Abstract
The origin of ferromagnetism observed in 2 at.%57Fe-dopedNiO samples is investigated. A precursor was produced from the co-precipitation of stoichiometric57Fe and Ni nitratesolutions, and the 2% 57Fe-doped NiO samples were prepared by heating this precursor in air at temperatures between 673 and873 K, for 1–25 h. X-ray diffraction, scanning electron microscopy, magnetization measurements and57Fe Mössbauer spectroscopy were used to study the properties of the prepared compounds. Thesamples are composed of crystalline nanoparticles with average sizes between 9 and 29 nm,depending on the heating conditions. Magnetization measurements show the appearance ofroom-temperature ferromagnetism for samples heated both at 673 and 873 K. The57Fe Mössbauer spectroscopy results indicate that this ferromagnetism is related toNi57Fe2O4 ferrimagnetic impurities for the samples heated at 873 K and to finite-size effects for the samples heated at673 K. The Ni57Fe2O4 impurities interact magnetically with the antiferromagnetic57Fe-doped NiO matrix and it is proposed that they result from a diffusion-type process of57Fe3+ ions during the heat treatment of the precursor.
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