Study On Preparation, Structure And Properties Of Perovskite Manganites

The colossal magnetoresistance (CMR) effect in rare-earth manganese perovskites has aroused considerable interest in scientific studies and potential technological applications. CMR effect, the intrinsic characteristic of manganites, is considered from the spins ordering of manganese under an applied field. However, the CMR effect can only be obtained under a field range of several teslas, which is too high for practical applications. In order to decrease the required magnetic fields, researchers thus focus on another type of magnetoresistance (MR) effect—extrinsic MR effect, which is often attributed to the spin-dependent scattering mechanism. By controlling the state of spins disordering near the grain boundaries or the heterostructure interface between manganites and other materials, the spin-dependent scattering process would be weakened under a lower applied field and thus the low field magnetoresistance (LFMR) achieved. One advantage of manganites is their nearly 100% spin polarization which may boost structural magnetoresistance associated with the spin-memory contribution to charge transport (e.g., spin-dependent tunneling) across the interfaces.The manganite powder of La_0.67Sr_0.33MnO₃ was synthesized by self-propagating high temperature synthesis (SHS) method. The results show that the product is single perovskite. The composition of the raw materials and the heat-treat temperature play an important role in the structure and the micromorpha of La_0.67Sr_0.33MnO₃ polycrystals. The powder whose average grain size is 1.2μm can be obtained when the relative content of KMnO₄ is equal to 0.4 and the heat treatment temperature is 1000°C.The La_0.67Sr_0.33MnO₃ powder synthesized by SHS method was sintered by three different sintering method respectively, that is conventional solid state sintering (CSS), high pressure sintering (HP) and spark plasma sintering (SPS). The resistivities of all the samples increased with the decrease of the sintering temperature, which can be attributed the grain size effects. The sample sintered by HP method at 1000°C exhibits the metal-insulator transition and the sample...