Enhanced metal-insulator transition and magnetoresistance in melt-processed La0.67Ca0.33MnO3 and Ho-doped manganites

We report significant enhancement of magnetoresistance (MR) in melt-process ed La0.67Ca0.33MnO3 (LCM) and in Ho-doped LCM samples. The LCM system exhib its surprising enhancement of metal-insulator transition (T-IM) that coinci des with the Curie transition temperature (T-c), T-IM approximate to T-c, i llustrating the enhanced percolative transport and spin-polarization throug h grain boundaries. Surprisingly, 1 mol % of addition of Ho enhances T-c of similar to 70 K with remarkable colossal MR. The nanoscale Ho distribution is consistent with the magnetic inhomogeneity-induced MR due to phase segr egation in Ho-doped sample whereas charge-segregation picture is valid for undoped LCM. (C) 2001 American Institute of Physics.