Electrical transport and magnetoresistance of double layered CMR manganites R_1.2Sr_1.8Mn₂O₇(R = La, Pr, Sm)

Polycrystalline bulk samples of double layered (DL) colossal magnetoresistive (CMR) manganites R_1.2Sr_1.8Mn₂O₇ (R = La, Pr, Sm) were prepared by sol-gel method to study the effect of size of lanthanide ion on their magnetotransport properties. The electrical resistivity of the samples was investigated in the temperature range of 70 K to 300 K at different magnetic fields. The samples LSMO and PSMO show insulator-to-metal transition (IMT) behavior, while SSMO sample exhibits insulating behavior in the entire temperature range with a very large value of resistivity. The insulator-to-metal transition temperature (T_IM) decreases from 123 K (LSMO) to 90 K (PSMO) and disappears in SSMO sample. To explain the electrical transport above T_IM, the temperature dependent resistivity data (T > T_IM) of all the samples were fitted to the equations of different conduction models. The results indicate that the conduction at T > T_IM is due to Mott variable range hopping (VRH) mechanism in the LSMO and PSMO samples, while Efros-Shkloskii (ES) type of VRH model dominates the conduction process in the SSMO sample. All the three samples show increasing magnetoresistance (MR) even below T_IM and the maximum MR is shown by LSMO (39 % at 75 K, 3 T).