HR: 16:00h
AN: GP24A-01    [Abstracts]
TI: Transition Warming and Cooling Remanences in Magnetite, Hematite and Pyrrhotite
AU: * Dunlop, D J
EM: dunlop@physics.utoronto.ca
AF: Physics Department, University of Toronto, Toronto, ON M5S 1A7, Canada
AB: One way of gaining insight into the size and morphology of assemblages of magnetite particles is to compare either remanence or susceptibility at low temperatures after zero-field cooling (ZFC) and after field cooling (FC) through the Verwey transition around Tv = 120 K. At 10 K a sample is demagnetized following ZFC, while in the FC initial state before warming the sample has a transition cooling remanence (TCRM), acquired largely but not entirely in crossing Tv. There is also a reciprocal remanence acquired as a result of heating a demagnetized sample from low temperature across Tv. This transition warming remanence (TWRM) is often called an inverse TRM. Hematite and pyrrhotite also acquire TCRM and TWRM in crossing a low-T magnetic phase transition in the presence of a field. In TCRM experiments, initially demagnetized samples were cooled in a 2 mT field from 300 K to 10 K and measured every 1 K to 5 K. At 10 K, the field was zeroed, and the remanence was then monitored during zero-field warming back to 300 K. The properties of TCRMs were generally similar to those of TWRMs produced by heating a ZFC sample in a 2 mT field from 10 K, but the increase in induced magnetization for the magnetites and pyrrhotites in cooling through their phase transitions was relatively small because of the very high coercivity of the low-temperature phase of both minerals. In the case of hematite, where the low-temperature phase is almost non-ferromagnetic, the induced magnetization actually decreased across the transition. On the other hand, 70-90 percent of the induced magnetization was retained as remanence when the field was zeroed at 10 K, and up to 20 percent of this remanence survived zero-field warming through the transition. This room- temperature memory was much larger than the remanence below the Morin transition. A remarkable property is the observed mirror-image symmetry between in-field warming curves tracking the acquisition of TWRM and zero- field warming curves of TCRM between 10 K and 300 K. The symmetry, with increases in the field-on induced + remanent magnetization curves exactly mirroring decreases in the field-off remanent magnetization curves, is almost perfect from 10-110 K.
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting