FMR Study of Surface-Tension-Related Stress Effects in Ultraclean Ni Thin Films

Abstract

Ni thin films, ranging in thickness from 19 to 837 Å, were deposited under ultrahigh vacuum conditions (10⁻⁹ to 10⁻¹⁰ Torr) onto extremely well degassed substrates. Intrinsic isotropic stress values in the films were determined by ferromagnetic resonance; these measurements were carried out while the films were in the vacuum system in which they had been prepared. Films deposited onto substrates at 25°–35°C were in a state of compressive stress when measured in UHV. The surface tension provides an explanation for these stresses; the stress levels are shown to obey a simple model. After adsorption of O₂, N₂O, or air, the stress levels in the films dropped to very low values, while adsorption of H₂, H₂O, CO, and pyridine resulted in slightly smaller effects. Admission of N₂ to the films caused no stress release. A small degree of reversibility of the latter effect was found for H₂ and H₂O. Adsorption of gases had a marked effect on the ferromagnetic resonance linewidths. Films thinner than 100 Å showed deviations from the surface tension model and the possible discontinuity of such films is indicated as the reason. Experiments on annealed films and experiments with films deposited onto substrates at elevated temperatures were performed. The influence of vacuum conditions other than UHV during film deposition was investigated. The relevance of this study to the solution of the problems of stress corrosion cracking and hydrogen embrittlement is pointed out.