HR: 09:05h
AN: GP11B-05    [Abstracts]
TI: Magnetostatic interactions in a natural magnetite-ulvospinel intergrowth system
AU: * Evans, M E
EM: evans@phys.ualberta.ca
AF: Institute for Geophysical Research, University of Alberta, Edmonton, AB T6G 2J1 Canada
AU: Krasa, D
EM:
AF: Earth and Environmental Sciences, Ludwig Maximilians University, Munich, 80333 Germany
AU: Williams, W
EM:
AF: School of Geociences, University of Edinburgh, Edinburgh, EH9 3JW United Kingdom
AU: Winklhofer, M
EM:
AF: Earth and Environmental Sciences, Ludwig Maximilians University, Munich, 80333 Germany
AB: The difficult problem of magnetostatic interactions in naturally-occurring minerals has a long history but a renewed attack on it is currently being driven by recent advances in instrumentation and computing power. We report a new investigation of a finely exsolved magnetite/ulvöspinel intergrowth first studied magnetically by Evans & Wayman (1974) and more recently by Harrison et al. (2002). Transmission electron micrographs reveal a rectilinear pattern of tiny magnetite blocks separated by ulvöspinel sheets. The magnetite blocks have a gaussian size distribution with mean and standard deviation of 193 and 46 nm, respectively (n ¦ 500), with the separation between nearest neighbours being typically 40 nm, but often much less. Thermomagnetic analysis yields a well-defined Curie point of 548°C indicating that the ``magnetite" actually has a compostion of Fe2.9Ti0.1O4. Routine hysteresis measurements immediately reflect the interaction between neighbouring ``magnetite" regions, with Mrs/Ms = 0.22, well below the expected value for non-interacting single-domain particles. The corresponding FORC diagram clearly reveals the interaction fields with Hi = 30 mT (full-width at half-maximum, FWHM) centred on a well-defined Hc peak at 20 mT. Furthermore, the maximum interaction field observed (~50 mT) agrees well with that expected from simple theory and micromagnetic calculations. Elimination of the intergrowth structure by heating in an evacuated quartz vial for 2 hours at 1000 °C leads to marked changes in the magnetic properties: Mrs/Ms drops to 0.11, Hcr/Hc increases from 1.98 to 2.73, the main peak on the FORC diagram shifts to 6 mT and the interaction field profile drastically narrows (FWHM Hi = 14 mT).
DE: 1519 Magnetic mineralogy and petrology
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
DE: 1595 Planetary magnetism: all frequencies and wavelengths
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005