J.H.J. Scott
Objective: To adapt nanoscale chemical characterization techniques in the analytical electron microscope (AEM) to materials challenges presented by advanced magnetic nanocomposite samples.
Problem: Advances in magnetic materials research rely increasingly on information about chemical distributions at near-atomic dimensions. Sometimes this information concerns elemental distributions in artificial heterostructures, such as the multilayers in a spin valve. In other cases, information is needed about chemical or structural gradients that occur naturally during materials processing, such as recrystalliztion or segregation in an amorphous magnetic metal upon annealing. Several AEM-based nanoscale characterization techniques can be applied to these analysis problems, including convergent beam electron diffraction (CBED), energy-dispersive x-ray spectroscopy (EDS), and electron energy-loss spectroscopy (EELS). The Microanalysis Research Group has been applying these tools to magnetic nanocomposites and investigating next-generation techniques such as electron spectroscopic imaging (ESI), including energy-filtered transmission electron microscopy (EFTEM) and spectrum imaging.
Approach: The AEM is capable of both imaging and chemical analysis with very high spatial resolution. In this case, an intermediate-voltage transmission electron micrscope/scanning transmission electron micrscope (TEM/STEM) is used in TEM mode to image samples with a point-to-point resolution better than 0.2 nm, and is used in STEM mode to interrogate the sample with a finely focused probe approximately 1 nm in diameter. Conventional electron beam microanalysis is made possible by an EDS detector and an EELS spectrometer. Powerful new techniques have also been enabled by the addition of an imaging energy filter and a hyperspectral data acquistion system, designed to acquire EDS and EELS spectra simultaneously at each point of a 2-dimensional field of interest. Using the imaging energy filter, a series of energy-selected images aquired above and below a core-loss ionization edge can be combined to produce an elemental map of the sample, providing important clues to the variations in magnetic properties at very fine length scales. Multiple-energy-window maps can also be adapted to study changes in valence state of transition metal ions (e.g. Mn3+ vs. Mn4+) in metals and magetic oxides. Using hyperspectral data acquisition techniques and a scanned fine electron probe, both an EDS and an EELS spectrum can be associated with each pixel in a 1-dimensional profile or a 2-dimensional map. This "data cube" can then be processed offline to extract information such as compositional maps or valence state maps.
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Figure 1. Elemental map of the spatial distribution of C in a Sm-Co-C magnetic nanoparticle materials |
Results and Future Plans: The novel techniques described above were applied to magnetic nanocomposite powders and magnetic heterostructures. Sm-Co-C nanocomposites produced in an RF plamsa torch from metal powder precursors and acetylene gas were mapped using EELS and energy-filtered imaging. Figure 1 shows an elemental map produced using a 3-window electron spectroscopic imaging technique and energy-selected images acquired in the neighborhood of the carbon K-edge at 284 eV. Investigations of multilayered samples for spin valves and advanced spintronic devices are underway, and we currently have samples of melt-spun amorphous magnetic tapes and nanocrystalline soft magnetic materials such as Fe44Co44Zr7B4Cu. |
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Last Updated
March 5, 2002
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