Layered wonder

Ongoing work on metallic film multilayers could facilitate their already extensive use in industry

 
Published: Thursday 31 October 1996

one of the most remarkable advances in material science has been the development and study of thin metallic films. These are made by evaporation of metals onto a substrate in highly controlled conditions. The unique properties of these films -- some of which are still being explored -- have been exploited in numerous electronic devices.

Metallic films, with two alternating different metals, form a multilayer which has many interesting properties, many of which are even now poorly understood. The multilayers are formed by evaporating two different metals alternately with a repeat distance of a few nanometres. Perhaps the most striking feature of some of these multilayers is the phenomenon of giant magneto-resistance.

This is the phenomenon of a change in the electrical resistance of some kinds of multilayers (like copper-cobalt) when a magnetic field is applied parallel to the film. The unique properties of multilayers depends both on the metallic interfaces and the repeat distance -- that is, the thickness of the metal.

Herein arises a problem: exposure of the film to high temperatures leads to a deterioration in sharpness of the metallic interfaces. But the most popular methods of making the films usually involve high temperatures, for instance in evaporation or chemical deposition. Thus, for the films to retain their useful properties, deposition methods are needed which do not require high temperatures. One such technique is electro-deposition where the metals are deposited by the process of electrolysis. Several groups in Japan and Russia have recently demonstrated electro-deposition of copper nickel and cobalt platinum multilayers ( Nature , Vol 382, 1996).

Another property of multilayers which is still controversial is the supermodulus effect. This is basically the occurrence of large changes in the stiffness of the layers when the period is changed. Discovered about a decade ago, this effect is still not very well established though there are now reports of a Japanese team observing it in a copper cobalt multilayer on a gallium arsenide crystal substrate.

Metallic multilayers have found many applications in industry. They are regularly used in constructing reflectors for extreme ultraviolet and even soft (low energy) x -rays. Lately, there has been considerable interest in the use of multilayers in computer memories, though commercial development is still some time off. A somewhat different kind of multilayers, called track-etched membranes, are being used for a variety of purposes like filtration of cancer cells from blood. Even the supermodulus effect could have potential applications in devices where anomalously high elastic moduli (stiffness) are needed. With many groups working on multilayers around the world, the number of applications seem all set to grow very soon.

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