Effects Of Introducting The Second Phases On Magnetic And Electronic Transport Properties In Colossal Magnetoresistance Materials

Recently growing attention has been paid on the grain boundary effect in polycrystalline perovskite manganites of the type Ln_1-xTxMnO₃ (Ln is rare earth, T is divalent cation). Last years, colossal magnetoresistance (CMR) effect was observed in this kind of materials. However, the intrinsic CMR effect in the perovskite manganites is only triggered at high magnetic fields of several tesla, which restrains its use for practical applications. Recently growing attention is being paid to polycrystalline manganites in which the grain boundary effects dramatically modify their physical properties. An attractive feature for the polycrystalline manganites is a large magnetoresistance(MR) at very low magnetic field over wide temperature range below Tp. The low field magnetoresistance in polycrystalline manganites is usually thought to be a result of the spin polarized tunnelling through energy barriers at the grain boundaries. The structural disordered interfaces play a role of the energy barrier for carriers. The magnetoresistance can be enhanced through controlling the grain boundary effect by forming composites of the CMR oxides with secondary phases. However, in these composite systems, the enhanced LFMR was usually observed at lower temperature, which restrains its use for practical applications. In this paper, CMR granular composite systems were investigated. Using different mothed, We discussed the effects of the second phase on the structure and transport properties of the CMR composites.La_2/3Sr_1/3MnO₃ (LSMO)/CuO and LSMO/Al₂O₃ multilayers granular thin films were prepared by using tandem deposition method via a magnetron sputtering system. The results show that the electrical transport and magnetoresistance(MR) strongly depends on the layers of the thin films and the concent of the second phase. For pure LSMO films, the insulator-metal transition temperature Tp is 236K and the max MR is 23%. For LSMO/CuO composite film with 40 layers, The Tp value decreases to 174K while the max MR increases to 31%. As compared to CuO, the MR increases at higher temperature as a result of the introduction of Al2O₃. In terms of the experiment and theory calculating results, We suggest that the high-T grain boundary magnetoresistance is possible to be obtained by selecting high activation energy insulator as the second phase in manganite-based composite films.La_2/3Ca_1/3MnO₃ (LCMO)/MgO granular composite systems were prepared by coating method. As compared to pure LCMO, MR is enhanced in the LCMO/MgO composites. For pure LCMO, the max low field MR is 8% measured at 0.3T. For LCMO/xMgO composite with x=0.07, the max low field MR increases to 28%. Furthermore, the electrical transport properties and MR of the LCMO/MgO composites are related to the grain size of the parent LCMO powers. In terms of the SEM images of the samples, we suggest that the transport properties of the composites are related to the coupling properties between the neighboring grains, which is approved by the two magnetic transitions observed in the composites. However, similar to the other LCMO composites, MR is enhanced only at low temperature range below Tp.The strong magnetoelectric coupling effect is observed in the composites of a ferromagnetic and a ferroelectric compound (called bi- or multiferroic), which suggests a new idea for designing the CMR composites. The experiment results observed in the LCMO/BaTiO₃ composites sintered at 900℃are different from other LCMO composites. The insulator-metal transition temperature (Tp) is shifted to higher temperature and resistivity decreases with increasing the BaTiO₃ content. The Tp value increases from 185 K for pure LCMO to 230 K for x = 0.3 sample. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO₃. The experimental results can be explained in terms of the magnetoelectric coupling effect. However, for the samples sintered at 1200℃, BaTiO₃ enters the LCMO lattice at the grain surface. The electrical transport properties and MR are similar to the general LCMO composites. For the composites sintered at 1000℃and 1100℃, the R-T and MR-T curves appear two peaks, which could be due to that partial BaTiO₃ enter the LCMO lattice at the grain surface at the medium sinter temperature. In this case, two kinds of grain boundary coexist in the composites.Moreover, the CMR composites with other insulator oxides were investigated in this paper. In the LSMO/CeO₂ composites, the resistivity varies asρ=ρTp exp( ?α(T ?Tp)2). The constantαdoes not vary for all samples with different CeO₂ content x. The experimental results can be interpreted in terms of a surface magnetization mechanism. Different electrical and magnetic transport properties were observed between LCMO/SrO and LCMO/MgO composites. As compared to MgO, the SrO content has little effect on Tp and the low field MR is enhanced at higher temperature range. The different properties could be attributed to the formation of the different interfacial phase at the grain boundary in two kinds of composites.In summary, the grain boundary effect in the CMR granular composites strongly depends on the second phase propertied and material preparation method. MR can be enhanced at higher temperature range by appropriate choice of the second phase.