Transition element nanoclusters embedded in dielectric matrix are important for their optical and magnetic properties. In this work, composite silica films with cobalt atomic concentration up to 10% were synthesized by radiofrequency (rf) magnetron co-sputtering deposition technique. The composite systems structure could be triggered by varying the deposition parameters and by using a subsequent thermal treatment to promote the nanocluster formation. Magnetic measurements showed that the embedded cobalt nanoparticles act as superparamagnets, with coercive field and blocking temperature related to their size and structure (hcp or fcc). (C) 2003 Elsevier B.V. All rights reserved.