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

The semi-empirical self-consistent tight-binding model of ammonia and hydro gen adsorption at Co(0001) and small Co clusters is used to study the chemi sorption 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 mu(B) due to an isolated hydrogen at om adsorption; for Co-13 clusters the change is somewhat smaller but less l ocalized. At H(1x1)-Co(0001) the magnetization of surface Co atoms drops to 0.88 mu(B). The hydrogen magnetic moment is very small and couples antifer romagnetically to Co. Ammonia adsorption is found to reduce the Co atom mag netization locally by similar to 0.1 mu(B) or less. We discuss the possibil ity of adsorbate-metal antiferromagnetic coupling in more detail. (C) 2000 Elsevier Science B.V. All rights reserved.