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 La 0.7Ca0.3MnOz powders having crystallite sizes of about 10 nm and various ox ygen content (2.68 less than or equal to z less than or equal to 3.35), adj ustable by changing the MnO2/Mn3O4 ratio, Magnetic measurements show that t he 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 maxim um M-s, observed for nu(Mn) similar to 3.3, is much smaller. Annealing in a ir at temperatures above 500 degrees C increases M-s for all the samples, b ut a marked increase in M, toward the bulk value (similar to 90 emu/g) occu rs in a lower temperature range for lower nu(Mn) than for higher nu(Mn). Th ese observations are discussed in terms of magnetic disorder resulting from defects induced by high-energy milling and surface effects dominant in sma ll crystals. (C) 2000 Academic Press.