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La0.67Ca0.33MnO3 (LCMO) nanoparticles were prepared by the sol–gel method. The composite samples of (LCMO)x/(ZrO2)1-x were obtained by mixed LCMO and ZrO2 powders and sintered at 1300°C. Detailed studies of magnetic and magnetotransport properties for composite samples have been performed. The x-ray diffraction and scanning electronic microscopy observations indicated that no reaction happened between LCMO and ZrO2 grains. The low-field magnetoresistance (LFMR) can be greatly improved by ZrO2 addition close to the percolation threshold of x=60%. The field sensitivity of MR at 77 K in the low field region for (LCMO)0.4/(ZrO2)0.6 is 7%/100 Oe, which is one order of magnitude larger than that obtained by pure LCMO. The enhancement of the spin-dependent tunnelling and scattering of electrons at the interfaces of LCMO and ZrO2 grains is responsible for the enhanced LFMR. [ABSTRACT FROM AUTHOR]

The effects of Ag addition in Ti-doped La0.67Ba0.33MnO3 (LBT) (abbreviated as LBT/Agx, where x is the nominal molar ratio of Ag) on the electric and magnetoresistance (MR) properties have been studied systematically. The results show that the resistivity of the samples decreases dramatically with increasing Ag addition and reaches its minimum for x = 0.27. The room-temperature MR ratio of LBT/Agx increases with increasing x and reaches its maximum at x = 0.27. The MR ratio at 280 K for the x = 0.27 sample at H = 10 kOe is as large as 41%, which is about 3.4 times larger than that for LBT. The Ag addition makes the LBT crystallites more perfect and uniform, and suppresses the magnetic scattering at the grain boundaries. The Ag conduction channels penetrating into the insulating regions can also lower the resistivity. The near room-temperature TC and low resistivity of the Ag-added LBT are responsible for the large intrinsic room-temperature MR. [ABSTRACT FROM AUTHOR]