A domain theory of magnetic grains in rocks

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Abstract
The activation theory of domain wall movements cannot explain the observed coercive forces of magnetite powders if the individual grains are considered to be divided into lamellar domains with alternately opposite directions of magnetization. Experimental data are shown to be consistent with a ‘rod model’ in which the grains are divided into domains of square cross section extending right across the grains. This model leads to a coercive force proportional to the -3/4 power of grain diameter, in agreement with observations on powdered magnetite. An approximate calculation of the energies of the two models indicates that the rod model has the lower energy if magnetostrictive strain is more important than magnetic anisotropy in determining domain structure. It appears probable that this is the case in magnetite, and therefore that the rod model gives E better picture of the domain structure of magnetic grab in rocks.

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