Defects boost disc capacity

By Kimberly Patch and Eric Smalley, Technology Research News

The solution to a major nuisance in the field of superconductivity could also prove useful in the quest to make higher capacity data storage media.

Microscopic vortices form in superconductor material when the material is in a magnetic field. These tiny magnetic tornadoes hinder the flow of electrons, which reduces the superconductor's efficiency. Introducing vertical defects into the superconductor material pins these vortices in place, which minimizes their effect.

A team at IBM Research has applied this pinning technique to magneto-optical recording media. By corralling the tiny magnetic fields that constitute bits, the researchers reduced the size of a viable bit, allowing more information to be stored on a disc.

If the technology can be scaled up, it could lead to data storage densities on the order of one terabit per square inch, said Lia Krusin-Elbaum, a research staff member at IBM Research. One terabit per square inch translates to a standard-size compact disc that could hold 23 terabits of data, which is enough to store 687 two-hour movies.

In magneto-optical recording, which is used in rewritable compact discs, the heating action of the recording laser changes the orientation of a disc's magnetic field. The data is read by shining a polarized laser beam on the disc. The polarization of the light that bounces back indicates whether the domain, or bit, hit by the laser is oriented up or down. Ones and zeros are represented by the two types of polarized light.

If the domains are too small they interfere with each other and they also become more susceptible to small increases in temperature, which can reverse their magnetic fields. Orienting the domains perpendicularly helps to a point, but researchers still face these problems when they try to further reduce the size of the domains.

One way around the problem is to pattern the media by setting up boundaries to contain each domain, or by embedding periodically spaced particles or nanowires, one per bit, in the material. But patterned media is more difficult to produce than ordinary media, and patterned media would require yet-to-be-developed recording technology that can be positioned precisely over the domains.

The IBM technique attempts to combine the best of both approaches by producing tightly confined, perpendicularly-oriented domains, but in unpatterned media. The technique "uses continuous films, [is] relatively easy to prepare, and it does not require nanometer-scale patterning or fabrication," said Krusin-Elbaum.

The recording medium consists of a 0.7-nanometer layer of cobalt sandwiched between 3-nanometer and 2-nanometer layers of platinum. The researchers make vertical, linear defects, or upward folds in the cobalt layer. The defects produce strain in the material around them, and domains that form in the areas of strain take on the shape and orientation of the defect.

A domain in media without defects is analogous to a strand of cooked spaghetti wriggling side to side. The effect of the defects is analogous to confining the spaghetti inside a straw. The result is the domains are 10 times narrower and so can be packed together 100 times more densely.

The strain induced by each defect extends hundreds of microns around the defect, said Krusin-Elbaum. In the IBM experiments, the average domain formed perpendicularly as far as 300 microns from the defect.

This means relatively few defects would be needed to prepare each CD, she said. For example, if the strain from each defect was effective for an area 300 microns in diameter, about 160,000 defects would be required to prepare a CD-size disc. Those defects would support more than 23 trillion domains.

Other researchers are working on improving the storage capacity of a rewritable CD to 100 gigabits per square inch, said Krusin-Elbaum. "Introducing linear defects may push this density into [the] terabit per square inch range," she said.

"This is nice work and potentially very useful," said Phillip N. First, a physics professor at the Georgia Institute of Technology. "Because the density of imposed defects would be much less than the bit density, this new work could provide a relatively simple path to media densities [on the order of] 1 terabit per square inch for perpendicular magnetic recording," he said.

There are, however, "many other technological hurdles to be overcome in reading and writing the data," he added.

One of those hurdles is the wavelength of light. Magneto-optical disks are read using light, and lightwaves cannot distinguish features smaller than a wavelength. Three hundred or 400 nanometers is the shortest wavelength researchers have been able to use so far. "That takes you into the ultraviolet," said Robert White, a professor of electrical engineering and materials science and engineering at Stanford University and director of the university's Center for Research on Information Storage Materials.

Even 100-nanometer domains, which would require 100-nanometer-wavelength light or smaller, only yield 65 gigabits per square inch, said White.

There are tricks to increasing storage capacity, like overlapping the round bit areas and reading the edge of the crescent that's showing, he added. If the defects act to smooth the edges of the crescents, that may help. "But it's not obvious to me that this is going to take us to the terabit [range]," he added.

So far the researchers have tested domains formed by individually-produced defects. The researchers' next step is to make media containing more than a single defect, said Krusin-Elbaum. The researchers hope to demonstrate the feasibility of their technique in the next couple of years and to put it to use in the next 5 years, she said.

Krusin-Elbaum's research colleagues were Takasada Shibauchi, Bernell Argyle, Lynne Gignac and Dieter Weller. They published the research in the March 22, 2001 issue of the journal Nature. The research was funded by IBM.

Timeline:  5 years
Funding:   Corporate
TRN Categories:   Materials Science and Engineering; Data Storage Technology
Story Type:   News
Related Elements:  Technical paper, "Stable ultrahigh-density magneto-optical recordings using introduced linear defects," Nature, March 22, 2001


April 18, 2001

Page One

Defects boost disc capacity

Alternative quantum bits go natural

Light powers molecular piston

Bumps could make better biochips

Crystal changes shape in ultraviolet light


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