Appearance of a Weak Ferromagnetism in Fine Particles of Antiferromagnetic Materials

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Abstract
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.

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