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Abstract: A significant enhancement of low-field magnetoresistance (LFMR) was obtainable from the polycrystalline composites between La0.7Sr0.3MnO3 (LSMO) and La2O3 phases. For this study, the magnetic and magnetotransport properties of samples with various compositions of (1− x)LSMO–xLa2O3 (x =0, 0.025, 0.05, 0.075, 0.1, 0.2, 0.3) were systematically investigated. Compared with pure LSMO (T C =362K), all composites exhibited a slightly depressed T C values of 355–358K, implying that the magnetic ordering of LSMO is insignificantly affected by chemically compatible La2O3. While the LFMR value at 300K in 500Oe was monotonously increased from 1.36 to 1.67% with increasing x from 0 to 0.075, it was abruptly increased to the maximum value of 2.22% for x =0.1 with only one order increase in its resistivity relative to x =0.075, and further increasing x from 0.1 to 0.3 resulted in a linear decrease to 1.8% with almost no variation in resistivity. This peculiar behavior, different from normal metallic percolation threshold effect, is closely related to the microstructure variation. Remarkably improved LFMR properties of the LSMO–La2O3 composite samples are attributed to high grain boundary areal density and sharpening of a disordered LSMO grain boundary region acting as more effective spin-dependent scattering centers. [Copyright &y& Elsevier]

The structure and electrical-magnetic transport properties of (La0.7Ca0.3MnO3)1-x/(Bi2O3)x has been investigated systematically by conventional solid-state reaction method. The composite samples were obtained by pure La0.7Ca0.3MnO3 (LCMO) and Bi2O3 powders and sintered at different temperature (Ts) 800°C and 1300°C for 2 h. For Ts=800°C samples, the results of X-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicate that Bi2O3 and LCMO coexist in the composites and Bi2O3 mainly segregates at the grain boundaries or grain surface of LCMO, and double peaks related to the metal-to-insulator (M–I) transition have been observed. Corresponding to the two M–I transition peaks, the curves of magnetoresistance (MR) against temperature also showed two peaks for all composites. But for Ts = 1300°C samples, Bi3+ ions mainly enter into the lattice of LCMO to form reaction products layer La0.7-xBix Ca0.3MnO3, only one peak occurs around the M–I transition temperature and the MR around TP has been enhanced. The results also indicate that the added Bi2O3 has an important effect on the LFMR. These phenomena can be explained by the double exchange (DE) mechanism, the grain boundary effect and the intrinsic transport properties together. And the grain surface state of the particles is a key factor influencing the magnetoresistance effects. [ABSTRACT FROM AUTHOR]

The (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x (x = 0–0.15) composites were prepared by conventional solid state reaction method. We have investigated the structural, magnetic and electrical properties of (La0.7Ca0.3MnO3)1-x/(CuFe2O4)x composites using X-ray diffraction (XRD), scanning electron microscopy, field cooled DC magnetization and magnetoresistance (MR) measurements. The resistance measured as a function of temperature demonstrates that pure LCMO samples display metal to semiconductor transition. On the other hand, all the LCMO/CFO composites samples clearly present the electrical behavior of insulator/semiconductor. It indicates that resistivity of the samples increase systemically with the increase of CFO content. The MR was measured in the presence of 0.5 T field. At low temperatures, it is clearly observed that the MR effect is enhanced with x = 0.03 of CFO composition. In summary, the spin-polarized tunneling and the spin-dependent scattering may be helpful to the improved low-field magnetoresistance (LFMR) effect. These phenomena can be explained by the segregation of a new phase related to CFO at the grain boundaries or surfaces of the LCMO grains. [ABSTRACT FROM AUTHOR]

Abstract: (La0.7Ca0.3MnO3)1− x /(MgO) x composites were prepared by traditional solid-state reaction procedure. The electrical transport properties, magnetic properties and low-field magnetoresistance (LFMR) of all samples with different MgO addition were studied systematically. With increasing MgO dopant, the resistivity increases, and the specific magnetic moment σ S decreases, but Curie temperature (T C) remains the same as that of the pure LCMO. Some specific effects of Mg dopant on the electrical transport properties were observed. A new metal–insulator (M–I) transition temperature (T P2) was observed at low temperature, except for the normal T P1, which was appeared at high temperature (about 270K). With increasing the doping, T P2 decreases monotonously. The intensity of resistivity at T P1 becomes weak and then turns to a platform. Furthermore, through the analysis of X-ray diffraction (XRD) and magnetic properties, we can see that MgO does not react with LCMO and only goes into the interface or the surface of LCMO grains. It is interesting to notice that the effect of MgO is to enhance the LFMR of the composites. According to the Double Exchange (DE) interaction theory, we can conclude that the effects of MgO addition on the electrical transport properties and LFMR for all the samples can be explained by the model of the grain boundary effect, the intrinsic transport properties and spin-dependent scattering. [Copyright &y& Elsevier]