The Effect Of MgO And Nd₂O₃ Doping On The Electric-Magneto Transport Properties And Magnetoresistance In LCMO System

Since the discovery of colossal magnetoresistance (CMR) effect in manganese perovskite, lots of researchers put great interests in this area. Usually, to gain a large CMR value need to apply a large magnetic field, which is limited its application. The study of low field magnetoresistance (LFMR) becomes a hot project. In this dissertation, we use the optimal hole-doping manganites as the mother material for our study, and compound it with some oxides. The electrical transport properties of the compound samples under zero and applied field are studied. And the LFMR is well studied in our dissertation. The dissertation consists of five chapters:First, we introduce the advance of manganese perovskite, theories, the destination and main content in chapter one.The chapter two, we first describe the synthesis methods for manganese perovskite and the effect of sintering process. The test technologies are introduced and the basic properties of mother material are also analyzed.In chapter three, we first prepared La_0.7Ca_0.3MnO₃(LCMO)and MgO composite. The composites have large magnetoresistance (MR) at much lower temperature than TP when applied a small field (⁰.3T). Through analysis we know that this MR is not the intrinsic MR of LCMO. The intrinsic MR effect can be explained by double exchange interaction between Mn3+ and Mn4+ ions in manganese perovskite. Non-intrinsic MR effect is mainly related to the coating MgO particles around LCMO grains and the conducting electrons will be affected by spin dependent scatting. The intrinsic peaks have a high sensitive to MgO particles. When adding MgO into LCMO system, the intrinsic peak weakened and turned to a platform quickly andρincreases gradually. As we know, MgO mainly segregated at the interface or surface of LCMO grains and dilute the magnetic properties of LCMO in the samples, soσS decreases gradually and TC unchanged.The next chapter, we change the oxide to Nd₂O₃ and to form LCMO/Nd₂O₃ composites. With increasing Nd₂O₃, we found that theρ-T curves and MR have clear double peaks. Theρ-T curves have an orderliness changes. We can be explained these phenomenon by double channel model and double exchange interaction. For LFMR, when adding Nd₂O₃ into LCMO system, the spin disorder will increases at the interface of LCMO grains. When applied a small magnetic field, the spin disorder of Nd₂O₃ layer at the surface of LCMO grains decreases and the non-intrinsic CMR appears.The last chapter is the conclusion.