HR: 11:10h
AN: GP12A-03    [Abstracts]
TI: Memory of the magnetic field applied during cooling in stoichiometric magnetite: Grain-size dependence and inferences on the physical processes
AU: * Smirnov, A V
EM: alexei@earth.rochester.edu
AF: Yale University, Dept of Geology and Geophysics PO Box 208109, New Haven, CT 06520 United States
AU: Tarduno, J A
EM: john@earth.rochester.edu
AF: University of Rochester, Dept of Earth and Environmental Sciences Hutchison Hall 227, Rochester, NY 14627 United States
AB: A memory effect was recently discovered in stoichiometric magnetite (Fe₃O₄) at temperatures below its transition from a cubic to monoclinic structure (the Verwey transition, ~120 K) (e.g., Smirnov and Tarduno, EOS, 2002). The effect is expressed as an inflection point observed in magnetic hysteresis loops after cooling magnetite through the Verwey transition in the presence of a magnetic field. The location of the inflection is controlled by the strength of the applied field during cooling (for fields between 0.01 and 0.09 T). Our working hypothesis is that the reorganization of magnetic domains and the formation of monoclinic twins may interact such that they give rise to the field-memory effect. The hypothesis predicts that the effect should not exist in single-domain magnetite (no domain walls). We also expect that the effect should be less well expressed in multidomain magnetite because of the larger number of magnetic domains (i.e. only a few of which will potentially interact with monoclinic twins). We tested these predictions by studying synthetic magnetite samples, which varied from single-domain to multidomain in size. Our experimental results to date confirmed the predictions. The effect was best pronounced for grain sizes between 0.5 and 5 microns, and less well expressed in samples with larger grain sizes. No effect was observed on single-domain magnetite. We will discuss implications of our results for the physical mechanisms of the memory of the field applied during cooling. We also discuss new experimental approaches (such as three-axial low-temperature magnetometry), which may provide additional insights into these mechanisms.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly