Closure domains in magnetite

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Abstract
We have observed clear arrays of closure domains at internal and external boundaries, such as crystal edges, cracks, and chemically altered regions, in a single crystal of magnetite, using the Bitter colloid technique with a {110} viewing plane. The common occurrence of closure domains in magnetite confirms the controlling role of magnetostatic energy in determining domain structures in strongly magnetic materials, first postulated by Landau and Lifschitz (1935). Closure domains also reconcile a long‐standing discrepancy between the numbers of body domains observed in magnetite and the numbers predicted, which we show are much reduced when closure domains are taken into account. We verify experimentally that closure domains change their shape and internal structure depending on the crystallographic orientation of the boundary at which they form. When the boundary intersects the {110} viewing plane along a easy axis, for example, at favorably oriented internal boundaries or cracks, the closure domains are asymmetric in shape and are bounded by 71° and 109° walls. However, when the boundary is a {111} crystal face, containing no easy axis, the closure domains are symmetric in form and bounded by ≈90° walls. Closure domains of this latter type collect colloid densely along one of the two bounding walls, suggesting concentrations of magnetic poles, and the two walls are at angles of 70°–80° to each other, rather than the expected 90°. Several possible models are proposed to explain this behavior. Our study shows that internal domain structures in magnetite are relatively simple. Much more complicated structures appear on viewing surfaces that do not contain two sets of easy axes. The lack of orientation of the viewing surface in most previous studies may account for the reported rarity of closure domains in magnetite. Closure domain arrays forming at the margin of a chemically altered area are the first reported evidence for a direct link between chemical alteration and domain structures that could result in remagnetization.