Spherical metallic nanoparticles have been formed in ${\mathrm{SiO}}_2$ layers by 160-keV implantation of Co ions at a fluence of ${10}^{17}$ ions ${\mathrm{cm}}^{-2}.$ The implanted samples were next irradiated with $200-\mathrm{MeV}{}^{127}\mathrm{I}$ at fluences ranging from ${10}^{11}$ to ${10}^{14}$ ions ${\mathrm{cm}}^{-2}$ at room temperature. Modifications of nanoparticle size and shape were observed by transmission-electron microscopy and have been characterized by magnetic measurements at 295 and 5 K. For ${}^{127}\mathrm{I}$ fluences below ${10}^{12}$ ions ${\mathrm{cm}}^{-2},$ the nanoparticles grow in size but remain spherical. For higher fluences an anisotropic growth occurs, with elongation and the preferential easy axis seen along the incident-beam direction. The nanoparticle’s growth and deformations are attributed to thermal spike effects.

• Received 10 April 2003
• Corrected 30 June 2003

DOI:https://doi.org/10.1103/PhysRevB.67.220101