Fe3O4 nanoparticles with size ∼ 13 nm have been prepared successfully in aqueous micellar medium at ∼ 80 ° C. To make Fe3O4 nanoparticles resistant to surface poisoning a new route is developed for coating Fe3O4 nanoparticles with noble metals such as gold or silver as shell. The shell thickness of the core-shell particles becomes tunable through the adjustment of the ratio of the constituents. Thus, the route yields well-defined core-shell structures of size from 18 to 30 nm with varying proportion of Fe3O4 to the noble metal precursor salts. These magnetic nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), MR, differential scanning calorimetry (DSC), Raman and temperature-dependent magnetic studies. © 2005 Elsevier Inc. All rights reserved.