Characterization of stoichiometric nanocrystalline spinel ferrites dispersed on porous silica Aerogel

Stoichiometric magnetic nanosized ferrites MFe(2)O(4) (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 Mossbauer 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 NiFe(2)O(4) sample, highly blocked (similar to 70%) in the MnFe(2)O(4) sample and nearly fully blocked in the CoFe(2)O(4) sample. A fitting procedure of the Mossbauer 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 CoFe(2)O(4) nanocrystals.