Magnetic Properties of Colloidal Nickelous Oxide

Magnetic susceptibility measurements by the Faraday method have been made in the temperature range 4-550^ for a series of iron-free nickelous oxide samples prepared by the heat-treatment of hydrous nickelous hydroxide in a current of nitrogen at temperatures of 250-1300^, the crystal size as given by electron diffraction methods varying from 80-2000 A. Samples (crystal size 2000 A) prepared at 1200-1300^ exhibit susceptibilities typical of an antiferromagnetic material, the observed N\'eel point of 523^ agreeing with the results of earlier investigators. In samples (crystal size 100-200 A) prepared at 300-500^ the susceptibility agrees with that for the larger crystals above the N\'eel point, but exhibit maxima at lower temperatures. The temperatures, T₂, at which the maxima appear, decrease regularly and the amplitudes of the maxima increase regularly, with decreasing crystal size. In the range studied, linear plots 10^-4T₂=1. 090f-1. 037 are obtained for T₂ as a function of f, the ratio of the average number of next nearest magnetic neighbors per nickel atom to the number of next nearest magnetic neighbors in an infinite crystal, as computed from the observed crystal sizes. Neutron diffraction patterns demonstrate that the magnetic structure is antiferromagnetic at temperatures both above and below T₂, and hence the maxima do not correspond to shifted N\'eel points. The magnetic susceptibility curves for the colloidal crystals of nickelous oxide resemble those obtained by others for certain iron and chromium sulfides. In iron sulfide, FeSₗ, the susceptibility increase to the maximum is attributed by N\'eel and others to a ferrimagnetic structure, analogous to certain ferrites. In view of the "active" oxygen (Bunsen test) contents of the nickelous oxide samples which vary from 0. 36 to 0. 00% as the crystal size increases from 80 to 2000 A, it is possible to attribute the ferrimagnetic behavior to the ordering of a sublattice of defects, as N\'eel does for FeSₗ. However, the subsequent decrease in susceptibility at temperatures below T₂ cannot be readily explained for either FeSₗ or NiO. In view of the linear relationship between T₂ and f, it is concluded that the unusual magnetic properties of colloidal nickelous oxide is attributable to the small size of the crystals and the accompanying enhanced specific surface and the decreased number of next nearest magnetic neighbors which results in an altered magnetic environment. Nickelous oxide prepared at 250^ contains a small amount of undecomposed nickelous hydroxide detectable by electron diffraction methods. The magnetic susceptibility of pure nickelous hydroxide follows a Curie-Weiss law ₌=1. 07 (T-36) from about 300^ to 90^. Below 90^, the susceptibility increases more slowly to a value of 200010^-6 emu at 4^, and some field dependence was observed, suggesting the desirability of additional studies concerning possible ferromagnetism.