| Magnetic and electrical behaviour are reported for two sodium substituted perovskites, viz., La0.9Na0.1MnO3 and La0.8Na0.2MnO3 (referred to as LSM10 and LSM20 respectively). The two crystallize in pseudo-cubic symmetry with a ~ 3.8Å. Magnetization measurements made in the temperature range 20K - 300K show that (i) LSM20 exhibits a sharp ferromagnetic - paramagnetic (FM - PM)phase transition at ~290K and (ii) LSM10 exhibits a much disordered magnetic state being FM at 20K with the transition to PM state spreading over a wide range begining at 50K and continuing right upto 300K and beyond. Measurements on electrical resistivity show that (i) the larger sodium containing sample LSM20 is electrically conducting in the region 20K - 295K and shows a tendency of going over to insulating state at little above 300K, (ii) the lesser sodium containing sample LSM10 shows metal - insulator (M - I) transition at ~210K which is well below Tc, where it would go over from FM to PM state and (iii) the resistivity of LSM10 is about 3 - 4 orders higher than of LSM20. The observations of LSM10 (i) being magnetically disordered, (ii) exhibiting much higher resistance than LSM20 and (iii) exhibiting M - I phase transition at a much lower temperature than where it would show FM - PM transition are all suggestive of the presence of complex exchange inteactions namely double exchange (Mn3+ - O - Mn4+) being frequently inhibited in LSM10. These observations on the two systems, with substitutions of monovalent cation for La3+, studied here are in line with the general observation on the requirement of substitution of more than 1/3rd of La3+ by divalent cations for evolution of long range magnetic ordering and electrical conduction (emanating from sufficient number of Mn3+ - O - Mn4+ pairs for double exchange). |
|