Nanocrystals May Give Boost to Data Storage

Robert F. Service

For companies that make magnetic disk drives, the future is scary. Over the past 5 decades, engineers have managed to control the magnetic orientation of smaller and smaller spaces on their disks. That's recently allowed them to increase data storage capacity by a staggering 100% a year. Industry experts aren't sure how much longer they can keep up that blistering pace, however. "Five years out, we don't know what will come next," says Christopher Murray, a chemist who works on new materials for future disk drives. "It's an unnerving situation."

Now, Murray and his IBM colleagues have hit upon an answer that may steady a few nerves. On page 1989, the researchers report creating tiny carbon-coated metallic particles--each just 4 nanometers, or billionths of a meter, across--that they assemble into a thin sheet and bake into a magnetic film that could be used in hard disk drives. Down the road, if each of the tiny particles can be made to store a bit of information as a magnetic field, the films have the potential to hold terabytes of data per square inch, hundreds of times the capacity of today's disk drives. The new nanoparticle films aren't about to hit the computer superstores: Researchers must still work out how to make them compatible with the technology used for writing and reading bits of data to the films. Still, Jim Heath, a chemist and nanoparticle expert at the University of California, Los Angeles, says the progress thus far is impressive. "This is a big deal," he says. "It means that magnetic recording could be carried down to near molecular length scales."

Capturing the $35-billion-a-year market for disk drives won't be easy, however. Today's hard disks owe their storage prowess to films made from a cobalt alloy that are rugged and cheap to make. Manufacturers essentially spray-paint magnetic material onto a surface under vacuum and bake it. That leaves a material full of 15- to 20-nanometer magnetic grains whose magnetic orientation can be aligned by a recording head positioned just above it. Typically, a bit of information is stored as the common orientation of hundreds of those grains.

Engineers have long increased storage capacity by shrinking the magnetic grains in the films, so each bit of stored data takes up less space. But there's a limit to this process: Many magnetic materials, such as cobalt, lose their magnetic behavior when particles shrink below about 10 nanometers. And particles that do maintain their strong magnetic behavior tend to clump together instead of forming an even sheet.

The IBM team--Shouheng Sun and Murray at IBM's T. J. Watson Research Center in Yorktown Heights, New York, along with Dieter Weller, Liesl Folks, and Andreas Moser at the Almaden Research Center in San Jose, California--managed to get around both problems at once with some clever chemistry. Their strategy was to make tiny particles from iron and platinum, which would start out as weakly magnetic--allowing them to form an array--but then transform them into stronger magnets at the end.

The researchers started by concocting a solution that included two metal salts--one containing iron atoms, which are hungry for electrons, the other platinum atoms capable of donating electrons. As the salts dissolved, the iron atoms turned to the platinums for electrons, causing the atoms to begin assembling themselves into a ball. Also in the brew were soap molecules, oleic acid, and oleyl amine. As the particles grew, the soap molecules glommed onto the metal particles and stopped them growing at 4 nanometers.

At this stage, the metal particles were weakly magnetic jumbles of iron and platinum atoms. To make an array, the IBM team simply poured the particles out of the beaker. As the solvent evaporated, the particles nestled down into a regular structure like oranges stacked in a box.

Next, the IBM researchers baked their array like a sheet of cookies, at 500ºC for about 30 minutes. The heat fused the organic molecules into a hard carbon coat that locked the particles in place, and it caused the iron and platinum atoms to segregate into distinct atomic planes, a change that dramatically boosted the magnetic strength of the materials.

The IBM team showed that these materials can store data faithfully at a density equivalent to that of hard disks on the market today. The particles' small size may even allow researchers to boost that density 10-fold using current read and write heads. But if heads can be improved to manipulate magnetic fields on single particles--and that's a big if--then the films could potentially store orders of magnitude more data.

Sun and Murray are quick to point out that the new materials need more work. The biggest problem, Murray says, is that conventional recording heads work only if all the magnetic grains or particles on a disk have their crystalline axes aligned with the disk's surface. For now, however, the tiny iron-platinum particles can freeze in place facing any direction. Murray says the IBM team is working on aligning the particles by applying an external magnetic field to their films as they bake. If they succeed, the future of data storage may soon become a little less unnerving.