disks won't hit quantum barrier
Technology Research News
One key in the continuing quest to increase
the capacity of computer disk drives is making sure the devices are stable
enough to hold information over time. Bigger disks are not much of an
advantage if there is a risk that the data can just seep away.
IBM researchers who are looking long-term have found the theoretical storage
capacity and stability of many of the materials used in magnetic drives.
Magnetic disks are made up of billions of magnetic regions that can point
in one of two directions corresponding to the ones and zeros computers
use to store information.
In a perfect disk drive, magnetic bits
will switch only when they are instructed to by the computer. In practice,
however, perfection is elusive. The pragmatic manufacturing standard ensures
a low probability that many bits will randomly change: each bit must maintain
95 percent of its magnetization over 10 years.
There are two mechanisms that cause bits to switch spontaneously. Heat
can provide enough energy to flip a bit, and in situations where there's
not a lot of other energy around -- like when things get very small and
very cold -- quantum tunneling has the potential to do so as well. Disk
drive manufacturers haven't paid much attention to the second problem
simply because it's not a practical consideration at room temperature.
In the next decade or so, however, as researchers try different ways of
cramming more information into smaller spaces, it could prove useful to
know how much of a problem quantum tunneling might be.
Quantum tunneling is one of the weird quantum properties of matter and
energy that comes into play when things are very small. Ordinarily particles
like electrons can be confined by energy barriers much like marbles a
Unlike marbles, however, each electron has a realistic probability of
disappearing and reappearing outside the barrier. Given enough electrons,
some are certain to tunnel through. Quantum tunneling can also happen
in small magnetic fields, which means that if the bits in a disk drive
were too small, they would interfere with each other.
IBM researchers looking into long-term basic research have found how much
of a problem quantum tunneling is likely to be, and the answer is a good
one: quantum tunneling becomes an issue only at an extremely low temperature
-- below 2 degrees Kelvin, or -271 Celsius.
"We knew that some temperature above absolute zero quantum tunneling would
become important. We just didn't know how high that temperature would
be for the magnetic media used in disk drives," said Geoff Held, a research
staff member at IBM Research.
The researchers found that for all materials currently used for magnetic
disks, this temperature was below 2 Kelvin, said Held. "This means
that one can continue to improve disk performance by cooling down the
operating temperature of the drive until one reaches very low temperatures,"
Using this information, the researchers also found the ultimate theoretical
limits of how much data can fit in a given area of magnetic disk drives
made from certain materials. For example, one promising medium of iron
platinum nanoparticles has a theoretical limit of 43 terabits per square
inch at 1.3 degrees Kelvin, according to Held. It takes about one terabit
to store sixty hours of full-motion video.
The work will probably not come into play for very long time, Held said.
"As long as researchers can improve aerial density without cooling the
disk drive they will continue to do so. If a point is reached where no
further improvement is possible at room temperature and there is still
a demand for increased aerial density, methods of cooling the disk drives
will be considered. At that point our work will allow researchers to determine
how much they can cool the disk drives and still see improvement in aerial
density," he said.
The research is good, but probably not of great practical use, said Caroline
Ross, an associate professor of materials science at the Massachusetts
Institute of Technology. Because quantum tunneling becomes an issue for
disk drives at such a low temperature, it may never be a practical concern,
she said. "It's a good thing to know, but we'll never be able to test
the theory in a disk drive because we can't run one at those temperatures,"
Held's research colleague was Geoff Grinstein. They published the research
in the September 3, 2001 issue of Applied Physics Letters. The research
was funded by IBM.
Timeline: > 10 years
TRN Categories: Data Storage Technology
Story Type: News
Related Elements: Technical paper, "Quantum Limit of Magnetic
Recording Density," Applied Physics Letters, September. 3, 2001.
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