A systematic theoretical study of the equilibrium geometries and energetics of cobalt atoms encapsulated in Si,, (n = 1 - 13) clusters and comparison with pure Si,, clusters have been performed by density functional theory-generalized gradient approximation calculations combined with a genetic algorithm. Our results reveal that the geometries of bare Si, clusters are substantially modified upon doping of Co atom. Co-doping improves the stability of the clusters after n >= 7. In general, the stability of Si,,Co clusters increases with increasing size n. The Si9Co was found as magic-number cluster, and the enhanced stability was explained by the 18-electron rule. The magnetic moment on Co atom inside Si,,Co cluster is quenched in all the clusters with n >= 4. (c) 2005 Elsevier B.V. All rights reserved.