HR: 0830h
AN: GP31C-0759    [PDF]
TI: Low-Temperature Magnetometry of Synthetic Titanohematite (y = 0.7): A Way to Get Closer to Resolving the Mechanism of rTRM?
AU: * Lagroix, F
EM: lagr0012@umn.edu
AF: University of Minnesota, Departement of Geology and Geophysics, 108 Pillsbury Hall 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AU: Banerjee, S K
EM: banerjee@umn.edu
AF: University of Minnesota, Departement of Geology and Geophysics, 108 Pillsbury Hall 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AU: Moskowitz, B M
EM: bmosk@umn.edu
AF: University of Minnesota, Departement of Geology and Geophysics, 108 Pillsbury Hall 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States
AB: Rob Hargraves once referred to self-reversed magnetizations as ``skeletons in paleomagnetists' closets'' and devoted part of his career to understanding this phenomena. He and colleagues (Lawson et al., Science, 213,1372-1374, 1981) were the first to observe by transmission electron microscopy (TEM) the cation ordered and disordered antiphase domains and boundaries in high titanium titanohematites (yFeTiO3ú1-yFe2O3). But the mechanism enabling titanohematite to acquire a reversed thermoremanence (rTRM) remains to the present day somewhat elusive. Following the first observation of rTRM in natural rocks in the early 1950's, numerous models have been proposed for the reversal mechanism. Concepts common to all models are: 1) microstructurally, the grain is composed of at least one cation ordered and one cation disordered phase, 2) the phase superexchange coupled to the cation ordered domain generating the rTRM is more Fe-enriched and therefore has a higher blocking temperature than the cation ordered domain. The model dependent concept is whether the phase superexchange coupled to the cation ordered domain generating rTRM is cation ordered and ferrimagnetic (Ishikawa, J. Phys. Soc. Jap., 13(8), 828-837, 1958; Pr\'{e}vot et al., PEPI, 126, 75-92, 2001) or cation disordered and antiferromagnetic (AF) (Hoffman, JGR, 97(B7), 10883-10895, 1992; Nord and Lawson, JGR, 97(B7), 10897-10910, 1992). We attempt to provide experimental validation to these currently proposed models for rTRM by analyzing the magnetic behavior of synthetic titanohematite samples, y = 0.7 (Tc $\approx$ 330 K), that have been annealed and quenched at various temperatures above and below the cation disorder - order transition. Our preliminary results, from remanent and induced magnetization experiments in weak and strong fields and across the temperature range of 3 K to 400 K, clearly show that 370 K is a critical temperature, when approached from above, at which superexchange interactions with the ordered domains commences. Moreover, on cooling through 370 K, magnetic susceptibility increases one order of magnitude in a 30 K interval or less depending on the sample. The larger the cation ordered domains, the sharper and greater is the increase in magnetic susceptibility. The magnetic moment measured in a strong field from 10 K to 400 K linearly decreases by one order of magnitude in the temperature range of 10 K to 400 K, showing no break in the slope at 370 K. This suggests that a low value of the anisotropy energy in the cation ordered domains is associated with the onset of superexchange interactions and critical to the rTRM. The increase in coercivity of remanence, the shape of hysteresis loops and the low values of remanent magnetization and magnetic susceptibility above 370 K suggest that an AF phase with a small parasitic moment is superexchange coupled to the cation ordered domains. This is in greater agreement with the cation disordered AF models proposed by Hoffman, Nord and Lawson.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 5109 Magnetic and electrical properties
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting