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Itsy-bitsy: Hard drives bumping up against physical limits

As they ponder creating hard-drive disks that can store more and more data, researchers enter territory that redefines 'small'

Monday, October 29, 2001

By Byron Spice, Science Editor, Post-Gazette

Correction/Clarification: (Published Oct. 30, 2001) The hard-disk drives in computers are fast, but not quite so fast as indicated by yesterday's article about the devices. It said hard disks revolve thousands of times each second; in reality, they revolve thousands of times each minute.

In today's computer-savvy age, computer chips have brand names, Internet users debate the merits of cable modems vs. DSL phone lines and the introduction of a new operating system, such as last week's release of Windows XP, is a national news story.

    Dense matters

Heat-assisted magnetic recording technology, now being developed by a consortium headed by Carnegie Mellon University's Data Storage Systems Center and Seagate Technology, might enable the storage of one terabit of computer data for every square inch of surface area on a hard disk.

How dense is a terabit per square inch?

One terabit is 1 trillion digits -- a series of 1s and 0s. If a person wrote a 1 or a 0 every second in each box on a standard piece of graph paper, it would take 31,709 years, 10 months, 14 days, 1 hour, 44 minutes and 41 seconds to write a terabit. It would require a square piece of graph paper measuring 3 miles on each side.

At a density of 1 terabit per square inch, a photo downloaded off the Internet could be stored in an area the same diameter as a human hair; the information from 16 floppy disks could be stored in an area the size of the period at the end of this sentence.

One trillion grains of salt would fill one cubic meter.

At a density of 1 terabit per square inch, almost six hours of movies could be stored in an area equivalent to a thumbnail; the head of a pin could hold the capacity of a music CD.


But nobody thinks much about the hard-disk drive. As critical to the popularity of PCs as fast, cheap microprocessors and the Internet, these data storage devices whirl away anonymously inside computer cabinets, filing away emails, computer games and downloaded photographs. And for the past decade, the amount of data that can be magnetically stored within a square inch of disk has more than doubled annually, a fact that is as amazing as it is taken for granted.

So it's news to most of us that the jig is up.

Hard disks aren't going away any time soon, but those days of spectacular growth in storage capacity will soon be over. Engineers, bumping up against the physical limits of conventional hard disks, are looking for new storage technologies.

"We don't know exactly when [traditional] recording will run out of steam," said Mark Kryder, senior vice president and research director for Seagate Technologies, a major hard-disk maker. "We have thousands of clever engineers who will extend it as far as we can." But within two or three years, advances in storage capacity will begin to taper off, he predicted.

"In the labs, we've all seen the effect," Kryder added.

Last week, Carnegie Mellon University announced that the U.S. Department of Commerce had agreed to help fund a five-year, $21.6 million effort led by Seagate and CMU's Data Storage Systems Center to develop an enhanced data storage technology, called heat-assisted magnetic recording.

If researchers can pull it off, the new technology could store at least 100 times more data in a square inch than has been achieved with today's hard disks.

"It's a huge challenge," said Ed Schlesinger, a professor of electrical and computer engineering who, with James Bain, will lead the CMU portion of the program. Researchers will have to sort out the strange physical properties that apply to materials and light at scales smaller than a virus.

Between the Data Storage Systems Center, the premier academic center for data storage research, and Seagate's soon-to-open Pittsburgh research center, about two-thirds of the consortium's work will be performed in Pittsburgh.

A ripple effect

A leveling-off of the exponential growth of hard-disk recording capability wouldn't seem to be a crisis. After all, hard-disk drives could continue to do everything that they do today.

But continued improvements in data storage, much like development of faster microprocessors, drive down the costs of computing and make new applications possible.

Take television, for instance. Cheap hard-disk storage makes personal video recorders -- such as Tivo -- possible. These devices make it possible to record multiple TV programs simultaneously, or to seemingly "pause" live TV.

"My expectation is that 10 years from now you will not buy a TV set without a personal video recorder built in," Kryder said. Placing hard disks in TVs could alone double the size of the $35-billion-a-year industry.

High-definition televisions now on the horizon will make such devices essential, said Robert White, director of the Data Storage Systems Center. Today's videocassette recorders simply won't work with HDTV, he noted, and even DVDs may not be good enough to match HDTV's demands.

As the density of data on a hard disk grows, the size of the disks themselves can shrink. IBM already is selling a Microdisk that can store a gigabyte of information -- 1 billion computer words, or "bytes" -- on a device measuring not much more than an inch square. Hard-disk drives could be built into cars, hand-held audio devices and any number of machines and appliances.

Some people envision a "memory prosthesis," a handheld device that could record everything a person sees, hears or does in the course of a day.

