Spin-glass behavior in 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 La2O3, CaO, MnO2, and Mn3O4, mixed in the stoichiometric cation ratio, yields single-phase La0.7Ca0.3MnOz powders ha ving crystallite sizes of about 10 nm and various oxygen stoichiometries (2 .68 less than or equal to z less than or equal to 3.35), adjustable by chan ging the MnO2/Mn3O4 ratio. The spontaneous magnetization (M-s) of the as-mi lled powder depends less on the nominal Mn valence (v(Mn)) than in bulk (La , Ca)MnO3 and the maximum M-s, observed for upsilon(Mn) = 3.3, is much smal ler. Annealing in air at temperatures above 500 degrees C increases M-s, bu t the way M-s approaches the bulk value (90 emu/g) depends on upsilon(Mn). These observations are discussed in terms of magnetic disorder resulting fr om defects induced by high-energy milling and surface effects dominant in s mall crystals. (C) 2000 American Institute of Physics. [S0021-8979(00)55208 -6].