Characterization of the dispersion process for NiFe₂O₄ nanocrystals in a silica matrix with infrared spectroscopy and electron paramagnetic resonance

The structural development of the NiFe₂O₄ nanocrystals dispersed in a silica matrix was followed by IR and EPR spectroscopies of the dried gel 10NiO-10Fe₂O₃-90SiO₂ after heat treatment. The dried gel obtained at 200°C was amorphous, in which Fe³⁺ and Ni²⁺ ions were distributed in the pores of silica matrix. When the dried gel was heat treated at 400°C, NiFe₂ O₄ clusters were partially formed, showing an enhanced interaction with the silica matrix. NiFe₂O₄ clusters were completely formed in silica matrix when the heat treatment was increased to 600°C, at which the interactions between the clusters and silica matrix reached a maximum. The formation reaction of NiFe₂O₄ clusters was accompanied by a rearrangement of the silica matrix network. Further increase of the heat treatment temperature to 800°c led to superparamagnetic single domain NiFe₂O₄ nanocrystals (ca. 4 nm) dispersed in the silica matrix with the elimination of the interactions between magnetic nanocrystals and silica matrix.