Tight-binding study of ammonia and hydrogen adsorption on magnetic cobalt systems

The semi-empirical self-consistent tight-binding model of ammonia and hydrogen adsorption at Co(0001) and small Co clusters is used to study the chemisorption role in surface magnetism. The adsorbate choice has been suggested by recent experiments. At the Co(0001) surface the atomic magnetization is predicted to diminish locally by 0.26 μB due to an isolated hydrogen atom adsorption; for Co13 clusters the change is somewhat smaller but less localized. At H(1×1)–Co(0001) the magnetization of surface Co atoms drops to 0.88 μB. The hydrogen magnetic moment is very small and couples antiferromagnetically to Co. Ammonia adsorption is found to reduce the Co atom magnetization locally by ∼0.1 μB or less. We discuss the possibility of adsorbate–metal antiferromagnetic coupling in more detail.