The effect of chemisorption on collective metal properties

Abstract

From a discussion of the effects of chemisorption on electrical conductance, ferromagnetic moment and ferromagnetic anisotropy of metals, conclusions are drawn about the state of the metal surface atoms after chemisorption. Hydrogen chemisorption resembles alloying of the surface layer of the metal; the state of the metal surface atoms is not appreciably changed. Chemisorption of gases such as oxygen, carbon monoxide and nitrogen substantially weakens the binding of the surface atoms to the remainder of the metal. However, as appears from the effect of hydrogen chemisorption on the ferro-magnetic moment, the influence of hydrogen on very small nickel particles is much stronger, which is ascribed to the lower degree of coordination of the metal atoms in these particles. The decrease in ferromagnetic moment brought about by chemisorption points to a decrease in ferromagnetic coupling energy to a value smaller than the thermal energy, which converts the metal surface atoms generally into a paramagnetic state. However, there remains an interaction between the metal surface atoms and the remainder as appears from the effects of chemisorption on the ferromagnetic anisotropy. Effects of chemisorption on the anisotropy of small nickel particles, of evaporated metal films and on the spin wave resonance properties of evaporated films are discussed. Owing to the coupling of the metal subsurface atoms to the surface atoms that are paramagnetic after chemisorption, the anisotropy decreases on chemisorption. When, however, an anti-ferromagnetic surface phase of appreciable size is formed, as happens on oxidation, no decrease in anisotropy is found, since in that case the metal atoms have their atomic moments coupled to moments the orientation of which remains fixed.