Nanorings self-assemble into 100-nm memories

Portland, Ore. - Nanoscale memories recently self-assembled into nonvolatile magnetic rings with diameters below 100 nanometers. Unlike conventional magnetic domains, which measure in microns, nanorings can be more closely packed, because crosstalk among nanorings is impossible. Future memory chips based on cobalt magnetic nanorings could harness nanowires to switch bits on and off.

"Our results show that cobalt nanoparticles can self-assemble into rings with stable magnetic properties at room temperature," said professor Alexander Wei at Purdue University (West Lafayette, Ind.). "Systems like ours could be just what the data storage industry is looking for."

Wei cautioned, however, that much work remains to be done to characterize and develop a system for addressing and programming such nanoscale memory rings before they can be commercialized. "This discovery will not make nonvolatile nanoscale memory available tomorrow, but it could be an important step toward its eventual development," Wei said.

Wei's research group (www.chem.purdue.edu/awei/) discovered, in collaboration with fellow Purdue professor Steven Tripp and electron microscopist Rafal Dunin-Borkowski at the University of Cambridge, that cobalt nanoparticles were dipoles that normally would form chains of plus-ends mated to minus-ends over and over. However, by carefully controlling the conditions under which they are grown, Wei was able to coax them into forming nanorings instead.

The most valuable characteristic of nanorings, besides their diminutive size, is that they emanate no magnetic field-or "flux closure." Any magnetic dipole produces a magnetic field, but due to the ring's shape the collective magnetic field outside the ring is zero.

Flux closure solves one of the eternal problems with magnetic media: The more densely they are packed, the more crosstalk between bits. In the worst case, adjacent one and zero bits can neutralize each other, resulting in catastrophic data loss. However, the flux closure of nanorings makes them immune to crosstalk; therefore they can be more tightly packed.

According to Wei, what differentiates his nanoring construction method from others is that the particles self-assemble into rings, rather than rely on a "cookie cutter" patterning method as is traditionally used in manufacturing. The self-assembly method was perfected in the lab for cobalt rings in collaboration with Dunin-Borkowski. Using electron holography, Dunin-Borkowski imaged the flux-closure states and verified that they are stable at room temperature, even with no external power supply.

Wei said preliminary research already shows that a nanoring's magnetic state-flowing either clockwise or counterclockwise-can be switched on or off by applying an external magnetic field. What Wei's team demonstrated for the first time was how nanorings can self-assemble. However, no research group has yet demonstrated how nanoring bits can be addressed and programmed by similarly sized nanoscale wires and turn an interesting research result into a usable memory technology.

Next up, nanowires

"Nonvolatile memory based on our nanorings could in theory be developed," Wei said. "But for now nanorings are simply a promising development." For the future, Wei said he wants to deliver a nanoring with an integrated nanowire that can program the flux direction and thus the bit value of each nanoring.

"We hope that with self-assembly we can someday integrate making cobalt nanorings and the electrically conductive nanowires that produce highly localized magnetic fields for switching flux-closure states," Wei said.

Wei's group works with the Birck Nanotechnology Center at Purdue. The National Science Foundation and Department of Defense funded Wei's research.