Portland, Ore. -- 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, said Frank Zhu, a doctoral candidate at Johns Hopkins who worked on the project with professor Chia-Ling Chien.
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.
''In traditional magnetic media, including MRAMs, the lines of flux are linear, so the domains must be spaced far enough apart that they don't demagnetize with each other. But with nanorings, they form circular magnetic vortexes that do not interact with each other, because the field does not extend outside the ring," said Zhu.
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.
Initial runs of prototype devices had dismal yields. "At first our nanorings had only about a 40 percent chance of forming a magnetic vortex," Zhu said. The group claims to have solved the problem over the past year by inserting a novel step into the fabrication process that improves yields without changing the nanorings' size. "We found that if we tilt the substrate during ion etching [by 10 to 14 degrees], we can make [the rings] asymmetrical and get nearly 100 percent yields," said Zhu.
The Johns Hopkins researchers cooperated with Jimmy Zhu, a Carnegie Mellon EE professor and director of that university's Data Storage Systems Center. Working with Carnegie Mellon's Zhu on the MRAM nanorings was EE Xiaochun Zhu, a doctoral candidate at the university.
To produce the rings, the two groups 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. "We found that the asymmetric nanorings have a much better chance of achieving the necessary magnetic vortex state that enables us to switch their states on demand by changing the direction of an external magnetic field," said Zhu of Johns Hopkins.
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.
