. All samples around Tim appear a peak of MR, the value of MR increases from-22%(x=0.0) to-73%(x=0.2)。With the increasing of doping amounts, the temperature zone of MR has been widened.In chapter four, Cr-doping La0.5Sr0.5Mn1-xCrxO3(x=0.0,0.1,0.2,0.3)samples have been prepared by conventional solid-state reaction method. The X-ray diffraction results revealed that all the samples exhibits simple cubic perovskite structure at room temperature. Because the ionic radius of Cr is smaller than Mn, with the increase of doping content, the lattice constant of materials decrease gradually. Cr ion has specific properties, the magnetism of Cr-doping materials do not present single variation trend. When the doping content reach30%, the magnetism of materials Shows a trend of rising. The experimental result shows that the exchange interaction between Mn3+-O-Cr3+and Mn3+-O-Mn4+is double exchange interaction.At the end of this paper we summarizes the research achievement and Proposes the research forecast. At the same time, we also point out some questions which are not been solved and need our Ongoing research... |
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