The origin of ferromagnetism in57Fe-doped NiO

The origin of ferromagnetism observed in 2 at.% 57Fe-doped NiO samples is investigated. A precursor was produced from the co-precipitation of stoichiometric 57Fe and Ni nitrate solutions, and the 2% 57Fe-doped NiO samples were prepared by heating this precursor in air at temperatures between 673 and 873 K, for 1–25 h. X-ray diffraction, scanning electron microscopy, magnetization measurements and 57Fe Mossbauer spectroscopy were used to study the properties of the prepared compounds. The samples are composed of crystalline nanoparticles with average sizes between 9 and 29 nm, depending on the heating conditions. Magnetization measurements show the appearance of room-temperature ferromagnetism for samples heated both at 673 and 873 K. The 57Fe Mossbauer spectroscopy results indicate that this ferromagnetism is related to Ni57Fe2O4 ferrimagnetic impurities for the samples heated at 873 K and to finite-size effects for the samples heated at 673 K. The Ni57Fe2O4 impurities interact magnetically with the antiferromagnetic 57Fe-doped NiO matrix and it is proposed that they result from a diffusion-type process of 57Fe3+ ions during the heat treatment of the precursor.