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We report significant enhancement of magnetoresistance (MR) in melt-processed La[sub 0.67]Ca[sub 0.33]MnO[sub 3] (LCM) and in Ho-doped LCM samples. The LCM system exhibits surprising enhancement of metal-insulator transition (T[sub IM]) that coincides with the Curie transition temperature (T[sub c]), T[sub IM]approx. T[sub c], illustrating the enhanced percolative transport and spin-polarization through grain boundaries. Surprisingly, 1 mol % of addition of Ho enhances T[sub c] of ∼70 K with remarkable colossal MR. The nanoscale Ho distribution is consistent with the magnetic inhomogeneity-induced MR due to phase segregation in Ho-doped sample whereas charge-segregation picture is valid for undoped LCM. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report measurements of the resistivity and magnetization of high-quality melt-processed La[sub 0.67]Ca[sub 0.33]MnO[sub 3] (LCM) and Sr-doped LCM materials that exhibit a sharp and enhanced metal–insulator transition (T[sub MI]) with significant colossal magnetoresistance (CMR) and two competing magnetic phases in the vicinity of T[sub MI]. Our results can well be explained using percolation in combination with the phase-separation scenario proposed for CMR. We propose that the spin polarization through grain boundaries and the enhanced grain connectivity between the ferromagnetic domains facilitate better percolation through a junction and filament-like resistor network. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have performed magnetic and transport measurements on La[sub 0.67]Ca[sub 0.33]MnO[sub 3] polycrystalline and partially melted samples. The magnetization and resistance decrease as sintering temperature is increased. The insulator-to-metal transition temperature is remarkably enhanced in the partially melted sample, enabling its possible applications at room temperature. The microscopic studies in melt samples show an excellent connectivity between grains, suggesting the enhancement of percolative transport by opening new conduction channels and the disappearance of magnetic phase boundary at elevated temperature by the ordering of Mn spins blocked at the grain boundary of sintered samples. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]