HR: 13:45h
AN: GP43B-01 INVITED     [Abstracts]
TI: Quantitative imaging of magnetic nanoparticles using off-axis electron holography
AU: * Harrison, R J
EM: rjh40@esc.cam.ac.uk
AF: Department of Earth Sciences, 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, Pembroke Street, Cambridge, CB2 3QZ United Kingdom
AU: Simpson, E T
EM: ets22@cam.ac.uk
AF: Department of Materials Science and Metallurgy, Pembroke Street, Cambridge, CB2 3QZ United Kingdom
AU: Kasama, T
EM: tk305@cam.ac.uk
AF: Frontier Research System, The Institute of Physical and Chemical Research Hatoyama, Saitama, 350-0395 Japan
AU: McEnroe, S A
EM: Suzanne.McEnroe@ngu.no
AF: Geological Survey of Norway, NGU, Trondheim, N-7491 Norway
AU: Brown, L
EM: lbrown@geo.umass.edu
AF: Department of Geosciences, Morrill Science Center University of Massachusetts 611 North Pleasant Street, Amherst, MA 01003-9297 United States
AU: Hirt, A M
EM: hirt@mag.ig.erdw.ethz.ch
AF: Institute of Geophysics, ETH-H”nggerberg, Zurich, CH 8093 Switzerland
AB: Very few techniques are capable of providing quantitative information about the local magnetic induction in a material with nanometre spatial resolution. We have recently demonstrated that the transmission electron microscopy (TEM) technique of off-axis electron holography can be used to measure the magnetic induction in a naturally occurring titanomagnetite mineral sample with a spatial resolution of approximately 5˙nm. Here we present an overview of the technique and its application to the quantitative measurement of magnetic induction in nanoscale maghemite particles embedded in a matrix of hematite. The technique of off-axis electron holography allows the measurement of the amplitude and phase shift of a high-energy electron wave that has passed through a sample in a TEM. A positive voltage is applied to an electron biprism (for example, a quartz wire coated with gold) to overlap an electron wave that has passed through the sample with one that has passed only through vacuum. Analysis of the phase shift (the local position) of the holographic interference fringes that form in the overlap region is used to provide the magnitude and direction of the electromagnetic field within the material quantitatively and non-invasively. The technique can be used to map long-range fields such as those between patterned magnetic nanostructures, and also more local fields such as those due to charge redistribution on an atomic scale at interfaces between materials. The phase shift of the electron wave is sensitive to the magnetic induction and the mean inner potential (MIP). By magnetising the sample in opposite directions in situ in the electron microscope, acquiring electron holograms of these oppositely magnetised regions, and finally subtracting the resulting phase images from each other during processing, the MIP contribution to the phase shift can be eliminated, leaving only the magnetic signal of interest. Contours can then be generated from this magnetic phase image to produce a map of the in-plane component of the magnetic induction in the sample. This approach relies on being able to reverse the direction of the magnetisation in the sample exactly, which is satisfied for certain sample geometries, such as chains of crystals. The technique is applied to the study of a natural coarse-grained hematite from El Laco, Chile, which is characterised by an unusually high value of the coercive force (450 mT). TEM analysis reveals the presence of numerous nanoparticles of an iron oxide spinel phase. Quantitative imaging of a 30 nm diameter nanoparticle demonstrates that it is strongly magnetic with a saturation induction of 0.48 T, in perfect agreement with the saturation induction of maghemite. In-situ application of magnetic fields indicates that the maghemite is strongly exchange coupled to the hematite host, in agreement with FORC analysis of the bulk sample.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005