HR: 0800h
AN: GP21B-0505    [Abstracts]
TI: Monoclinic c-axis selection at the Verwey transition: new insights from off-axis electron holography and the delta-ratio magnetosome detection method
AU: * Chen, A P
EM: chen0653@umn.edu
AF: Institute for Rock Magnetism, Department of Geology and Geophysics, University of Minnesota, 291 Shepherd Labs 100 Union Street S.E., Minneapolis, MN 55455, United States
AU: Feinberg, J M
EM: jfei05@esc.cam.ac.uk
AF: Institute for Rock Magnetism, Department of Geology and Geophysics, University of Minnesota, 291 Shepherd Labs 100 Union Street S.E., Minneapolis, MN 55455, United States
AU: Feinberg, J M
EM: jfei05@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Kasama, T
EM: tk305@cam.ac.uk
AF: Department of Material Sciences and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom
AU: Simpson, E T
EM: ets22@cam.ac.uk
AF: Department of Material Sciences and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom
AU: Harrison, R J
EM: rjh40@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AU: Moskowitz, B M
EM: bmosk@umn.edu
AF: Institute for Rock Magnetism, Department of Geology and Geophysics, University of Minnesota, 291 Shepherd Labs 100 Union Street S.E., Minneapolis, MN 55455, United States
AU: Dunin-Borkowski, R E
EM: rdb@cen.dtu.dk
AF: Department of Material Sciences and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom
AU: Dunin-Borkowski, R E
EM: rdb@cen.dtu.dk
AF: Center for Elektron Nanoscopy, Technical University of Denmark, CEN DTU building 307, Kgs Lyngby, DK-2800, Denmark
AB: Given their stable single domain (SSD) configuration at room temperature and world-wide occurrence, magnetosomes produced by magnetotactic bacteria are potentially an important source for stable (post) depositional remanence. One way to detect their presence is by the so-called low-temperature delta-ratio test proposed by Moskowitz et al. [1993]. When magnetite is cooled through the Verwey transition at ~120K (Tv) it transforms from cubic to monoclinic symmetry. Associated with this crystallographic transition is the selection of a new magnetic easy axis along the c-axis in the monoclinic phase, which corresponds to one of the three original cubic [100] directions. The selection of the new c-axis is at least a function of the presence or absence of an external field during cooling. In fact, this cooling field dependence forms the basis for the delta-ratio test for detecting chains of magnetite magnetosomes. In essence, the application of the external field biases the c-axis selection and effectively produces an assemblage of partially aligned SD particles below Tv. Whereas samples containing high concentrations of magnetite magnetosomes chains are known to yield high delta ratios (>2.0), the details remain unclear. For instance, we do not know how physical deformation of the chain structure may modify the c-axis selection. Nor do we have a good understanding of how partial oxidation or magnetic relaxation affects the reorientation of magnetic easy axes on warming remanences. Here we attempt to duplicate the delta- ratio test inside a transmission electron microscope while monitoring the magnetic microstructures within magnetite magnetosome chains using off-axis electron holography. Our results will shed light on the c-axis selection process and the possible reasons for elevated delta ratios in magnetite magnetosomes.
DE: 1505 Biogenic magnetic minerals
DE: 1519 Magnetic mineralogy and petrology
DE: 1522 Paleomagnetic secular variation
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting