Structural and magnetic properties of ultrahigh vacuum-grown cobalt particles deposited on self-organized Ru buffer layers are presented and correlated. The Co structures formed this way present fascinating examples of how magnetism is influenced by mesoscopic geometrical constraints. Especially, the variation of the magnetic anisotropies at low temperature suggests effects of large interparticle interactions. (C) 1999 American Institute of Physics. [S0021-8979(99)66108-4].