HR: 08:05h
AN: GP11B-01 INVITED     [Abstracts]
TI: Low-temperature cooling behavior of single-domain magnetite: forcing of the crystallographic axes and interactions
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: Low-temperature cycling (LTC) of magnetic remanences carried by rocks has become a standard technique in palaeo-, rock and environmental magnetism as a means of identifying mineralogy and grain-size. LTC usually involves measuring low-temperature thermomagnetic curves on cooling through crystallographic transitions, such as magnetite's Verwey transition at 120 K. The literature suggests that single domain (SD) magnetite grains are not affected by cooling through the cubic/monoclinic Verwey transition whereas larger multidomain (MD) magnetite grains partially demagnetize. However, Carter et al (2004) recently pointed out that the maximum shape anisotropy for SD grains (an infinitely long cylinder) is approximately three times smaller than the monoclinic magnetocrystalline anisotropy along the hardest axis, i.e., SD grains are not impervious to LTC. Using a simple model for uniaxial SD grains they showed non-interacting grains should display abrupt reversible behavior at the Verwey transition. Initially we extended their model to include cubic anisotropy and magnetostatic interactions, however, there are certain key features observed experimentally, which this model does not explain. We developed a new "controlled switching" mechanism; the orientation of the low-temperature monoclinic axes are not chosen randomly, but instead are controlled by the direction of the magnetic moment on cooling through the Verwey transition. The model of Carter et al. (2004) assumes that relationship between the cubic and monoclinic symmetry is randomly chosen. This new model correctly predicts experimentally observed low-temperature trends that the "random" model of Carter et al. (2004) does not. We therefore propose a new model for the mechanism controlling the behavior of SD grains at the Verwey transition, and show that the low-temperature behavior of SD and MD grains can yield ambiguously similar behavior.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 4260 Ocean data assimilation and reanalysis (3225)
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005