HR: 1340h
AN: GP23A-0169    [Abstracts]
TI: Modeling of 6-line Ferrihydrite Low-Temperature Magnetic Properties for Environmental Change Studies
AU: * Guyodo, Y
EM: guyodo@lsce.cnrs-gif.fr
AF: LSCE, Campus du CNRS, 12 Avenue de la Terrasse, Gif-sur-Yvette, 91198 France
AU: Banerjee, S K
EM: banerjee@umn.edu
AF: IRM, Dept. of Geology and Geophysics, Univ. of Minnesota, 108 Pillsbury Hall, Minneapolis, MN 55455 United States
AU: Berquo, T S
EM: berqu013@umn.edu
AF: IRM, Dept. of Geology and Geophysics, Univ. of Minnesota, 108 Pillsbury Hall, Minneapolis, MN 55455 United States
AU: Penn, R L
EM: penn@chem.umn.edu
AF: Dept. of Chemistry, Univ. of Minnesota, 139 Smith Hall, Minneapolis, MN 55455 United States
AU: Seda, T
EM: sedat@physics.wwu.edu
AF: Dept. of Physics and Astromony, Western Washington Univ., Bellingham, WA 98225 United States
AU: Solheid, P
EM: peat@umn.edu
AF: IRM, Dept. of Geology and Geophysics, Univ. of Minnesota, 108 Pillsbury Hall, Minneapolis, MN 55455 United States
AB: Ferrihydrite (5Fe$_{2}$O$_{3}$.9H$_{2}$O) is a poorly crystalline, highly reactive mineral that occurs in a wide variety of natural settings and has been identified as a key element in the biogeochemical cycle of iron in the environment. Magnetically, it is considered as an antiferromagnetic mineral with a weak magnetic moment possibly due to uncompensated spins. In our presentation, we will focus on the modeling and interpretation of low-temperature ($<$300K) magnetic data obtained on three samples of 6-line ferrihydrite nanoparticles that have been synthesized by precipitation from homogeneous solution. The three samples, which possess similar composition and crystallinity (probed by x-ray diffraction and extended x-ray absorption fine structure), are characterized by three different particle size distributions with average diameters between 3 and 5.5 nm. Using the particle size distributions obtained from transmission electron microscopy images, we performed numerical fits of the high-field (0-5T) induced magnetization data for several temperatures above the unblocking temperatures of the samples. The magnetization was modeled as the superposition of an antiferromagnetic signal and a magnetization due to uncompensated moments (which are assumed coupled to the antiferromagnetic moments). Fits of the low-field magnetic susceptibility data as a function of temperature were also obtained, using the low-field approximation of the Langevin function. Combined with a detailed study of the thermal evolution of the M”ssbauer spectra, our modeling of the magnetic data allowed us to quantify the sensitivity of the magnetic properties of 6-line ferrihydrite to temperature and particle size, useful as environmental signatures.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting