期刊
EARTH AND PLANETARY SCIENCE LETTERS
卷 194, 期 3-4, 页码 343-358出版社
ELSEVIER SCIENCE BV
DOI: 10.1016/S0012-821X(01)00562-3
关键词
magnetite; hysteresis; magnetic domains; coercivity; remanent magnetization
Submicron magnetite crystals with mean sizes of 0.037, 0.10 and 0.22 mum undergo major changes in hysteresis properties and domain states in crossing the Verwey transition (T-v approximate to 120 K). The 0,037 mum crystals are single-domain (SD) both in the cubic phase at room temperature T-0 and in the monoclinic phase below T-v. The 0.10 and 0.22 mum crystals have a mixture of SD and two-domain (2D) states at room temperature T-0 but mainly SD structures below T-v, in agreement with micromagnetic calculations. Coercive force H-c increases on cooling through T-v, by a factor 3-5 in the submicron magnetites and 40 in a 1.3 mm, single crystal, because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite. As a result, domain walls and SD moments are so effectively pinned below T-v that all remanence variations in warming or cooling are reversible. However, between approximate to 100 K and T-0, remanence behavior is variable. Saturation remanence (SIRM) produced in monoclinic magnetite at 5 K drops by 70-100% in warming across T-v, with minor recovery in cooling back through T-v (ultimate levels at 5 K of 23-37% for the submicron crystals and 3% for the 1.3 mm crystal). In contrast, SIRM produced in the cubic phase at 300 K decreases 5-35% (submicron) or > 95% (1.3 mm) during cooling from 300 to 120 K due to continuous re-equilibration of domain walls, but there is little further change in cooling through T-v itself. However, the submicron magnetites lose a further 5-15% of their remanence when reheated through T-v. These irreversible changes in cycling across T-v, and the amounts of the changes, have potential value in determining submicron magnetite grain sizes. The irreversibility is mainly caused by 2D-->SD transformations on cooling through T-v, which preserve or enhance remanence, while SD-->2D transformations on warming through T-v cause remanence to demagnetize. (C) 2002 Elsevier Science B.V. All rights reserved.
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