Effects of Cr doping in La 0.67 Ca 0.33 MnO 3 : Magnetization, resistivity, and thermopower

The effects of Cr substitution on Mn sites in the colossal magnetoresistance (CMR) compound ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ have been studied by preparing the series ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3} (xl~0.3).$ A careful study in the magnetic and electrical transport properties has been carried out by the measurements of magnetization, resistivity, magnetoresistance, and thermopower. It was found that Cr is impotent in lowering ${T}_{C}$ when $xl~0.2.$ An extraordinary magnetotransport behavior, characterized by double bumps, was observed around $x=0.1.$ As a result, the temperature range of CMR is greatly broadened. The analysis of resistivity and thermopower data in the paramagnetic state enable us to identify the polaronic transport mechanism. Morever, it is found that the polaron activation energy as well as polaron binding energy are almost constant within a broad Cr content. We suggest these peculiar effects of Cr doping could be the consequence of the possible double exchange interaction between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Cr}}^{3+}.$