Anomalous electronic transport in metallic nanomultilayer at all length scales: Influence of grain boundary and interfac
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e-dependent microstructure and electronic transportation of Ni/Al-type nanomultilayers as a function of the bilayers number, the modulated ratio, and the periodicity were investigated. The deposited multilayers have anisotropic nanocrystalline structure and asymmetrical interfaces. This special interfacial feature is the result of asymmetrical diffusion of Ni to Al lattice near the Ni–Al interface. Anomalous resistivity enhancement increases with decreasing both the periodicity and the modulated ratio, but is insensitive to the number of bilayers. Accounting for the effects of grain boundary and interface boundary, the dominative mechanism at distinct length scales can be interpreted with the modified model of those of Fuchs– Sondheimer and Mayadas–Shatzkes. Especially for the thinnest film with smallest modulated ratio, the intermixing effect turns out to be the crucial mechanism in the electronic transportation of metallic nanomultilayers.
I. INTRODUCTION
Metallic multilayered materials composed of alternating nanoscale layers have been extensively studied as one of the significant research topics in recent years. Numerous experiments have proven that on the nanometer scale, the transport properties of metallic multilayers are easily affected by many inherent complexities of the materials.1–3 For highlighting anomalous resistance changes quite a few potential mechanisms have been proposed including Fuchs–Sondheimer model, 4,5 Mayadas–Shatzkes model,6 intermixing effect,7 quantized miniband theory,8,9 percolation theory,10,11 and Bloch–Grüneisen theory,12,13 of which the models of Fuchs–Sondheimer and Mayadas–Shatzkes have provided the best interpretations in many cases. As a matter of fact, metallic multilayers are also well congruent for fundamental studies of length–scale effect on resistance enhancement of layered composites, especially in a nanometer length range.1,3,14 It was reported that the resistance would be altered with the variation of modulation periodicity, which could be explained by the interface effect,4,15 the grain boundaries (GBs) effect,16 and the jellium-based quantum-size effect.17 Although the proposed theories and models are suc-
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Address all correspondence to this author. e-mail: [email protected]; [email protected] DOI: 10.1557/JMR.2008.0193 1658
J. Mater. Res., Vol. 23, No. 6, Jun 2008
cessful to some degree, the viewpoint of an anomalous resistance enhancement remains controversial, and there has not yet been reached a consensus regarding the dominative mechanism of the changes in the electronic transportation.14 For instance, the influence of surface on the resistance is usually masked by the GB effect,6,18 in which the most of transported electrons are believed to scatter from the GBs rather than the point defects and impurities.19 Nevertheless, the resistance enhancement beyond the predicted value of Fuchs–Sondheimer model has also been reported in some multilayered systems.16,20 Also, the intermixing effect, based on different diffusion behaviors at the heterostructure
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