In order to promote the technological applications of perovskite manganites, a great fundamental interest has been devoted to tailoring and/or enhancing their magnetotransport properties. Design and fabrication of manganite-based nanocomposites offer great potential to tailor the magnetotransport properties. In this work, we illustrate the promising concept of 0-3 type manganite/insulator composites (where manganite nanoparticles are uniformly and discretely embedded in a three-dimensional-connectivity insulator matrix) through the example of the La0.67Ca0.33MnO3 (LCMO)/MgO system. We present a promising strategy, which is based on the creation of core (LCMO)–shell (MgO) composite powders, for the synthesis of 0-3 type LCMO/MgO composites. A modified polyacrylamide gel method has been developed to prepare the core-shell structured LCMO/MgO composite powders. Besides its ability to create well-defined core-shell composite structures, the present gel method also allows the production of nanopowders with uniform particle size and in spherical shape. In our (1−x)LCMO/xMgO composite system, the lattice of LCMO is found to expand with rising MgO concentration x, yielding a bulk tensile strain. The increase in the tensile strain with x yields to a structural phase transition in the LCMO phase from an orthorhombic Pnma structure (x≤0.2) to a rhombohedral R3¯c structure (x≥0.3), and this structural transition leads to a relaxation of the strain. The strain effects induced by the MgO second phase exert a great influence on the ferromagnetic transition temperature TC. The composite system exhibits a percolative behavior in the conductivity, and the metal-insulator transition temperature TM-I decreases with x and is finally suppressed as the MgO content increases up to x=0.3. An enhancement in magnetoresistance (MR) is observed at low temperatures for the composites, and the low-field sensitivity of MR gets enhanced with the increase in MgO content. This work provides a promising method for manipulating the magnetotransport properties of manganites by composites with a proper insulator phase in a 0-3 connectivity fashion.
In this work, (1-x)La 2/3 Ca 1/3 MnO 3 (LCMO)/xMgO (x=0, 0.1, 0.2, 0.3, 0.4, 1) core-shell composite particles were first synthesized by a polyacrylamide gel route, and then the obtained composite particles were pelletized and sintered into the bulk composite samples. It is believed that the MgO phase was selfwelded to form a three-dimensional-connectivity matrix and the LCMO particles were discretely embedded in the MgO matrix without adhesion. The magnetoresistance (MR) properties of the composite samples were investigated. It is demonstrated that the MR value increases with raising MgO concentration x at low temperatures (T<80 K), and also the low-field sensitivity of MR, defined as d(MR)/dH, gets enhanced for the composites. The enhancement in MR can be attributed to the enhanced spinpolarized tunneling of conduction electrons at the grain boundaries in the composite samples.
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