HR: 1340h
AN: GP13A-0016    [Abstracts]
TI: Low-temperature viscous magnetization of multidomain magnetite: evidence for disaccommodation contribution
AU: * Muxworthy, A R
EM: adrian.muxworthy@gmail.com
AF: University of Southampton, NOC, European Way, Southampton, SO14 3ZH United Kingdom
AU: Williams, W
EM: wyn.williams@ed.ac.uk
AF: University of Edinburgh, Geology, King's Buildings, West Mains Road, Edinburgh, EH9 3JW United Kingdom
AB: There are large gaps in our understanding of multidomain (MD) viscous behavior and MD remanence theory in general. For example, Dunlop and ™zdemir (2000) have demonstrated that multidomain (MD) viscous remanent magnetization (VRM) acquired at 200 C in crushed and sized natural crystals of magnetite persists on thermal demagnetization 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-organized by small increases in temperatures. That is, theory predicts that any VRM acquired by domain walls at 200 C will not persist to the Curie temperature. Recent viscosity experiments at temperatures above room-temperature have revealed that MD viscosity has a complex dependency on the magnetization state, dislocation density, disaccommodation, stoichiometry and thermal history. In particular the contribution of disaccommodation to viscosity has been difficult to isolate. Below room-temperature, disaccommodation displays a large increase, therefore by measuring viscosity below room temperature, then its contribution can be more readily assessed. Low-temperature viscous acquisition and decay measurements above and below the Verwey transition have been measured for a selection of natural and synthetic multidomain magnetite samples. A strong correlation between the viscosity spectra and published disaccommodation spectra was found, where disaccommodation reflects electron mobility. Assuming the viscosity is controlled by identical mechanisms as disaccommodation, the reduction in electron mobility below the Verwey transition is found to significantly increase viscous acquisition and decay rates over the time scales measured (1-3000 seconds). Although strongly affecting the viscosity, disaccommodation processes do not appear to control the rate of change of viscosity with time, i.e., the viscosity curvature. It is suggested that the curvature is controlled by the shape of relaxation-time distributions, which is approximately the same for all the magnetite samples studied. In addition, the acquisition and decay curvature parameters mirror each other when plotted as a function of temperature, inferring that at any given temperature the acquisition and decay processes are identical.
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005