Silicon-based magnets boost spintronics
By Celeste Biever
A family of silicon-based semiconductors that exhibit magnetic properties has been discovered, paving the way for "spintronic" computer chips that are compatible with existing silicon manufacturing technology.
Spintronics is an emerging field that exploits the fact that current flowing through transistors made of certain materials can store data not only as charge but also as magnetic moment.
Today, computer chips store bits as a current that is either on or off. But scientists hope that in future the current in transistors could also be magnetically polarised as either "up" or "down", allowing more information to be stored in the same space.
These chips would require less power, as switching the current from on to off could be triggered by a low-power flip in magnetisation, rather than by impeding the current using a voltage, which is how the current technology works.
Magnetised materials also retain their spin even when a current is switched off, so silicon-based magnetic chips could be the basis of micro-processors with non-volatile memory. They would not boot up from a hard drive but would provide immeditate access to the operating system when switched on. "Switching on a computer would be like switching on a TV," says Steve Pearton, a materials scientist at the University of Florida in Gainesville.
Disrupted structure
Spintronic transistors have not been used so far to build computer chips because they have been made of gallium or indium-based semi-conductors, not silicon. These are not compatible with today's chip manufacturing methods. Silicon itself is not magnetic and efforts so far to dope it with magnetic metals have disrupted its crystal structure, vastly reducing its ability to conduct electricity.
However, an international team of scientists reported on Sunday that a blend of silicon and iron doped with cobalt exhibits the necessary magnetic properties without disrupting the crystal structure.
The secret was finding two different metals that combine with silicon to form crystals of the same shape and size, preserving a uniform structure throughout.
"For spintronics to be useful, you have to come up with something really compatible with silicon technology," says Pearton. "Intel has an outstanding product, they are not going to take up a new exotic material. They want something they are pretty much using already."
Temperature trouble
However, one problem that still remains is temperature. The cobalt-doped iron silicide that the researchers produced only exhibits magnetism below -220 °C. Above this temperature the material's magnetic domains respond to tiny temperature fluctuations rather than to the inherent spin of their electrons, and so lose their magnetism.
But to be used in a computer chip, which heats up due to the large amounts of current passing through it, spintronic transistors would have to be magnetic well above 50 °C.
The scientists' next task is to investigate the temperature at which other materials in the same family lose their magnetic properties, in the hope of finding a high-temperature silicon magnet.
"We are not jumping up and down and saying we solved the problem. This is proof of principle," explains John DiTusa, of Louisiana State University in Baton Rouge and one of the research team. He expects to see spintronic computer chips within the next 10 years.
Journal reference: Nature Materials (DOI:10.1038/nmat1103)