Characterization of Stoichiometric Nanocrystalline Spinel Ferrites Dispersed on Porous Silica Aerogel

Stoichiometric magnetic nanosized ferrites MFe₂O₄ (M = Mn, Co, Ni) were prepared in form of nearly spherical nanocrystals supported on a highly porous silica aerogel matrix, by a sol–gel procedure. X-ray diffraction and transmission electron microscopy indicate that these materials are made out of non-agglomerated ferrite nanocrystals having size in the 5-10 nm range. Investigation by Mössbauer Spectroscopy was used to gain insights on the superparamagnetic relaxation and on the inversion degree. Magnetic ordering at room temperature varies from superparamagnetic in the NiFe₂O₄ sample, highly blocked (∼70%) in the MnFe₂O₄ sample and nearly fully blocked in the CoFe₂O₄ sample. A fitting procedure of the Mössbauer data has been used in order to resolve the spectrum into the tetrahedral and octahedral components; in this way, an inversion degree of 0.68 (very close to bulk values) was obtained for 6 nm silica-supported CoFe₂O₄ nanocrystals.