Magnetic Properties of Colloidal Nickelous Oxide

Magnetic susceptibility measurements by the Faraday method have been made in the temperature range 4-550°K 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°C, the crystal size as given by electron diffraction methods varying from 80-2000 A. Samples (crystal size ∼2000 A) prepared at 1200-1300°C exhibit susceptibilities typical of an antiferromagnetic material, the observed Néel point of 523°K agreeing with the results of earlier investigators. In samples (crystal size ∼100-200 A) prepared at 300-500°C the susceptibility agrees with that for the larger crystals above the Néel point, but exhibit maxima at lower temperatures. The temperatures, $T_c$, 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 [${10}^{-4\mathrm{}}{T}_c=1.090f-1.037\mathrm{}$] are obtained for $T_c$ 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 $T_c$, and hence the maxima do not correspond to shifted Néel points.