The effects of hydrogen and oxygen chemisorption on the ferromagnetic anisotropy of nickel-on-silica particles

Abstract

After a consideration of the results so far obtained on the effects which oxygen and hydrogen chemisorption have on the ferromagnetism of nickel, it is shown that investigation of the effect of adsorption on the ferromagnetic anisotropy of nickel can be a considerable aid in forming an understanding of the chemisorptive bond on nickel. Then follows a brief discussion on the origin of ferromagnetic anisotropy and on the setup of experiments furnishing information about the effect of chemisorption on the ferromagnetic anisotropy. Experiments are presented in which the effect of hydrogen chemisorption on the magnetization of nickel-on-silica particles is determined by measurements in fields alternating with frequencies of 1 to 20 kc/sec and at temperatures from 77° to 373°K. It is demonstrated that the experimental results can be accounted for to a high degree. To permit a reliable comparison to be made between the effects of hydrogen and oxygen, the distribution of oxygen over the nickel particles in the samples must be equal to that of hydrogen. This has been achieved by deriving the oxygen from nitrous oxide. It appears that hydrogen chemisorption decreases the anisotropy of nickel particles up to about 50 A in diameter much more than does oxygen chemisorption. This suggests that the anisotropy is affected by a surface layer which is modified differently by hydrogen adsorption than by oxygen adsorption. Measurements on larger nickel particles (diameters about 70 A) demonstrate that, in a relative sense, hydrogen adsorption has a much smaller effect on the anisotropy of these particles. The cause of this difference, which is related to the size of the nickel particles, is discussed. A detailed analysis of the variation of the effect of oxygen adsorption with the frequency of the magnetizing field shows that oxygen adsorption decreases the anisotropy of larger nickel particles at low coverages only. At higher coverage, no further decrease in anisotropy is observed. Finally, an explanation is developed for the difference between the effects of hydrogen and oxygen adsorption on the anisotropy, as well as for the absence of a further decreasse in anisotropy energy on oxygen adsorption at higher coverages on larger nickel particles.