Effects of Ga doping in the colossal magnetoresistance materialLa0.67Ca0.33MnO3

Abstract

The effects of Ga doping on Mn site in the colossal magnetoresistance material ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ have been studied by preparing the series ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}$ $(x=0.0,0.02,0.05,0.1).$ Upon doping, no appreciable structure changes have been found. However, the Curie temperature and the associated metal-insulator transition decrease drastically with Ga doping. The resistivity in the paramagnetic regime for all these samples follows Emin-Holstein's model of small polaron. A large variation of the resistivity coefficient B and the polaron activation energy ${E}_{a}$ with Ga doping was found. The variation of B implies the change of both polaron concentration and average hopping distance with Ga doping. Considering the possibility of on-site Coulomb repulsion and polaron-polaron interactions, we suggest a combination of polaron nearest-neighbor hopping and non-nearest-neighbor hopping dominating the transport process in ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Ca}}_{y}{\mathrm{MnO}}_{3}.$ Combining with the data of thermopower, we found a large increase of polaron binding energy with nonmagnetic Ga doping, which strongly suggests the additional magnetic nature of the lattice polaron in ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}.$