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8/17/2006
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Atomic Switch Could Yield Higher-Density Memories

Though they're years away, arrays of atomic-scale devices could provide the enhanced density needed for future memory devices.

PORTLAND, Ore. — A single-atom oscillator demonstrated by the Center for Nanoscale Science and Technology could become the basis for a future bistable atomic switch.

Scientists at the Center, part of the Commerce Department's National Institute of Standards and Technology (NIST), injected a charge that switched a single cobalt atom between two locations on a crystalline substrate.

"A single cobalt atom attached to two copper atoms starts switching at a threshold voltage of roughly 15 millivolts," said NIST scientist Jason Crain. "The frequency of the switching can be controlled by varying the electron current, reaching a rate of around 20 KHz at a current of 1 nanoamp."

Using a "tunneling noise spectroscopy" measurement technique pioneered by the group, the researchers characterized the cobalt atom's switching characteristics in order to understand its underlying mechanism.

Previous research explored the dynamics of isolated cobalt atoms, Crain said. "Our task was to understand how the switching takes place when the cobalt atom is incorporated into a nanostructure."

The NIST scientists used a scanning tunneling microscope to line up copper atoms. The line ended with a single cobalt atom. When they placed the microscope's tip beside the cobalt atom and injected current, the atom started switching between two positions—either directly in line with the copper atoms or off to one side at about a 45-degree angle.

Theoretical calculations of the electronic structure showed that a particular molecular orbital of the cobalt atom allowed it to switch positions, while the copper atoms to which it was attached remained relatively stationary. Calculations of other electronic properties of the molecules supported the experimental results, the researchers said.

The next step will be building a working device in a circuit. That will require a semiconducting or insulating substrate. Though they are years away, arrays of atomic-scale devices could provide the enhanced density needed for future memory devices.

The researchers noted that the number of copper atoms in the switch affected switching rates (two was fastest, three or four slowed its frequency and more than five terminated switching altogether).

Even though the researchers used a cobalt atom, which is magnetic, as its switching element, it did not harvest the magnetic properties of its atomic switch. When building arrays of devices, however, the magnetic properties of cobalt could enable novel read-out mechanisms for future atomic-scale memories.

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