Bigger disks won't hit quantum barrier

By Kimberly Patch, 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 box.

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," he said.

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," she said.

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
Funding:   corporate
TRN Categories:   Data Storage Technology
Story Type:   News
Related Elements:  Technical paper, "Quantum Limit of Magnetic Recording Density," Applied Physics Letters, September. 3, 2001.


November 28, 2001

Page One

Programmable DNA debuts

Device would boost quantum messages

Virtual computers reconfigure on the fly

Software sorts video soundtracks

Bigger disks won't hit quantum barrier


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