HR: 08:20h
AN: GP11B-02    [Abstracts]
TI: Low Temperature Magnetic Properties of Multidomain Magnetite: Insights From Single Grain Measurements
AU: * Kosterov, A
EM: kosterov@umn.edu
AF: Universität Bremen, FB Geowissenschaften, Postfach 330440, Bremen, D-28334 Germany
AU: * Kosterov, A
EM: kosterov@umn.edu
AF: IRM, University of Minnesota - Twin Cities, 100 Union Street S.E., Minneapolis, MN 55455 United States
AU: Fabian, K
EM: karl.fabian@uni-bremen.de
AF: Universität Bremen, FB Geowissenschaften, Postfach 330440, Bremen, D-28334 Germany
AB: Low temperature (LT) measurements have been performed on two single multidomain (MD) magnetite grains of ~ 75 μm and ~ 120×80×40 μm in size. Both show a well defined Verwey transition at a transition temperature T_V which has a small thermal hysteresis: 123.33(2) K on cooling vs. 123.53(2) K on heating for grain 1, and 116.33(6) K vs. 116.69(8) K for grain 2. Several indications lead us to the conclusion that two different mechanisms of twin formation exist in MD magnetite at low temperatures:
(1) Transformation twinning on cooling through T_V;
(2) Magnetostrictive deformation twinning below T_V. Two twinning processes are necessary to account for the following crucial observations:
1. Below T_V more numerous and larger magnetization jumps occur than above. This suggests that the pinning sites are formed on cooling through the transition and are due to twinning. Diminishing of jumps with increase of cooling field also fits into this picture.
2. At 10 K H_c(ZFC) > H_c(FC) and SIRM(ZFC) > SIRM(FC). This suggests that during ZFC through T_V each single grain is subdivided by transformation twinning into regions with different c-axis directions.
3. We observe that (a) Low-field cooling (LFC, H < H*) leads to ZFC-like LT SIRM with a maximum LT SIRM around H* (~ 50 mT); (b) High-field cooling (HFC, H > H*) produces FC-like LT SIRM. H* appears to be the maximum field which a given grain can compensate by its self-demagnetizing field. Above H* a sharp (grain 1) or gradual (grain 2) transition into the HFC (no-twinning) regime occurs.
4. The fully stable FC state only forms by cooling in field to below 70 K. Switching off the field at T > 70 K leads to deformation twinning. Twin domains below T_V substitute energetically unfavorable magnetic closure domains.
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005