Scientists build 'magnetic semiconductors' one atom at a time

"Chips might take on many new capabilities once such 'spintronic' technology is perfected," Yazdani said. "One thing we might be able to do is make chips that can both manipulate data and store it as well, which right now generally requires two separate parts of a computer working together."

Computers use two different kinds of technology to calculate results and store data. While semiconductor chips -- often based on silicon or more advanced materials such as gallium arsenide -- do the calculating, data storage has generally been accomplished with magnetic materials within floppy disks or reels of tape. Combining these functions into a single device could reduce the size and energy requirements of computer hardware, a perennial goal of the industry.

Although gallium arsenide "doped" with manganese has been a promising candidate material for such dual-function chips for a decade, working with the material has proven frustrating for a number of reasons. One difficulty is that researchers have not been able to engineer the material with optimal magnetic properties.

"Up until now, we have not had a way to control how the manganese sits in the gallium arsenide substrate," Yazdani said. "We could not specify, for example, how large the bits of manganese would be, or how far apart they would be located. And because we couldn't study how changing these variables affected the semiconductor's performance, it was hard to know what its ideal specifications should be. For the most part, we had to just crystallize the material -- with the dopant arranged more or less randomly -- and hope."

Dale Kitchen, a reasearcher in Yazdani's lab and first author of the Nature paper, hit upon a solution while working with a high-tech tool used to explore complex materials called a scanning tunneling microscope, which operates very differently than a desktop optical microscope. The device has a finely-pointed electrical probe that passes over a surface in order to detect variations with a weak electric field. The team, however, found that the charged tip could also be used to eject a single gallium atom from the surface, replacing it with one of manganese that was waiting nearby.

"The important thing technically was that we could incorporate the manganese into the underlying crystal lattice," Yazdani said. "If you want to study how the semiconductor functions, it would not have been enough merely to deposit the manganese on the surface. They needed to become a single integrated material."

Using their new technique, the team was able to find the precise arrangements of manganese atoms that exhibited magnetic properties, the important factor in developing spin-based electronics. The experimental data agreed with theoretical work that had been performed by Michael Flatté and his group at the University of Iowa, which had anticipated the atomic arrangement that optimized magnetism in the experiments.