Investigation Of Electrical And Magnetic Transport Properties In Manganese Perovskite Two-Phase Composites

As a strongly correlated electron system, the rare earth doped perovskite manganese which has colossal magnetoresistance (CMR) effect is the hot topic in the research of condensed physics and materials physics, due to exhibiting intriguing physical properties such as insulator-metal transition, charge ordering, orbital ordering and phase separation. In recent years, low field magnetoresistance (LFMR) has become a new research aspect in the research area of CMR. Very different from CMR, LFMR is an extrinsic characteristic which commonly happens in polycrystalline material. The advantage of LFMR-low drive magnetic field and non temperature sensitive-favors the practical application of CMR materials. Ordinarily, enhanced LFMR is obtained by making a composite of the perovskite manganese oxides with a secondary phase. This is called the manganese-based two-phase composition. In this work, the main research content is enhanced low field magnetoresistance in two-phase perovskite manganese composition. The perovskite manganese composition containing different second phase is prepared using conventional sintering method and newly bonded method, and the corresponding electrical and magnetic properties are studied.The main contents are as follows:1, The perovskite manganese composition containing different second phase, that is LaNiO₃/La_0.7Sr_0.3MnO₃, Ta₂O₅/La_0.7Sr_0.3MnO₃ andLa_0.7Ca_0.3MnO₃/La_0.7Sr_0.3MnO₃, is p repared using conventional ceramic sintering method.In the LaNiO₃/La_0.7Sr_0.3MnO₃ composite with sol-gel precursor powders, if LaNiO₃ contention is low, the value of saturate magnetization increases with the concentration of LaNiO₃. The PM-FM transition temperature varies slightly with LaNiO₃ content. The insulator-metal transition temperature is smaller and the variation is much larger. The LFMR enhancement is observed over a wide range of temperature up the room temperature. The composite of (LSMO)_0.95/(LNO)_0.05 had remarkable MR value and it is not sensitive to temperature near room temperature.In the Ta₂O₅/La_0.7Sr_0.3MnO₃ composite with sol-gel precursor powders, a small amount of Ta ions enter into LSMO grains near the grain surface region, resulting in the deduction of the cell volume, and consequently, an extra deduction of the saturate magnetization. The paramagnetism-to-ferromagnetism transition temperature is increased. The LFMR enhancement is observed over a wide range of temperature from 50 to 350K when the second phase material (Ta₂O₅) is introduced. The magnetic disorder caused by the secondary phase at grain boundaries and the Ta doping effect on the surface of LSMO grains are believed to contribute to the enhanced MR. And in the Ta₂O₅/La_0.7Sr_0.3MnO₃ composite with sol-gel precursor powders, if the sintering temperature and time is low, two peaks are observed in the dM/dT－T curve. The peak temperature T_c1 is the grainboudary contribution and T_c2 is the grain contribution, correspondingly. In the Ta₂O₅/La_0.7Sr_0.3MnO₃ composite with solid state reaction precursor powders, the variation of cell volume insulator-metal transition temperature is slightly.As for the La_0.7Ca_0.3MnO₃/La_0.7Sr_0.3MnO₃ composite with solid state reaction precursor powders, the insulator-metal transition is only observed with low content of second phase. The resistivity increases with the content of La_0.7Ca_0.3MnO₃. The intrinsic CMR contribution is observed in the composite with high content ofLa_0.7Ca_0.3MnO₃.2, The properties of sintering composite of ZnO/La_xSr_1-xMnO₃ and ZnO/ La_xCa_1-xMnO₃ are studies.The La_0.7Sr_0.3MnO₃/ZnO composites with low content of LSMO about 1%-5% exhibit nonlinear voltage-current properties. Applied magnetic filed could change the electrical property. Resistance increased after magnetic field was applied, which was a positive magnetoresistance (PMR) phenomenon. The existence of PMR is duo to the broadening of barrier at the grain boundaries caused by the magnetic field.In the composite of (La_xCa_1-xMnO₃)_0.2/(ZnO)_0.8, the insulator-metal transition temperature is much smaller than the PM-FM transition temperature due to the existence of large content of ZnO. For the sample of x=0.7, intrinsic CMR is observed with temperature-broadening effect.The La_0.7Sr_0.3MnO₃/ZnO composites with high content of LSMO above 15% exhibit obvious magnetoresistant effect. The insulator-metal transition temperature increases with the sintering temperature. The composite with La_0.7Sr_0.3MnO₃ content of 33% and sintering temperature of 1100℃has the biggest value of MR.In the composite of (ZnO)_0.8/(La_0.4Ca_0.6MnO₃)_0.2, a giant CMR effect is observed for the measuring field from low to high up to 3 T and measuring temperature down to 50 K. The giant CMR effect is meta-stable which vanishes for different measuring process. The charge ordering state and capacity effect caused by the insulation ZnO between La_0.4Ca_0.6MnO₃ grains contribute to the electrical relaxation behavior.3, The concept of bonded perovskite manganite oxide is firstly putted forward in this thesis. Three different types of bonder samples, i.e. polymer (epoxy resin), metal (tin) and complex (epoxy resin and tin), are prepared.In the polymer bonded samples, all the samples show purely semiconducting behavior in the whole temperature range. There is a slight peak in the MR-T curve for the sample containing 2% binder and no peak is observed for the samples containing lower concentration of binder (0.5% and 1%). MR value for the sample containing 1% binder keeps at the basically same value in the temperature range of 250K-350K.In the metal (Sn) bonded samples, the insulator-metal transition is only observed with low content of Sn. If Sn content exceeds 10%, the samples show basically pure metal behavior. The bonded samples with Sn content of 5% and 8% exhibit obviously low field magnetoresistance properties.In the complex bonded samples with fixed Sn content and alterable epoxy resin content (1%-4%), the insulator-metal transition is observed with polymer content of 2%. Other samples do not show insulator-metal transition. The complex bonded sample with polymer content of 1% has biggest value of resistivity. The complex bonded sample with polymer content of 2% does not show obvious magnetoresistant property. In the complex bonded sample with polymer content of 4%, enhanced LFMR and intrinsic CMR are simultaneously observed.And if another kind of material, such as insulating oxide and metal, is added to this bonded sample, the real manganite-based two-phase composition could be made, as there is no high-temperature reaction in the preparing process.