HR: 10:35h
AN: GP22A-02    [Abstracts]
TI: Viscous behaviour of multidomain magnetite
AU: * Muxworthy, A R
EM: adrian.muxworthy@ed.ac.uk
AF: University of Edinburgh, Institute of Earth Science University of Edinburgh King's Buildings West Mains Road, Edinburgh, EH9 3JW United Kingdom
AU: Williams, W
EM: wyn.willliams@ed.ac.uk
AF: University of Edinburgh, Institute of Earth Science University of Edinburgh King's Buildings West Mains Road, Edinburgh, EH9 3JW United Kingdom
AB: Magnetic materials acquire viscous remanent magnetisation (VRM) by exposure to ambient fields over long time-scales. This VRM is usually identified in rocks by a component of magnetisation parallel to the direction of the Earth's magnetic field at the site. VRM masks the original palaeomagnetic signal in the rock and must be removed. The expectation in palaeomagnetic studies is that this VRM component will demagnetise between ambient temperatures and a maximum temperature that is not likely to exceed 150-200 \deg C. However, Dunlop and \"{O}zdemir (2000) have demonstrated that multidomain (MD) VRM acquired at 200 \deg C in crushed and sized natural crystals of magnetite persists on thermal demagnetisation up to the Curie temperature, that is, there is a MD VRM component which is metastable. This goes against the classic MD theory of N‚el which predicts that domain walls which move at low-temperatures in the earth's field, are easily re-organised by small increases in temperatures. Which means that any VRM acquired by domain walls at 200 \deg C will not persist to the Curie temperature. Clearly, there are large gaps in our understanding of MD VRM behaviour and MD remanence theory in general. In this paper we report the study of MD magnetic viscosity using high-temperature controlled-field magnetometers. To examine the contribution of disaccomodation to viscous behaviour we measured the viscosity both before and after high-temperature thermal equilibration. In such a study it is very important to have well-characterised samples. So, to conduct these experiments we have examined a synthetic and selected natural multidomain magnetite samples. In particular we have grown a new set of sized (mean grain sizes between 1-100 $\mu$m) MD magnetites by hydrothermal recrystallisation. In addition to these samples, we have considered sized MD synthetic magnetites of commercial origin, and large natural individual crystals of magnetite.
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting