CLOSURE DOMAINS IN MAGNETITE

We have observed clear arrays of closure domains at internal and exter nal boundaries, such as crystal edges, cracks, and chemically altered regions, in a single crystal of magnetite, using the Bitter colloid te chnique with a {110} viewing plane, The common occurrence of closure d omains in magnetite confirms the controlling role of magnetostatic ene rgy in determining domain structures in strongly magnetic materials, f irst postulated by Landau and Lifschitz (1935). Closure domains also r econcile a long-standing discrepancy between the numbers of body domai ns observed in magnetite and the numbers predicted, which we show are much reduced when closure domains are taken into account. We verify ex perimentally that closure domains change their shape and internal stru cture depending on the crystallographic orientation of the boundary at which they form. When the boundary intersects the {110} viewing plane along a <111> easy axis, for example, at favorably oriented internal boundaries or cracks, the closure domains are asymmetric in shape and are bounded by 71 degrees and 109 degrees walls. However, when the bou ndary is a {111} crystal face, containing no <111> easy axis, the clos ure domains are symmetric in form and bounded by approximate to 90 deg rees walls. Closure domains of this latter type collect colloid densel y along one of the two bounding walls, suggesting concentrations of ma gnetic poles, and the two walls are at angles of 70 degrees-80 degrees to each other, rather than the expected 90 degrees. Several possible models are proposed to explain this behavior. Our study shows that int ernal domain structures in magnetite are relatively simple. Much more complicated structures appear on viewing surfaces that do not contain two sets of <111> easy axes. The lack of orientation of the viewing su rface in most previous studies may account for the reported rarity of closure domains in magnetite. Closure domain arrays forming at the mar gin of a chemically altered area are the first reported evidence for a direct link between chemical alteration and domain structures that co uld result in remagnetization.