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New magnetic semiconductor material spins hope for quantum computing

Scientists at the Department of Energy's Pacific Northwest National Laboratory have created a semiconductor material that has superior magnetic properties at room temperature—and that may propel research one step closer to quantum computing



Scott Chambers, a PNNL senior chief scientist, created a thin-film semiconductor material that is magnetic at room temperature in the molecular beam epitaxy lab shown here.

August 2—The future of quantum computing offers the potential for substantially greater data storage and faster processing speeds, but its advancement has been limited by the absence of certain critically important materials—in particular, a semiconductor that is magnetic at room temperature. Now, scientists at the Pacific Northwest National Laboratory (PNNL) have created a semiconductor material that has superior magnetic properties at room temperature.

Using a special synthesis technique, PNNL scientists created a thin-film semiconductor material made of titanium, oxygen, and cobalt. In collaboration with scientists at the IBM Almaden Research Center in San Jose, California, they showed that the materials required for quantum computing and the emerging area of "spintronics" (the exploitation of an electron's spin to carry information, rather than its charge) likely can be obtained.

The current generation of computers uses an electron's charge to store and process information, but this approach limits the ultimate speed and storage density that can be achieved. Magnetic storage, such as that found in a computer hard drive, relies on the magnetic properties created by an electron's spin. However, if an electron's spin can be harnessed within a semiconductor, the potential exists to create entirely new ways of computing and signal processing that will greatly increase speed and data storage densities.

Spintronics would provide the basic properties required for advanced technologies, such as on-chip integration of magnetic storage and electronic processing functions and quantum computing, which depends on coherent spin states to transmit and store information. A material is permanently magnetic if the majority of its electrons spin in the same direction.

To be practical, spintronics will need to use semiconductors that maintain their magnetic properties at room temperature. This is a challenge because most magnetic semiconductors lose their magnetic properties at temperatures well below room temperature, and would require expensive and impractical refrigeration to work in an actual computer.

Chambers and his team of scientists achieved these properties in a crystalline oxide film known as anatase titanium dioxide that is infused with a small amount of cobalt, a magnetic impurity. These results will be presented at the 2001 Spintronics Workshop in Washington, D.C., August 9 to 11.

"Although other scientists have created similar materials, their films had considerably poorer magnetic properties," said Scott Chambers, a chemist and PNNL senior chief scientist. "Our material has superior magnetic strength—an improvement of nearly a factor of five."

Chambers says that PNNL's synthesis technique, while difficult to perform, is more controllable at the atomic level, and therefore yields better results. The next step is to refine the growth process.

The PNNL work builds upon experiments conducted by scientists in Japan who created the same material using laser ablation, an effective but less-controlled thin-film synthesis method. The strength of laser ablation is that it allows researchers to cover a wide range of growth conditions and film compositions rather quickly when used in what is known as the combinatorial mode. In this way, several materials can be screened relatively rapidly to determine promising candidates.

The combinatorial mode search by the Japanese group revealed that the material the PNNL team has synthesized in a more-controlled fashion has significant potential for the applications at hand. A description of the Japanese research was published in the Feb. 2, 2001, issue of Science.

Chambers and his team created this magnetic semiconductor material using a synthesis method called molecular beam epitaxy. In this growth method, individual beams of atoms-in this case, titanium, oxygen and cobalt-are generated in a highly controlled vacuum environment and directed onto a crystalline surface of strontium titanate where the atoms condense and form a crystalline film with dimensions on the nanoscale.

Chambers designed and built this equipment in the early and mid-90s, and his particular system was the first of its kind in the world when installed at the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE user facility at PNNL.

After the material was created, a team of scientists at IBM, led by research staff scientist Robin Farrow, validated the results by characterizing the material's magnetic properties. In the material synthesized at PNNL and characterized at IBM, each cobalt atom's magnetic moment, which is a measure of the material's magnetic strength, is about five times larger than in the Japanese scientists' material.

This research is in the first year of a three-year program funded by PNNL's Nanoscience and Nanotechnology Initiative. While early results are promising, PNNL scientists will continue their research to determine the material's ideal growth temperature, growth rate, composition and choice of substrate, and then optimize the structural properties required for achieving the desired magnetic properties.—by Staci Maloof

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