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 having crystallite sizes of about 10 nm and various oxygen stoichiometries (2.68⩽z⩽3.35), adjustable by changing the MnO2/Mn3O4 ratio. The spontaneous magnetization (Ms) of the as-milled powder depends less on the nominal Mn valence (vMn) than in bulk (La, Ca)MnO3 and the maximum Ms, observed for vMn=3.3, is much smaller. Annealing in air at temperatures above 500 °C increases Ms, but the way Ms approaches the bulk value (90 emu/g) depends on vMn. These observations are discussed in terms of magnetic disorder resulting from defects induced by high-energy milling and surface effects dominant in small crystals.