Structural and Magnetic Properties of Ultrafine La0.7Ca0.3MnOz Powders Prepared by Mechanical Alloying

Ultrafine La0.7Ca0.3MnOz powders with controlled oxygen stoichiometry have been synthesized by mechanical alloying at ambient temperature. It is found that high-energy ball milling of the starting materials, La2O3, CaO, MnO2, and Mn3O4 mixed in the stoichiometric cation ratio, yields single-phase La0.7Ca0.3MnOz powders having crystallite sizes of about 10 nm and various oxygen content (2.68 less than or equal to z less than or equal to 3.35), adjustable by changing the MnO2/Mn3O4 ratio, Magnetic measurements show that the spontaneous magnetization (M-s) of the as-milled powder depends less on the nominal Mn valence (nu(Mn)) than in bulk (La,Ca)MnO3 and that the maximum M-s, observed for nu(Mn) similar to 3.3, is much smaller. Annealing in air at temperatures above 500 degrees C increases M-s for all the samples, but a marked increase in M, toward the bulk value (similar to 90 emu/g) occurs in a lower temperature range for lower nu(Mn) than for higher nu(Mn). These observations are discussed in terms of magnetic disorder resulting from defects induced by high-energy milling and surface effects dominant in small crystals. (C) 2000 Academic Press.