Monday, January 23, 2006

"NANOTECH: Nanorings promise MRAM density boost"

Researchers have developed magnetic nanorings that promise to enable magnetic RAM densities to rival or surpass those of flash memories. MRAMs can store bits when the power is off, but their comparatively low densities have precluded their development for anything more than niche markets. Now researchers from Johns Hopkins and Carnegie Mellon Universities have found a way to form magnetic domains into asymmetrical 100-nanometer-diameter cobalt rings. The results are magnetic vortexes that become completely self-contained, permitting tightly packed densities of up to 30 Gbits of storage per square inch--ten times higher than flash's 3 Gbits/square inch. Other experimental MRAMs store bits in a magnetic tunnel junction--a sandwich structure that includes a pinned magnetic layer, an oxide tunnel barrier and a free magnetic layer that can switch between two linear magnetic polarization states. A bit is detected when the cell's electrical resistance changes. But MRAMs have only been successfully fabricated in samples at 1-Mbit and 4-Mbit densities, a far cry from the 1-Gbit flash chips fabricated today. Further, the magnetic vortexes can be harnessed to store bits in two states--either the vortex state or a neutral state of two opposing, onion-skin magnetic fields. Ironically, in order for the nanorings to encode the two states for memory cells, they must be so small that no magnetic field can exist inside each core. To ensure that no vortex could exist within the cores of the nanorings, the researchers kept the nanorings' diameter below 50 nm. To produce the rings, the researchers first coated the single-crystal silicon substrate with a monolayer of 100-nm-diameter polystyrene spheres. The spheres did not contact each other and had controllable average separation distances. Next, a 40-nm-thick film of cobalt was deposited by magnetron sputtering to cover all the spheres and open substrate areas. Finally, an argon ion beam was used to etch away the cobalt, including what was on top of the polystyrene spheres. The cobalt was protected by the bottom side of the spheres, however, resulting in 100-nm-diameter cobalt nanorings on the silicon substrate. The devices were found to switch reliably between two stable states. The asymmetry of the nanorings was also found to improve their switching behaviors, and the parameters that achieve optimal performance were explored, he said. Next, the researchers plan to prepattern a substrate with polystyrene spheres positioned over circuitry that can read and write the nanorings, thereby turning them into 100-nm- bit cells for MRAMs. The research was funded by the National Science Foundation.