Studies On Field Dependence Of Magnetoresistance And Magnetic Annealing Effects In Half-metallic Manganite

Half-metallic manganites have attracted intense attention due to their abundant physical mechanisms and extensive applications. The perovskite manganite La2/3Sr1/3MnO3 (LSMO), with environmental stability and high spin polarization, is one of the well-known half-metallic ferromagnets. The relation between magnetization (M) and magnetoresistance (MR) in LSMO is usually applied to describe the inherent coupling between transport property and magnetic state. However, the studies of magnetic field (H) dependence of MR are insufficient in LSMO. The purpose of this dissertation is to give some formular description of MR(H) for comprehending the micromechanism of transport property in LSMO at the microscopic scale. Since the MR effect of polycrystalline LSMO is relatively low, we annealed La0.7Sr0.3MnO3 under magnetic field to enhance its MR effect. The main results are listed below:1. Magnetic field dependence of magnetoresistance in half-metallic manganiteFormular description of the magnetic field dependence of magnetoresistance is of great importance for not only comprehending the origin of magnetoresistance but also designing the spintronic devices. Within the framework of micromagnetic theory combined with charge carrier hoping and spin-polarized tunneling models, the magnetic field dependence of resistivity in half-metallic ferromagnet LSMO has been studied systematically. It is shown that the resistivity has a linear dependence with magnetic field in a single grain. As for the resistivity of grain boundary, namely the resistivity of spin transport across the boundary, it has an exponential relationship with magnetic field at low field regime. Finally we give the analytical MR formulas of poly crystalline LSMO, which agrees well with the experimental results.2. Research on the enhanced magnetoresistive response in magnetic-annealed La0.7Sr0.3MnO3Polycrystalline perovskite La0.7Sr0.3MnO3 was annealed under magnetic field in order to increase the low-field magnetoresistance effect. The experimental results show that the magnetic annealing would not affect the magnetic moments but remarkably change the magnetic anisotropy of the sample. Compared with the perovskite without magnetic annealing, the magnetic-annealed La0.7Sr0.3MnO3 exhibits large magnetoresistance effect, which can be more obviously observed in low magnetic field. At room temperature, a 140% enhancement of magnetoresistive response at 0.1 T was observed in the magnetic-annealed sample, which can be explained with the mechanism of spin-polarized intergrain tunneling.