Applying a strong electric field to a nano-scale piece of magnetite yielded unexpected results, reports Dick Ahlstrom.
An old dog is delivering surprising new tricks in a research collaboration involving Trinity College Dublin and Rice University in Texas. The "dog" in this case is magnetite, a substance known and used for thousands of years. But scientists have discovered unexpected characteristics that could lead to new kinds of inexpensive electronic devices.
The work combined a well-known material, magnetite, with the latest techniques in nano-scale production and delivered a completely unexpected result, explains Trinity's Prof Igor Shvets, who led the Dublin team. The research was published last month in the online version of the Nature Materials journal and will be in next month's print edition.
"We worked with iron oxide, magnetite, the grandfather magnetic material," Shvets explains. Magnetite, or lodestone, is naturally magnetic and was used in early compasses to find magnetic north. "This material has been known and studied for 2,000 years and was used by Greek and Chinese sailors to navigate."
Magnetite is unusual in that it can be both a conductor of electricity at higher temperatures and a strong insulator at low temperatures.
"Most oxides are insulators, so why this oxide conducts has been debated for years," Shvets says. "At low temperatures, minus 150 degrees, the material becomes an insulator. There is a transition from its conductivity phase to its insulating phase."
Shvets holds Trinity's chair of applied physics within the university's School of Physics. He is also involved in Trinity's new nanotechnology lab, Crann. This gives him access to the specialised equipment needed to produce nano-materials, including highly structured and controlled oxide materials such as magnetite. Rice University has similar expertise and the two institutions came together to study properties of magnetite produced to these exacting standards.
There is a constant demand for novel electronic materials for next-generation computers and this can sometimes involve the study of something as familiar as magnetite. Yet when nanotechnogy is involved, surprises can arise. Such was the case with this collaboration, according to Shvets.
"We had a nano-scale piece of magnetite and applied a strong electric field to it," he says. These experiments were carried out at Rice using the magnetite materials produced in Dublin.
The researchers found that, with a specific high voltage, the chilled magnetite transitioned abruptly from an insulator into a conductor. There was a very tight threshold voltage at which the magnetite suddenly snapped back to being an insulator.
"This is a very peculiar observation in its own right," says Shvets. Yet it should help add to our understanding of oxide-based conductor insulators. It also opens up new uses for this very old material.
"You can use conducting oxides for many applications where ordinary semiconductors can't be used," he says. "If you can understand how to turn oxides into semiconductors, then these would be very cheap."
For example, silicon-based solar cells are dear to produce, but if cheap iron oxide could be used as a substitute, costs would drop dramatically. Shvets believes that it should be possible to make a semiconductor from magnetite.
"In principle this is a very exciting area," he says. "Oxides potentially are very suitable materials for these applications."
The research team will now try to understand what it is within magnetite's structure that causes the switching between insulator and conductor to occur. With this should come a better understanding of the nature of its insulating state.