Elaboration by ion implantation of cobalt nano-particles in silica layers and modifications of their properties by electron and swift heavy ion irradiations

This work aims to investigate the capability of ion irradiations to elaborate magnetic nano-particles in silica layers, and to modify their properties. Co+ ions have been implanted at 160 keV at fluences of 2.1016, 5.1016 and 1017 at/cm2, and at temperatures of 77, 295 and 873 K. The dependence of the particle size on the implantation fluence, and more significantly on the implantation temperature has been shown. TEM (transmission electronic microscopy) observations have shown a mean diameter varying from 1 nm for implantations at 2.1016 Co+/cm2 at 77 K, to 9.7 nm at 1017 Co+/cm2 at 873 K. For high temperature implantations, two regions of particles appear. Simulations based on a kinetic 3-dimensional lattice Monte Carlo method reproduce quantitatively the features observed for implantations. Thermal treatments induce the ripening of the particles. Electron irradiations at 873 K induce an important increase in mean particle sizes. Swift heavy ion irradiations also induce the ripening of the particles for low fluences, and an elongation of the particles in the incident beam direction for high fluences, resulting in a magnetic anisotropy. Mechanisms invoked in thermal spike model could also explain this anisotropic growth. (author)