Effects of oxygen sorption on the ferromagnetic properties of small nickel particles

Abstract

Measurements on the effects of oxygen sorption on the low-field magnetization of nickel are reported. An investigation was made of three types of nickel-on-inert-support catalysts containing very small or larger (up to 140 Å diameter) nickel particles. The oxygen sorbed was taken either from nitrous oxide, which decomposes on nickel into oxygen and nitrogen, or from molecular oxygen. It is shown that oxygen is adsorbed homogeneously by the nickel particles interacting with nitrous oxide, and the interaction remains restricted to about the first layer of nickel atoms at room temperature. Admission of molecular oxygen to a nickel catalyst causes the nickel particles to be partially oxidized successively. From the experimental results it is concluded that the oxygen sorption affects the ferromagnetism of nickel by decoupling the magnetic moments of the chemisorbing metal surface atoms from the moment of the remainder of the metal. In completely superparamagnetic systems this brings about a decrease in magnetization that is larger on adsorption from nitrous oxide than on sorption from molecular oxygen owing to the difference in distribution of the adsorbed atoms. Measurements with magnetizing fields of different frequencies clearly demonstrate the effect of oxygen sorption on both the ferromagnetic moment and the ferromagnetic anisotropy. It is shown that the distribution of the oxygen atoms sorbed can even determine whether the resulting change in magnetization will be positive or negative, when incompletely superparamagnetic systems are investigated. The effect on the anisotropy can be ascribed to a decrease in the surface area of the ferromagnetic phase caused by the sorption of oxygen. Finally, the implications of a remaining weak coupling of the moments of the chemisorbing nickel atoms to the ferromagnetic core are discussed.