Ultrafine MnFe_2O_4 powders have been synthesised via the displacement reaction 2FeCl_3 +4MnO-> MnFe_2O_4 + 3MnCl_2 activated by high-energy ball milling. Single-phase MnFe_2O_4 powders having crystallite sizes ranging between 9.5 and 40 nm have been obtained after removing the by-product (MnCl_2) and unreacted (FeCl_3 and MnO) phases from the as-milled or annealed powders by washing and magnetic separation. Magnetic measurements show tha the spontaneous magnetisation decreases with crystallite size; that the coercivity increases with decreasing temperature; and that below about 20K, the temperature dependence of coercivity becomes very strong and the hysteresis loop becomes asymmetric with an enhanced remanence and coercivity 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 are coupled through exchange interactions at the interface.
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