boost disc capacity
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
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
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,"
"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,"
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
TRN Categories: Materials Science and Engineering; Data
Story Type: News
Related Elements: Technical paper, "Stable ultrahigh-density
magneto-optical recordings using introduced linear defects," Nature, March
Defects boost disc capacity
bits go natural
Light powers molecular
Bumps could make
changes shape in ultraviolet light
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