MAGNETIC DOMAIN-STRUCTURES ON A NATURAL SINGLE-CRYSTAL OF MAGNETITE

Well-defined domain structures with the theoretically expected angles of 71-degrees, 109-degrees and 180-degrees between adjacent domain magnetizations are observed on a 3-mm octahedral crystal of magnetite sectioned parallel to a {110} crystallographic plane. A regular and readily interpretable array of closure domains, of the style predicted by Landau and Lifschitz [1935], is seen at free surfaces such as crystal edges and cracks. Closure and body domains form along adjacent [111] easy axes in the interior of magnetite crystals, but the closure domains are suppressed near surfaces that do not contain two sets of [111] axes, that is if the domain structure is viewed on a plane that makes a large angle with {110}. From the measured correlation between the width of a domain and its length, we obtain estimates of (0.91 +/- 0.17) x 10(-3) J/m2 for the domain wall energy and 1.28 x 10(-11) J/m for the exchange constant in magnetite. The latter figure is close to the value 1.33 x 10(-11) J/m reported by Heider and Williams [1988]. Our study shows that the internal domain structures in multidomain magnetite are generally simple. More complicated patterns appear on unfavourably oriented viewing surfaces, but are not representative of the interior structure.