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Background

The attempt to develop and understand materials with new and useful properties is perhaps the most fundamental of all scientific endeavors; the use of the terms "stone age," "bronze age," and "iron age" is indicative of the value of materials to society, and the extent to which a society is defined by the materials it uses. Recent years have demonstrated this point more firmly than ever before as materials such as aluminum, teflon, and silicon have found their way into every aspect of our lives. Most recently, condensed matter science has engendered a new and compelling member in this progression: artificially structured materials.

Comprised of amorphous metals, granular materials, metastable crystalline alloys, superlattices, etc., artificially structured materials have grown rapidly in importance during the last decade. There is an underlying reason for the widespread interest in these materials. They each offer one or more degrees of freedom not found in ordinary materials--degrees of freedom over which the investigator has a great deal of control. Using this capability, a researcher may tailor a new material to meet his or her needs--either for the investigation of fundamental phenomena or to effect some practical application. For fundamental studies, we may tailor the material to suppress some well known phenomenon; this allows us to study subtle effects which are usually overshadowed in natural materials. For applications, the properties of the material may be tailored to suit some technological need.

Artificially Structured Materials

In my research, I study the magnetic properties of artificially structured materials (ASM's). For now, I will focus on amorphous metals, also known as metallic glasses. Metallic glasses are metals (they conduct electricity, and may be magnetic) yet they are also glasses. That is, they have no crystal structures, and thus share some of the properties normally associated with glass. Some display a great resistance to corrosion, and many are brittle compared to other metals. These materials are interesting both as the subjects of fundamental research (for example, on the origins of magnetism and electrical conductivity) and because they have many technological applications. For instance, metallic glasses are now being used to improve the efficiency of power distribution transformers.*

As an example, let me turn to metallic glasses again. Because the atoms do not have to lie on an orderly crystal lattice, it is possible to make mixtures of different metals (alloys) which cannot be made in crystalline form thus the composition is a degree of freedom that the researcher can control. Also, by heating the metallic glass, it is often possible to partially crystallize the material. By changing the amount of heat, the degree of crystallinity is also controllable. This process is used to fine tune the metallic glass used in high frequency transformers. These are similar to the power transformers mentioned earlier, but are smaller and are used in high speed electronics.

Other examples of ASM's include multilayered structures, quaicrystals, and granular metals. I cannot discuss all of these here, so I will focus on multilayers, which are atomic level "club sandwiches." These structures contain alternating layers of two or more elements. These layers are extremely thin--sometimes only a few atoms across. In this case, the degrees of freedom include the compositions and thicknesses of the layers. Structures such as these are now used as read head elements in high-capacity hard disk drives. They have also been used in fundamental studies, as a means of understanding the transition from two dimensional to three dimensional behavior. Other applications include the velocity sensors that are used in antilock brakes.

* These are the transformers that convert high voltage electricity in power lines to 120 and 240 V power used in people's homes. They usually look like a big cylindrical can located at the top of a telephone pole, or like a large green box on a concrete pad near the edge of a property.