Theoretical determination of two critical sizes for strain relaxation during Co/Pt(111) heteroepitaxy

By means of tight-binding quenched-molecular-dynamics simulations, we show that Co/Pt(111) growth involves two different relaxation lengths and mechanisms. First, below completion of the first monolayer, a lateral critical size appears for two-dimensional fcc and hcp quasi-pseudomorphic islands, which release their tensile stress by a dislocation lattice, allowing an over-closepacking of the film (as for 5d metal surface reconstructions). Then, for a thicker deposit, a vertical critical thickness of 2 ML is found, beyond which the whole Co film recovers its bulk lattice parameter, inducing a vertical height modulation. These results are successfully compared with most of the existing experimental data on this system.