Magnetic properties of ultrafine MnFe2O4 powders prepared by mechanochemical processing - art. no. 184414

Ultrafine MnFe2O4 powders have been synthesized via the displacement reacti on 2FeCl(3)+4MnO --> MnFe2O4 + 3 MnCl2 activated by high-energy ball millin g. Single-phase MnFe2O4 powders having crystallite sizes ranging between 9. 5 and 40 nm have been obtained after removing the by-product (MnCl2) and un reacted (FeCl3 and MnO) phases from the as-milled or annealed powders by wa shing and magnetic separation. Magnetic measurements show that the spontane ous magnetization decreases with crystallite size; that the coercivity incr eases with decreasing temperature; and that below about 50 K the temperatur e dependence of coercivity becomes very strong, and the hysteresis loop bec omes asymmetric with the remanent magnetization and the coercivity both enh anced in the upper branch when the sample is cooled in the maximum field. T hese observations are discussed in terms of a model in which it is assumed that each crystallite consists of a ferrimagnetic core surrounded by a spin -glass shell and the spins in the core and shell an exchange coupled.