University of Minnesota
University of Minnesota

Physics and Astronomy
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   Our group works at the interface between magnetism and semiconductor physics. Most people are familiar with the fact that semiconductors contain mobile carriers, which can be either electrons, which are negatively charged, or holes, which carry a positive charge. The control of charge transfer between semiconductors is the basis for much of today's useful technology, including all of the transistors in your computer. On the other hand, magnetic materials are ubiquitous in today's world, particularly in information technology, where they are the basis for the media used in disk drives. The basic component of magnetic materials is the spin of the electron, which gives it a magnetic moment. In a crude sense, one can think of electrons as miniature bar magnets. Generally speaking, a bit in a hard drive consists of an ensemble of these miniature magnets.

  What happens when you combine the worlds of magnetism and traditional semiconductors? This is an interesting question, because although the charge carriers in semiconductors carry a spin, semiconductors are not thought of as magnetic materials. The concentration of electrons is small relative to metals, and, unlike iron, cobalt, or nickel, common semiconductors like silicon or gallium arsenide cannot be permanently magnetized. Nonetheless, there are some features of semiconductors that could make them very useful in magnetic devices. First, a spin introduced into a semiconductor can last a long time and travel distances up to 100 microns, which is much longer than the typical length scales used in the either semiconductor electronics or magnetic storage media. Second, semiconductors interact strongly with light, which can be used as a sensitive generator and detector of magnetic information. Above all, unlike metals such as iron, the carriers can in a semiconductor can be added or removed by applying voltages. If those same carriers possess a magnetic moment, it could be possible to have a magnetic storage technology that can be combined with traditional semiconductor electronics. This potential technology is referred to as "spintronics."

  Alas, there are some big science problems on the way to this new technology. First, it is not so easy to get a spin into a semiconductor in the first place. This problem of spin injection is one of the research topics in our group. Second, it is not so easy to find materials that can be genuinely magnetic and semiconducting at the same time. We are studying a class of materials known as ferromagnetic semiconductors using a combination of transport and optical techniques. In particular, we are interested in spin dynamics that occur on time scales less than one nanosecond (10-9 seconds).

  It turns out that spin dynamics become even more interesting in systems that are spatially confined on length scales on the order of one micron or less. We are working with a technique to film "movies" of small magnetic particles on a picosecond scale. Among the phenomena that we are looking at are unique excitations that are due to the confinement.