Spherical nanoparticles have been formed in SiO 2 layers by 160 keV Co + ion implantation. Different particle sizes have been obtained depending on implantation fluence (2 × 10 16, 5 × 10 16 and 1 × 10 17 Co + cm −2) and substrate temperature (77, 295 and 873 K). These spherical particles have been subsequently irradiated with 200 MeV 127I ions at fluences from 10 11 to 10 14 cm −2 at 300 K. Modifications of magnetic properties of the irradiated samples, studied by a SQUID magnetometer at 5 and 295 K in a magnetic field applied perpendicular or parallel to the incident beams, have been related to the modifications of the particle size and shape observed by transmission electron microscopy (TEM). The results are in good agreement with calculations based on a model describing two different regimes: a spherical growth at low irradiation fluences and a prolate deformation along the beam direction for the high fluences.