Indiana University       Research & Creative Activity       September 2000 • Volume XXIII Number 2


The Era of Spin

Arrives in Swain Hall

by Michael Wilkerson

 

The laboratory in the basement of IU Bloomington’s historic Swain Hall where David Baxter works might seem a long way from the boardrooms of high-technology corporations in California, but as Baxter and his colleagues in the IUB Department of Physics continue to produce new theoretical and experimental discoveries, the gap between the two environments is shrinking.

Not many years ago, physicists like Baxter used to describe their discoveries and experiments as being far removed from any kind of application in business or consumer technology. Now, however, the rapid discovery of semiconducting and other technologically useful materials has led to new avenues for Baxter and his colleagues to explore.

“I look at problems that are related to materials or processes that are of interest to technology at some level,” says Baxter, an associate professor of physics who has worked at IU since 1987. “I concentrate not on the technology itself, but on the underlying physics issues that people who are trying to push the technology don’t have the time or the opportunity to investigate.”

Take something called the Fractional Quantum Hall Effect as an example. “This is a completely esoteric thing that had no application at the moment of its discovery,” Baxter says. “It just appeared when people looked carefully at the low-temperature properties of high-quality transistors being developed.”

When cooled to extremely low temperatures, these transistors displayed remarkable and unexpected properties that were difficult to explain. Along the way, this research won Nobel Prizes, not only for those who discovered and explained the effect, but also for the scientist whose contribution was growing the high-quality crystals needed to see the effect in the first place.

In fairly short order, these new transistors made possible the creation of cellular telephones and today’s petite, window-sized satellite dishes. “It’s a great example of how fundamental research and technology work together,” notes Baxter.

In his own laboratory, Baxter concentrates on two key areas: the study of spin electronics (also called spintronics) and the growth of microscopically thin films of both conducting and insulating materials.

Spin electronics is a fascinating new technology growing out of long-standing physics studies of the properties of electrons. “An electron is a tiny subatomic particle with basically three properties,” Baxter explains. “It has some mass, it has an electric charge, which is the property exploited by conventional electronics, and it has spin. In recent years, interest in the electron’s spin has risen, partly because more sensitive instruments are now available to study it, but also because the orientation of an electron’s spin has a substantial effect on the subtle properties of a material.”

The best example of this is magnetism. “The reason a magnet acts as it does is that the majority of its electrons have their spin lined up in the same direction,” Baxter says. A material that is naturally magnetic, such as iron, is called ferromagnetic. Like the poles of a magnet, which can be north or south, the spin of an electron can be characterized as going one direction or the other—in physics jargon, up or down.

Indiana University Professor of Physics David Baxter uses the cryostat, at left, to conduct resistance measurements in a magnetic field at very low temperatures. Photo Tyagan Miller.

Baxter and his colleagues exploit this discovery: inserting extremely thin, nonmagnetic, metallic film between two layers of ferromagnetic materials results in a structure whose electrical resistance changes dramatically as a magnetic field shifts the relative orientation of the two magnetic layers. In other words, the composite structure can be utilized as an extremely sensitive magnetic sensor.

This effect is now being incorporated into the read heads of computer disk drives, particularly high-density ones. The more sensitive a read head is, the more densely the magnetic domains—the zeros and ones that define computer data—can be packed on a medium such as a hard drive.

“This is an important advance,” Baxter says, “but we’re trying to take it a step further by using the same effect as an actual element of logic. We’re trying to make one magnetic layer out of a material whose orientation is difficult to change, a property known as high coercivity. Then you make the other magnetic layer out of a material that can be easily changed by a much smaller magnetic field, one of low coercivity.”

A two-state device can be made from these layers, using the less malleable material for storage of information and the more malleable material for the device to read the information. The reading is, essentially, examining whether the preponderance of the electrons in the high-coercivity material have an up or down spin or, translated to machine language, a one or a zero.

The physical phenomenon behind these devices is called the giant magnetoresistance effect (GMR), which is one of the core areas of Baxter’s research. He is experimenting with the creation and testing of thin film materials that will utilize the GMR and a related effect called tunnel junction magnetoresistance. In this kind of experiment, the material between the two magnetic layers is insulating, but it is so thin—less than twenty angstroms, or about ten layers of atoms thick—that electrons can tunnel through it using one of the bizarre facets of quantum mechanics.

According to Baxter, companies such as IBM are working feverishly to produce fast, high-density, nonvolatile random access memory for computers using this effect.

The production challenge is to manufacture these new materials so that each of the billion or more elements on a single chip behaves in precisely the same manner. If perfected, this technology could provide computer users with the advantages of speed and capacity now available only separately in different kinds of computer memory. According to Baxter, with the new technology, programs would remain in memory even with the power shut off.

Although it would seem evident that high-technology corporations such as Intel and Seagate would be quick to fundfundamental GMR and thin-film research, that’s not the case in today’s environment.

“I’m only speculating, but I wonder if the high performance of the stock market has made these companies even more focused on the short term,” Baxter says. “Whatever the reason, the traditional balance between funding of research and development—however a company defines it—has shifted dramatically toward development. I don’t know of any corporate funds available for this kind of research at universities, even though we’re talking about industrial applications that might be worth in the hundreds of billions of dollars.”

