Metallic multilayers

2 Metallic multilayers. In Section 7.4, we have met the recent discovery of multilayers of two kinds of metal, or of a metal and a non-metal, that exhibit the phenomenon of giant magnetoresistance. This discovery is one reason why the preparation and exploitation of such multilayers have recently grown into a major research field. 

The original motivation for the preparation of regular metallic multilayers of carefully controlled periodicity was the need for X-ray reflectors, both to calibrate unknown X-ray wavelengths and to function as large and efficient monochromators, especially for soft X-rays of wavelengths of several A. This was first done by 

Deubner (1930) and analysed in detail in a famous paper by DuMond and Youtz (1940). A typical modern multilayer for this purpose would be of W/Si. 

An intriguing recent review of size effects in materials due to microstructural and dimensional constraints with a focus on mechanical properties, including those of multilayers, is by Arzt (1998). 

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Greer, A.L. and Somekh, R.E. (1991) Metallic multilayers, in Proce.ssing of Metals and Alloys, ed. Cahn, R.W. Materials Science and Technology A Comprehensive Treatment, vol. 15 (VCH, Weinheim) p. 329. 

Krzanowski, J.E. (1991), The effect of composition profile shape on the strength of metallic multilayer structures , Scripta Materialia, 25, 1465-1470. 

Z.S. Shan and D.J. Sellmyer, Nanoscale rare earth-transition metal multilayers magnetic structure and... 

Specimens that contain materials with very different ion-milling rates, such as metallic multilayers grown on silicon substrates, often tend to form bridges of material across the perforated area. Ion-milling at very low angles of incidence ( 1-2°) in a direction parallel to the interface can sometimes be used to overcome or at least alleviate these bridging problems. Finally, it should be noted that the use of a crystalline substrate such as silicon provides a convenient reference material for specimen orientation purposes in the TEM. Examination of the substrate EDP can be used to ensure that the substrate normal is aligned exactly perpendicular to the electron beam direction. The thin-film microstructure can then be easily determined. 

Polymer/metal multilayer devices are used in the microelectronics industry. These devices are composed of alternating layers of polymer and metal, the metal is etched into lines and, except where via holes permit the contact of different metal layers, the polymer serves as an insulator. Because the polymer must withstand rather hostile environments during fabrication, the choice is narrowed to those which are stable to chemical treatment, high temperature (for short periods of time) and humidity. The polymers of choice here are the polyimides, although others are certainly used. 

Enhanced magnetoresistive effect, called giant magnetoresistance (GMR) effect, was observed in magnetic layered structures consisting of magnetic/nonmagnetic metal multilayers [131,132], First, GMR multilayers were produced by vacuum deposition... 

This work was followed by a number of researchers who confirmed the experimental results on various metallic multilayer systems (e.g. Cu/Ni [78-80] as well as on hard epitaxial and polycrystalline superlattices of nitrides, such as TiN/VN... 

Although this chapter is limited to electrodeposition of semiconductors, it is only fair to mention, even if briefly, some examples of electrodeposition of metal nanostructures. This is important because the principles and techniques used in electrodepositing metals are essentially the same as those used for depositing semiconductors - the main difference is that almost all studies on electrodeposition of nanocrystalline semiconductors involve compound semiconductors, with the added comphcations this entails. Examples include pulsed electrodeposition of metal multilayers [1, 2], porous membrane-templated electrodeposition of gold nanotubes [3], and Ni nanowires [4]. 

Bioactive glass (Bioglass )/metal multilayer composites Fracture strength 100 MPa Work of fracture 5.7 10 N.m Structural applications in the biomedical field [53.. 54]... 

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