A monolayer of gamma-Fe2O3 nanoparticles embedded in a polyimide (PI) matrix was fabricated by oxidizing an Fe metal film between two PI precursor layers. There was a critical Fe thickness (similar to7 nm) above which a continuous layer of gamma-Fe2O3 film was formed in the PI film. Below the critical Fe thickness, the oxide film broke up into fine particles whose size was similar to8 nm with narrow size distribution. It was further shown that these nanoparticles could have metallic cores, surrounded by an oxide layer. This method offers a unique way of covering a large surface area with fine magnetic oxide nanoparticles for potential application in high-density data-storage media. (C) 2004 Elsevier Inc. All rights reserved.