HR: 08:00h
AN: GP21C-01 INVITED     [Abstracts]
TI: Magnetism and Microscopy: Applications to mineral magnetism at the nanometre scale
AU: * Harrison, R J
EM: rjh40@esc.cam.ac.uk
AF: Department of Earth Science, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Dunin-Borkowski, R E
EM: red10@cam.ac.uk
AF: Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ United Kingdom
AU: Simpson, E T
EM: ets22@cam.ac.uk
AF: Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ United Kingdom
AU: Kasama, T
EM: tk305@cam.ac.uk
AF: Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ United Kingdom
AU: McEnroe, S A
EM: Suzanne.McEnroe@ngu.no
AF: Geological Survery of Norway, Leiv Eirikssons vei 39, Trondhiem, N-7492 Norway
AB: Several recent advances in electron microscopy are poised to revolutionise mineral magnetism over the next five to ten years. In this talk I review some of these advances and their application to the study of mineral magnetism at the nanometre scale. Arguably the most significant advance is the application of off-axis electron holography, a technique that yields a two-dimensional vector map of magnetic flux with nanometre resolution. The technique is capable of imaging the magnetization state within individual magnetic particles, as well as the magnetostatic interaction fields between neighbouring particles. Most imaging modes in a TEM suffer from the same drawback: the final recorded image is a spatial distribution of intensity; all information about the phase shift of the electron waves passing through the sample is lost. Electron holography provides an interference pattern from which the phase information can be recovered. Once corrections have been made for variations in sample thickness and mean inner potential, the gradient of the phase shift is proportional to the magnetic flux. Contour lines placed on a holographic image provide a quantitative image of the lines of magnetic flux with approaching nanometre spatial resolution. TEM offers two possibilities to yield compositional information with nanometre spatial resolution. Characteristic X-rays can be analyzed using conventional energy dispersive X-ray analysis (EDX) with a spatial resolution of around 10-20 nm. Alternatively, one can measure the intensity of electrons transmitted through the sample as a function of their energy loss. The resulting electron energy loss spectrum (EELS) contains a number of ionization edges. Using a post-column imaging filter it is possible to form an image of the sample using electrons that have suffered a specific energy loss. This technique can be used to produce quantitative elemental distribution maps with a spatial resolution approaching 1 nm or less. The shape of the Fe L$_{2-3}$ edge in an EELS spectrum is highly sensitive to the valence state of Fe, and can be used to determine the Fe$^{2+}$/Fe$^{3+}$ ratio to an accuracy approaching that of M”ssbauer spectroscopy. Nanoscale microstructures in the ilmenite-hematite solid solution are thought to be responsible for lamellar magnetism in slowly-cooled metamorphic rocks. We present the results of preliminary experiments to test the lamellar magnetism hypothesis directly using off-axis electron holography. The magnetization of a hematite host containing fine scale ilmenite precipitates has been compared quantitatively with the magnetization of a region containing no exsolution lamellae. The magnetization is determined from the holographic phase shift accumulated across each region. Analysis yields estimates of 3.8-4.7 mT for the exsolved region and 0.4-0.7 mT for the precipitate-free region, corresponding to an enhancement in magnetization by a factor of $\sim$ 5. This is much lower than the theoretical maximum enhancement factor of $\sim$ 22, indicating that there is a $\sim$ 60:40 ratio of in-phase to out-of-phase lamellae. These results represent some of the smallest magnetic fields ever quantified using any technique at this spatial resolution, and further demonstrate why holography is at the forefront of attempts to understand mineral magnetism at the nanometre scale.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting