Self-diffusion in nanoscale structures measured by neutron reflectometry
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Section I: Basic and Applied Research
Self-Diffusion in Nanoscale Structures Measured by Neutron Reflectometry Mukul Gupta, Ajay Gupta, Rachana Gupta, and Thomas Gutberlet
(Submitted July 12, 2005) Neutron reflectometry (NR) is an attractive tool for probing self-diffusion on a nanometerlength scale. The depth resolution available with NR is in the subnanometer range, which is due to contrast among the isotopes of an element, and NR provides a unique opportunity to probe self-diffusion in nanometer-range structures. Self-diffusion measurements in FeZr amorphous and nanocrystalline chemically homogeneous multilayers of type [Fe100-x Zrx /57 Fe100-x Zrx]10, were performed using neutron reflectivity. On the basis of the results obtained, the self-diffusion mechanism in nanometer-range structures is discussed in this article.
1. Introduction Amorphous and nanocrystalline metals and alloys, in which the tailoring of properties over a wide range by controlling particle size and morphology is possible, are an important class of materials.[1-4] Many physical properties, such as structural and magnetic properties, glass-to-crystal transition, and surface morphology, are strongly influenced by self-diffusion of the constituents. Further, the longstanding application of devices based on amorphous and nanocrystalline alloys is fundamentally controlled by the self-diffusion process. Therefore, an understanding of the self-diffusion mechanism is very important from both a scientific as well as technological point of view. To determine the self-diffusion mechanism, one needs to fabricate structures with proper isotopic labeling. Further, to probe structures with isotopic labeling, techniques capable of distinguishing contrast among isotopes are required. Mass spectrometry and radioactive tracer techniques have been used widely to probe self-diffusion.[5,6] These techniques essentially use cross sectioning and/or depth profiling to probe the buried layers. The depth resolution attained with these techniques is limited to about 5 nm, which poses a limit to the diffusion lengths that could be probed using these techniques. To probe self-diffusion lengths below this limit, a technique with much lower resolution is required. It is known that the grazing incidence reflection of x-ray or neutron reflectometry (NR) is a nondestructive way to probe surfaces and even buried interfaces. The depth resolution available with reflectometry techniques is in the subnanometer-length scale (i.e., as low as 0.1 nm), and it makes them ideal for probing self-diffusion or interdiffusion in This article is a revised version of the paper printed in the Proceedings of the First International Conference on Diffusion in Solids and Liquids—DSL-2005, Aveiro, Portugal, July 6-8, 2005, Andreas Öchsner, José Grácio and Frédéric Barlat, eds., University of Aveiro, 2005. Mukul Gupta, Thomas Gutberlet, and Rachana Gupta, Laboratory for Neutron Scattering, ETH Zürich and Paul Scherrer Institute, CH5232 Villigen PSI, Switzerland; and Ajay Gupta, UGC-DAE Consortium
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