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

Ultrafine MnFe2O4 powders have been synthesized via the displacement reaction 2FeCl(3)+4MnO --> MnFe2O4 + 3 MnCl2 activated by high-energy ball milling. Single-phase MnFe2O4 powders having crystallite sizes ranging between 9.5 and 40 nm have been obtained after removing the by-product (MnCl2) and unreacted (FeCl3 and MnO) phases from the as-milled or annealed powders by washing and magnetic separation. Magnetic measurements show that the spontaneous magnetization decreases with crystallite size; that the coercivity increases with decreasing temperature; and that below about 50 K the temperature dependence of coercivity becomes very strong, and the hysteresis loop becomes asymmetric with the remanent magnetization and the coercivity both enhanced in the upper branch when the sample is cooled in the maximum field. These 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.