Instead, funding continues to come, albeit in small amounts, from federal and state governments. Even there, the shift from the physical sciences to life and health sciences has been significant in recent years—in 1998, the amount of the increase in the research budget for the National Institutes of Health was greater than the entire budget for the National Science Foundation, prime sponsor of a great deal of physics research.

Other major players in the funding of physics are the Department of Energy and the Department of Defense, particularly the Defense Advanced Research Projects Agency. DARPA, which had a major role in the creation of the Internet, is now funding research related to spin electronics at a high level—more than $100 million over the next five years.

“They want to know how well electrons can be made to remember, in effect, the way they were spinning while traveling over great distances—a property known as spin memory,” Baxter says. “Much of my research is related to spin memory. If electrons can remember their spin as they travel over long distances, and they remain coherent, it may be possible to do some very elaborate computations with them, which would be a very significant advance.”

In DARPA’s case, the advance might include the breaking of codes and encryptions once thought impossible to decipher. The creation of spintronic RAM would help not only computer users in business and education but also the Defense Department, which could have digital devices that perform even in volatile military environments.

Although Baxter hopes that he and his IU colleagues will be able to get some of their spin memory research funded by DARPA, he’s currently excited by the State of Indiana’s 21st Century Research and Technology Fund. As the only Bloomington campus scientist funded by this initiative, Baxter hopes to explore the spintronic properties of the new magnetic semiconductors, notably gallium arsenide doped with manganese, which have made recent advances in computing possible.

Although he’s not a grower of crystals, Baxter tests the new substances to determine their fundamental properties. He uses crystals grown at a laboratory at Notre Dame in a process called molecular beam epitaxy, or MBE (the joke in the field, says Baxter, is that the MBE machine is so expensive it stands for MegaBuck Evaporator). The crystals incorporate magnetic elements within the gallium arsenide. Baxter then tests and measures the spin and spin memory of the substances.

“The technological possibilities of these materials are fascinating, as is the fundamental physics,” Baxter says. The materials could be utilized to make high-speed and possibly tunable laser and fiber-optic communications devices, and other devices that would combine the practical advances of high-tech entrepreneurs with the foundational science of Baxter and colleagues. At this point, however, the materials display useful magnetic properties only when super-cooled, but Baxter believes he and others can tweak them to surmount that problem, as was done in the early days of transistors.

The equipment in Baxter’s lab (although it doesn’t rise to the level of a MegaBuck Evaporator) is powerful. Baxter supervises devices that include vacuum chambers in which a plate of magnetic material two inches in diameter is bombarded by ionized argon, causing its atoms to spray throughout the chamber and condense onto a substrate elsewhere in the chamber. With this procedure, called magnetron sputtering, Baxter can control the thickness of the film to within one or two atomic layers.

For another process, Baxter collaborates closely with members of the chemistry faculty. During his first year at IU, Baxter gave a talk at a science symposium for local industry, and he and then-IUB professor Malcolm Chisholm learned of each other’s work in parallel areas. Their communications grew into DOE and NSF-funded experiments in what’s called chemical vapor deposition, which is a way of building up thin films through chemical reaction. Interestingly, the process has been used for 150 years in industry to create such things as diamond-tipped drill bits, but its underlying fundamentals have not been deeply studied. Baxter’s collaboration with Chisholm and Kenneth Caulton, Distinguished Professor in the IUB chemistry department, has produced exciting results, not just in terms of scientific papers, but also in terms of combined visions of how to proceed with a research project.

“We need more of this,” Baxter says of his joint research. “The new science building (which IU is proposing in the 2001–2003 budget request) will foster the kind of communication we need. Physics is building up a collaboration with biology by hiring Jay Tang (see “Cells on the Run,” page 12), a biophysicist. But just getting people to talk more is the first step. Collaboration was one of the best things that’s ever happened to me.”

In addition to considerable time spent in the lab and writing papers for scientific publications, Baxter serves as undergraduate faculty adviser for the Department of Physics and teaches the General Physics course for science majors.

“My teaching takes a lot of time, but my teaching and my research reinforce each other. I’ve involved some very bright undergraduate students who want to join my group for their own research projects,” he says. “Teaching also gives me an opportunity to talk with students about the relationships between fundamental research and applied technology; I don’t spend whole lectures on my own work, but there’s no question that the themes from what’s currently happening in my lab get sprinkled throughout the course.”

Baxter’s students come from all over the world, although most of the undergraduates are from Indiana. “Physics is a great preprofessional degree,” Baxter says. His graduates go on to study law, business, math, and other fields, and many find jobs in high-technology businesses. One popular avenue of employment for physics graduates—even including a former IU faculty member—is investment banking.

“I think the connection there is that people at the cutting edge in banking are trying to find new models for predicting future prices of stocks, commodities, what have you,” Baxter says. “That requires more and more sophisticated mathematical modeling, which is what physicists know how to do.”

What physicists like David Baxter know how to do might seem esoteric, but in the end, the quality of our lives is enriched by the kind of research that goes on in the basement of Swain Hall—and that’s no spin.

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