Influence of the grain-size and oxygen stoichiometry on magnetic and transport properties of polycrystalline La0.67Ca0.33MnO3±δ perovskites

Effects of grain size and oxygen stoichiometry in sol-gel and ceramics samples of La0.67Ca0.33MnO3+/-delta have been studied. Grain size (90 nm versus > 10 mu m) does not change either the Curie temperature (T-c) or the metal-insulator transition temperature (TM-I), but great differences in low-field magnetoresistance are observed. These differences have been explained by a spin polarized tunneling model for conduction between grains. Oxygen content (delta) could be changed in La deficient samples via an electrochemical method at room temperature (i.e., without changing grain size). We observe great differences in TM-I when decreasing delta, while T-c remains unchanged. Broken ways between Mn ions leads to zones with ferromagnetic behavior, but inhibits conductivity. (C) 1998 Elsevier Science B.V. All rights reserved.