Physics
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusm.m32a..05h&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #M32A-05
Physics
3600 Mineralogy And Petrology (Replaces, 3900 Mineral Physics, 1500 Geomagnetism And Paleomagnetism, 1519 Magnetic Mineralogy And Petrology, 1540 Rock And Mineral Magnetism
Scientific paper
The ilmenite-hematite solid solution is one of the most important magnetic phases in nature. Unusual magnetic behaviour is often observed in intermediate compositions due to the interaction between cation ordering, magnetic ordering, and exsolution. This presentation provides significant new insight into the effect of exsolution microstructures ("lamellar magnetism") on the magnetic properties of this system through a combination of experimental and computational techniques. Metamorphic titanohematites from the Swedish Granulite Region carry an unusually strong and stable remanent magnetization. We argue on the basis of detailed rock-magnetic and petrographic analysis that the stable remanence is caused by the presence of exsolution microstructures at the nanometer scale. Transmission electron microscopy (TEM) reveals a microstructure consisting of unit cell-scale disk-shaped precipitates with diameters around 10-20 nm and thickness' of 1-2 nm or less. Quantitative chemical analyses are obtained using electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). The precipitates are shown to be almost pure ilmenite within an almost pure hematite host. To provide insight into the effect of these precipitates on the bulk magnetic properties, we have performed a detailed series of Monte Carlo simulations, which successfully model the interaction between cation ordering, magnetic ordering, and exsolution in the ilmenite-hematite system. Cation exchange interaction parameters were constrained by the temperature- and composition-dependence of the long-range cation order parameter, determined by Rietveld refinement of in-situ powder neutron diffraction data. Magnetic exchange interaction parameters were taken from the literature (Burton and Kikuchi 1984). All previously known features of the equilibrium phase diagram are reproduced. The simulations allow the magnetic transition in chemically heterogeneous systems to be studied. At low temperatures (less than 525 C) they predict an intergrowth of antiferromagnetic hematite and paramagnetic ilmenite. Despite the antiferromagnetic nature of the hematite component, the simulation cells as a whole are ferrimagnetic, with small but significant net magnetization associated with the interface between exsolved phases. Competition between chemical and magnetic interactions leads to a characteristic cation distribution at the interface, which ensures a local imbalance between the numbers of up and down magnetic spins. The interface moment acts as a defect moment of the antiferromagnetic hematite, and has both the magnitude, coercivity, and thermal stability required to explain the remanence properties of the metamorphic samples, thus establishing exsolved titanomagnetite as a potential carrier of strong remanence on the Earth and other planets.
Harrison Richard J.
McEnroe S.
Robinson Patricia
No associations
LandOfFree
Lamellar magnetism: A source of unusually strong and stable magnetization on the Earth and other planets? does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Lamellar magnetism: A source of unusually strong and stable magnetization on the Earth and other planets?, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Lamellar magnetism: A source of unusually strong and stable magnetization on the Earth and other planets? will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1721562