Work is under way at CMU to develop wireless hard-disk drives, White said, a development that, combined with ever-cheaper disk drives, could usher in an era of "ubiquitous computing." Rather than relying solely on a computer's own hard disk, future computer users would tap into a shared storage utility, consisting of interlinked, wireless hard drives that could provide data storage on demand.

And even if people forgo new applications, the information age is producing new data at ever-growing rates.

At the end of World War II, all recorded knowledge was estimated at about 10 terabytes -- 10 trillion bytes -- of text. Today, 100 terabytes of new text is created each year. Physicist and essayist Philip Morrison estimated three years ago that adding film, audio and video to the mix would bring the total information at hand to perhaps 10 exabytes, or 10 million terabytes.

"We couldn't possibly store all the information we have on books anymore," Schlesinger said.

Already pretty special

Today's hard-disk drive is an engineering marvel. A stack of 3.5-inch-diameter disks revolves thousands of times a second, while actuator arms -- similar to the arm of a record player -- whiz back and forth. At the end of each arm is a tiny electromagnetic head, almost too small to see, that records data on the disk and later reads the data when needed.

The read/write heads fly across the ultrasmooth disks at a distance of just 20 nanometers -- 20 billionths of a meter, about twice as thick as a bacterium's cell wall. At these scales, a speck of dust might as well be a boulder, so the whole mechanism is encased in a protective box.

"I've been working in this field for almost 40 years," said Horacio Mendez, a former IBM research executive who is executive director of the Data Storage Systems Center, "and I'm still almost amazed that these things work."

Researchers at the University of California, Berkeley, in 1948 developed the first magnetic storage device for computers -- a revolving drum two feet long and eight inches in diameter -- that could hold 10,000 computer words. In 1957, IBM introduced the first commercial hard-disk drive, a two-ton machine with 50 24-inch-diameter disks; it could store 5 million bytes of information.

Today's largest capacity drive, small enough to hold in one hand, can store 180 billion bytes of information, enough to store about 40 DVD movies.

Only one thing is preventing engineers from packing even more data onto these disks. It is called the superparamagnetic limit -- an instability that afflicts magnetic materials at very small scales.

Hard disks are coated with ferromagnetic films that contain tiny magnetic grains -- each a little magnet with north and south poles. Data from a digital computer -- strings of 1s and 0s called bits -- is stored on the disk in the form of magnetic bits. Each magnetic bit is an area of the disk containing hundreds of grains; the electromagnet in the read/write head induces all of the grains in the bit area to flip in the same direction. As the head proceeds along the disk, it can flip the polarity of other bits in the same or opposite direction. Two adjacent bits with opposite polarities encode a 1, two adjacent bits with the same polarity encode a 0.

The Berkeley drum contained 800 of these magnetic bits for every square inch of surface. IBM's first hard disks had a few thousand bits per square inch. By making the grains smaller, packing them tighter and using just 100 to 200 grains per bit, rather than thousands of grains per bit, engineers have achieved densities of 33 billion bits, or gigabits, per square inch.

But these features are getting so small that the bits interact and become unstable, so that even room temperature heat is enough to cause the polarity of some magnetic grains to spontaneously flip. That's called the superparamagnetic effect. If enough grains flip within a magnetic bit, that bit eventually becomes unreadable and the data is lost.

Some people thought the superparamagnetic limit would be around 40 gigabits per square inch; recent tricks discovered by researchers now make it appear the actual limit may be at hundreds of gigabits per square inch. Either way, with densities now doubling about every nine months, that limit will be reached soon.

One solution in the near term, Kryder said, may be to re-orient the grains. They now are aligned longitudinally, with their north-south poles in the same plane as the disk. By arranging them with their poles perpendicular to the plane of the disk, it should be possible to position them closer together.

A longer term solution being pursued by many researchers is to etch the disk surface so that each bit is physically isolated and less like to perturb another.

Another possible remedy is the heat-assisted magnetic recording technology being studied by the Seagate/CMU consortium, which includes the University of Arizona and several disk makers.

In this approach, the disks are coated with alloys whose magnetic grains are more resistant to the effects of heat and thus resist the flip-flopping that bedevils conventional disks. The problem, Schlesinger noted, is that the same property also makes it difficult to record data on the disks.

To record, the researchers propose to use a tiny laser to momentarily heat each bit by hundreds of degrees. When the bit is hot, the read/write head can be used to record data; when it cools off again, the data is safely stored.

The heat-assisted technique could push densities to 1 trillion bits, or one terabit, per square inch and perhaps much further, Schlesinger said. But first researchers have to figure out how to build a read/write head that incorporates a tiny laser, how to focus a laser to a spot just 20 or 30 nanometers in diameter, how close bits can be spaced, how to design disk lubricants that can withstand heat shifts and many other technical challenges.

The dimensions are so tiny, Schlesinger said, that just measuring these devices becomes a problem.

"How do I even verify that I've made this thing?"

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