Effect of sintering temperature on electronic transport and low-field colossal magnetoresistance in sol-gel prepared polycrystals of nominal La 2 / 3 Ca 1 / 3 Mn 0.96 Cu 0.04 O 3

It is shown that transport and magnetoresistive properties of sol-gel prepared polycrystalline samples of nominal ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{O}}_{3}$ $(x=4%)$ strongly depend on sintering temperature ${(T}_{s}).$ Lowering ${T}_{s}$ causes a large decrease of the insulator-metal transition temperature ${(T}_{\mathrm{IM}})$ and a substantial increase in resistivity at $Tl~{T}_{\mathrm{IM}}.$ Above ${T}_{\mathrm{IM}},$ resistivity data can be fitted to the variable-range hopping model, $\ensuremath{\rho}\ensuremath{\propto}{e}^{{(T}_{0}{/T)}^{1/4}}$, with a strong dependence of ${T}_{0}$ on ${T}_{s}.$ For ${T}_{s}l~1200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ upon application of a low magnetic field (0.3 T), the samples exhibit colossal magnetoresistance (CMR) effect near ${T}_{\mathrm{IM}}.$ The largest CMR with a value $\ensuremath{\sim}74%$ is obtained at ${T}_{s}\ensuremath{\sim}1100\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ and decreases on varying ${T}_{s}$ from $\ensuremath{\sim}1100\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ We also show that the sample at ${T}_{s}=1000\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ exhibits an almost constant magnetoresistance with a value $\ensuremath{\sim}65%$ for the 0.3 T field over the whole low-temperature range of $Tl77\mathrm{K}.$