Anisotropy of Co nanoparticles induced by swift heavy ions

Spherical metallic nanoparticles have been formed in SiO2 layers by 160-keV implantation of Co ions at a fluence of 1017 ions cm-2. The implanted samples were next irradiated with 200-MeV 127I at fluences ranging from 1011 to 1014 ions 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 127I fluences below 1012 ions 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