Appearance of a Weak Ferromagnetism in Fine Particles of Antiferromagnetic Materials

This is the authors' abstract. We don't add key points for this paper.

Several experiments have been carried out on NiO which may be interpreted as confirmation of the suggestion of Néel that antiferromagnetic material in fine particle form should exhibit a weak ferromagnetism and superparamagnetism. The initial susceptibility of the nickel oxide between 22 A and > 1000 A increases with decreasing particle size. The susceptibility decreases with decrease in temperature below room temperature for all but the smallest size sample. This sample had a very large susceptibility and an ``S''-shaped magnetization curve, characteristic of superparamagnetic material. In the intermediate size range, cooling through the Néel temperature in a field introduces an additional magnetization which is field independent.