Study On Synthesis, Magnetic And Electrical Properties Of Perovskite Manganites Composite

Recently, growing attention has been paid on the grain boundary effect in polycrystalline R1-xAxMnO3 (R is rare earth, A is divalent cation) perovskite manganites. In the early 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 tunneling through energy barriers at 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 phase. In this paper, based on the polycrystalline manganese oxide of La_0.67Sr_0.33MnO₃ (LSMO), we modified the grain boundaries in these manganites by introducing the secondary phase with different properties. The electrical and magnetic properties of the composites have been investigated, which provides experimental and theoretical basis for the enhancement of low-field magnetoresistance. The main results are shown as following:1. The polycrystalline samples of La_0.67Sr_0.33MnO₃, Pr0.67Ca0.33MnO3(PCMO) and Sm_0.67Ca_0.33MnO₃(SCMO) were synthesized by sol-gel method and the structure, magnetic and electrical transport properties were studied.2. After mixed the manganese oxides of La_0.67Sr_0.33MnO₃ and Pr0.67Ca0.33MnO3 in a certain proportion, we ground, pressed, and sintered the samples at a certain temperature, and got the two-phase coexistence of complex manganese oxides (1-x)La_0.67Sr_0.33MnO₃ /xPr0.67Ca0.33MnO3(LSMO/PCMO) composites finally. We found that with increasing the content of PCMO, the magnetization of the LSMO/PCMO composite reduces significantly. The experimental results indicate that there is a strong magnetic coupling around and under PCMO's charge ordering temperature. The experimental results show that with the increase of PCMO content, the conductivity of composite systems decreases, the electrical resistivity increases, the resistivity peak ofρTpmoves to lower temperature, and the metal-insulator transition peak width becomes narrower. At low temperature, the low-field magnetoresistance increases a bit, while the high-field magnetoresistance increases a lot. There is a minimum in resistivity at low-temperature because of the tunneling effect between particles.3. The two-phase coexistence of complex manganese oxides for (1-x)La_0.67Sr_0.33MnO₃ /xSm67Ca0.33MnO3 (LSMO/SCMO) were prepared using the similar method. There are two magnetic transitions in the composite samples. The magnetization of the LSMO/PCMO composite samples reduces with increasing the content of SCMO. The exchange coupling between adjacent composite particles weakens with the increase of SCMO content. The conductivity of composite systems decreases, the electrical resistivity increases, the resistivity peak ofρTpmoves to lower temperature, and the metal-insulator transition peak width becomes narrower with increasing the content of SCMO.