Growth and properties of small Co islands on a strained Pt surface

The effect of strain on the surface diffusion of Co on a Pt surface is studied using first-principles density functional theory. We employed the projector augmented wave method in the 7-layer slab geometry. Co tends to occupy FCC sites except for surfaces under large tensile strain where HCP sites are energetically favorable. We find that energy barrier for single atom diffusion increases linearly with strain. The total energy of adatoms decreases with increase of strain. The magnetization of the Co adatom is around 2 μ_B. Pt atoms are strongly polarized making the total magnetic moment per Co atom at FCC sites 3.6 μ_B. The magnetic moment of the system is sensitive to the adatom position and the strain. For the saddle point, for example, the magnetization changes from 3.1 μ_B per Co atom for an unstrained lattice to 3.8 μ_B for a 3% compressed lattice. The results suggest that both FCC and HCP islands formation is possible on the strained Pt(1 1 1) surface. Exchange coupling between adatoms is analyzed using Green's function method. There is a strong exchange interaction between Co moments in the small clusters on the surface, suggesting that they should behave like giant moments.