Unusual Phenomena in Exchange-Biased Nanostructures
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Unusual Phenomena
in Exchange-Biased Nanostructures Ivan K. Schuller Abstract
The following article is an edited transcript based on the MRS Medalist presentation given by Ivan K. Schuller of the University of California, San Diego, on December 3, 2003, at the Materials Research Society Fall Meeting in Boston. Schuller received the MRS Medal for “his innovative studies of exchange bias in magnetic heterostructures and nanostructures.” Magnetic nanostructures have received increasing attention in recent years, motivated by the interesting phenomena that are apparent when physical size becomes comparable with relevant magnetic length scales. In addition, a number of important potential applications in the sensors and storage industries have emerged. When magnetic nanostructures are in contact with dissimilar magnetic materials, and because their magnetic fields extend considerably outside the physical structure, they are very susceptible to interaction with the surrounding environment. A particularly interesting situation is a ferromagnetic nanostructure in contact with an antiferromagnetic substrate. In this “exchange-biased” configuration, a variety of unusual phenomena arise: The reversal mode of the ferromagnet changes considerably, the superparamagnetic transition temperature is affected, and there is a noticeable change in the microscopic spin configuration. A series of experiments will be described involving these phenomena in nanostructured ferromagnets prepared by electron-beam lithography and self-assembly. Keywords: exchange bias, magnetic properties, thin films.
Introduction I hope that you will take with you the following general conclusions from this talk. The first lesson is that basic research pays. Over the years, basic research—especially in the physical sciences—has produced applications in unexpected ways. This is true of the well-known discoveries of x-rays and nuclear magnetic resonance, and now, history repeats itself with exchange bias and giant magnetoresistance. In many cases, revolutionary applications have arisen in the most unexpected ways, and targeted research would have never found them. The second lesson is that it is extremely difficult in materials science—especially with these novel types of materials—to be successful with any kind of “hit-and-run” research. Systematic studies are essential. The third lesson—especially for my younger colleagues—is that solid-state physics and materials science are here to
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stay and that this is one of the most exciting times to be in these fields. Today, we can manipulate, characterize, and measure materials at the atomic level and on short time scales. Thus, we can actually see atoms, which I was told in high school we would never be able to do. So much for the popular opinion, propagated even by politicians, that you have to be in biology to do interesting, exciting, novel, and—yes—really useful research! The vitality of these fields is proven by the large number of young people in the audience at this meeting. I will provide an elementar
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