| The manganese-based perovskite oxide which has colossal magnetoresistance (CMR) effect had been extensive studied, due to its plentiful physical connotation and potential applications in various devices. In recent years, the low-field magnetoresistance (LFMR) effect has become the hot topic in the research of CMR. LFMR only requires hundreds or dozens of gauss, which has application prospects in actual spintronic devices. Different from CMR, LFMR is an extrinsic characteristic originating from spin-polarized electron tunneling instead of the double-exchange mechanism. Ordinarily, enhanced LFMR is obtained by making a composite of the perovskite manganese oxides with a secondary phase. To make two-phase composition, low sintering temperature is needed. The interfacial diffusion reaction between perovskite manganite oxide and the secondary phase materials, however, cannot be entirely excluded in the sintering process. In our group, perovskite manganese oxides were prepared using novel bonded method instead of sintering process. The advantages of bonded magnets are their greatly superior mechanical properties and the fact that net shape parts of high tolerance can be easily prepared. And as sintering process is excluded, manganite-based two-phase composition with clear phase boundary could be made, this will help further improve the low-field enhanced magnetoresistance theory. Moreover, if another kind of material, such as insulating oxide and metal, is added to this bonded sample and the technique is improved, practical magnetoresistance materials with higher MR and non-sensitive to temperature could be obtained.This thesis aims at obtaining enhanced low field magnetoresistance by researching bonded perovskite manganese oxides. The effect of fabricating technique, properties of bonded composites, and transport mechanism are systematically analyzed. The main research work and obtained conclusions are as follows:1. Firstly, the influence of binder has been studied. The binders include insulator (epoxy resin, polyamide-imide and silicone rubber) and conductor (zinc and graphite). There is no insulator-metal (I-M) phase transition for epoxy resin/polyamide-imide bonded sample; for silicone rubber bonded samples resistance peak appears around230K, and the peak intensity increases with increasing of silicone rubber content. MR for the samples containing1%and2%epoxy resin is no-sensitivity to temperature in the temperature range of270-340K; intrinsic CMR peak appears in the sample containing10%polyamide-imide; LFMR characteristic is shown in the sample containing1%silicon rubber, magnetoresistance peak around230K appears in the samples with2%&3%silicone rubber.When using conductive binder, the samples containing low content of Zn (<30%) show purely semiconducting behavior, but if Zn content reaches40%, the sample showed pure metal behavior; the graphite bonded samples exhibit approximately linear resistance-temperature properties. Neither intrinsic CMR nor LFMR characteristic appears in MR-T curve for graphite bonded samples.2. Two-phase composites have been prepared using bonded method by introducing different kind of materials, which are metals (Sn/Zn/Ag), graphite (flaked and powdered morphology) and Bi2Se3.In samples using precursor powders with larger grain size (4.13μm), the binder can’t achieve better dispersion/partly reunite due to the larger of grain size, weakening the role of grain boundaries and then reduce LFMR.In Lao.gSr0.2MnO3/Zn composite system, I-M transition is observed with3%&5%epoxy resin and samples show enhanced LFMR.In La0.8Sr0.2MnO3/Ag composite system, the I-M transition is only observed in the sample of10%Ag+2%epoxy resin. At low temperature region, sample containint10%Ag+6%epoxy resin shows maximum MR (18.19%); but at the high temperature region, sample containing20%Ag+6%epoxy resin not only shows the largest MR, but also shows non-sensitive-to-temperature MR in the temperature range of200～320K.LSMO/graphite composites using epoxy resin as the binder show semiconducting behavior. Samples with low content graphite exhibit higher LFMR than samples with high content graphite. For samples using polyamide-imide as the binder, the insulator-metal transition is observed with high content of polyamide-imide (powdered shape); the intrinsic CMR with temperature-broadening effect for all samples is observed. For silicone rubber bonded system using flake graphite as the second phase materials, there is a resistance peak around230K due to the existence silicone rubber.In LSMO/Bi2Se3composite system, appropriate proportion of epoxy resin can induce intrinsic magnetoresistance reappear. For silicone rubber bonded composite system, the resistivity decreased with Bi2Se3content increasing; all samples show resistance peak around230K; for samples containing low content of Bi2Se3, MR peaks about200K and305K are observed; for the samples with high concentration of Bi2Se3, there are no peak appear; MR is more than30%in a wide temperature range from50to200K for the sample with10%Bi2Se3.3. The analysis and comparison on the transport properties of different binder system and different second phase (metal/graphite) composite system are given. We also study the AC transport properties for bonded perovskite manganese oxide composites.For samples with different binders system, high content polyamide-imide can make the intrinsic CMR and the extrinsic CMR enhance simultaneously. For LSMO/metal bonded composites, enhanced LFMR is higher using Sn as the binder than others; in samples using graphite as the second phase material bonded composites, high content of graphite will reduce MR.AC transport properties. Firstly, we study the effect of frequency on bonded perovskite manganese oxide composites:(I) The sample with5%polyamide-imide exhibits approximately linear resistivity-temperature properties; the sample with8%polyamide-imine shows semiconductor characteristic; resistance peak appears in the sample with10%polyamide-imine at high frequency; the sample with15%polyamide-imine exhibits I-M phase transition.(II) MR for the samples containing low concentration of polyamide-imide exhibit temperature-insensitive characteristics at the temperature below250K.Low frequency can make the intrinsic CMR and the extrinsic CMR enhanced simultaneously.Then, the effect of applied magnetic field at constant frequency is carried out. At133Hz, the resistivity for the samples containing polyamide-imide content of8%&15%is no-sensitivity to high field; the resistivity for the sample with10%polyamide-imide keeps almost equivalent at the low-temperature and low-field (0T,0.3T,0.5T); samples containing8%,10%&15%polyamide-imide exhibit I-M phase transition at low field. For the sample with15%polyamide-imide, the highest MR and obvious intrinsic CMR characteristics in the vicinity of the300K are obtained. |
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