Effect-of bandwidth and size disorder on the electrical and magnetotransport properties of doped LaMnO3 perovskite

Structural, electrical, magnetic and magnetotransport measurements have been-carried out on La1-2xPrxCaxMnO3; x 0.20 and 0.30 manganite compounds. These samples form in a distorted orthorhombic structure and behave as nearly low bandwidth systems. Both these samples exhibit insulator-metal transition below 120 K with a large colossal magnetoresistance (CMR) effect at relatively weaker fields (MR=75% at 1 Tesla for x 0.30 sample). We have also studied the structural, transport and magnetotransport properties of a larger size cation, Sr2+ substituted La0.5Pr0.2Ca0.3-xSrxMnO3 (0.0less than or equal toxless than or equal to0.20) compounds. It is observed that increasing Sr2+ substitution results in an enhancement-in transition temperature with a subsequent fall in MR%. The insulator-metal transition temperature for these samples increases from 138 K for x=0.0 to 252 K for x=0.20. The CMR effect fall from 46% for x=0.0 to less than 10% for x=0.20 at a field of I Tesla. Also, for samples With higher Sr content the MR% at low temperature is comparable to the MR% in-the vicinity of peak resistance. The resistance data in the semiconducting region of all the samples obeys the Mott's Variable Range Hopping type (InR proportional to T-0.25) of conduction mechanism.