An Interpretation of the Magnetic Properties of some Iron-Oxide Powders

The observed magnetic properties of fine dispersed powders of the ferromagnetic iron oxides are discussed in the light of modern theories of the magnetization of ferrites and of ferromagnetics containing non-magnetic inclusions. The individual powder particles are shown to be probably of single-domain size, and the relative coercivities of γ-ferric oxide powders of nearly spherical and of markedly acicular grains are qualitatively in agreement with theory, on the grounds of shape anisotropy. After allowance is made for volume concentration and small departures from stoichiometric composition, the ratio of the coercivities of apparently identical acicular powders of γ-ferric oxide and of magnetite is found to be approximately that of the saturation magnetization per ferric ion rather than that of the value per unit volume, and both coercivities are lower than theory would predict It is concluded that magneto-crystalline anisotropy is also involved. In contrast, relative and actual values of intrinsic induction at the beginning of the approach to saturation are in satisfactory agreement with theory, as are the relative values at remanence although the actual values here are rather low. It is hoped to extend the study to larger departures from stoichiometric composition involving partial transformation to the antiferromagnetic iron oxides α-Fe 2 O 3 and FeO.