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 mag netizations are observed on a 3-mm octahedral crystal of magnetite sec tioned parallel to a {110} crystallographic plane. A regular and readi ly interpretable array of closure domains, of the style predicted by L andau and Lifschitz [1935], is seen at free surfaces such as crystal e dges and cracks. Closure and body domains form along adjacent [111] ea sy axes in the interior of magnetite crystals, but the closure domains are suppressed near surfaces that do not contain two sets of [111] ax es, that is if the domain structure is viewed on a plane that makes a large angle with {110}. From the measured correlation between the widt h 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 val ue 1.33 x 10(-11) J/m reported by Heider and Williams [1988]. Our stud y shows that the internal domain structures in multidomain magnetite a re generally simple. More complicated patterns appear on unfavourably oriented viewing surfaces, but are not representative of the interior structure.