CATION DISORDER AND SIZE EFFECTS IN MAGNETORESISTIVE MANGANESE OXIDE PEROVSKITES

Large disorder effects due to size differences between A-site R(3+) (R =La,PT,Nd,Sm) and M(2+) (M = Ca,Sr,Ba) cations have been found in magn etoresistive (R(0.7)M(0.3))MnO3 perovskites. The ferromagnetic-metal-p aramagnetic-insulator transition temperature T-m varies as T-m = T-m(0 )-pQ(2) due to strain fields resulting from ordered or disordered oxyg en displacements Q that are parametrized by the statistical mean and v ariance of the A cation radius, respectively. The value of p is relate d to the Mn-O force constant showing that Mn3+ Jahn-Teller distortions assist electron localization at T-m. The maximum possible T-m is esti mated to be similar to 530 K although experimentally observable values are less than or equal to 360 K. A large suppression of magnetoresist ance due to cation disorder is also evidenced.