Ultrafine magnetic particles dispersed in a silica matrix were successfully obtained by treatment of a cross-linked cobalt iron citrate precursor, synthesized by a modified Pechini route, with 0.001 M K2Cr2O7 at 130 degrees C. The IR and NMR spectroscopic characterization of the precursor gel containing Co2+ and Fe3+ shows that the citric acid reacts with the metallic ions by coordination, the ethylene glycol by esterification and the tetraethylorthosilicate by substitution. SQUID measurements of the composite indicate superparamagnetic behavior. The blocking temperature, from the peak of the zero-field-cooled measurements, was 3 K at 1000 Oe. The magnetic diameter calculated using Langevin's equation was 4 mu. (C) 2007 Elsevier Ltd. All rights reserved.