The Magic of Pixie Dust

IBM solves media storage problem

IBM says a layer of ruthenium "pixie dust" applied to its next-generation disk drives fools recording heads into thinking the magnetic layers are thinner than they really are. And that may eventually allow 100 gigabits of information — or the equivalent of 13 hours of MPEG-4 compressed digital video — to be stored on a square inch of disk space.

IBM scientists say the new coating is a solution to a problem thought to be impossible to solve, allowing them to quadruple the amount of information a disk drive can store and, perhaps, hastening the arrival of interactive digital media applications for home entertainment.

Heres how: A disk drives recording head reads data as it flies over the rapidly rotating disk, and senses the magnetic orientation of the material that coats the disk. Its essential that the signal is strong enough for the recording head to read it, because each change in magnetic polarization represents a one or zero in the binary code that builds letters, musical tones and images. The signals strength depends on the "magnetic thickness" of coating. To increase the amount of data that can be written on a drive, engineers have had to make the coating thinner and shrink the diameters of the "magnetic grains" that make up the coating. Unfortunately, this makes the coating vulnerable to changes in temperature, which can affect signal strength and the readability of the data.

IBM scientists attacked this problem by using two magnetic layers instead of one. They first cover the whole disk with a very thin magnetic layer. Before applying the second, thicker layer, they put a substance between the two layers that is astonishingly thin, yet ensures that the two layers will be anti-parallel — the top oriented in one direction, the bottom in the opposite direction. Ten years ago researchers found that, of all the nonmagnetic elements, ruthenium is best at creating that anti-parallel coupling.

Once the ruthenium is in place, the top layer is applied. This layer is only 150 angstroms to 200 angstroms thick. An angstrom is one ten-millionth of a millimeter, which means that the whole coating is incredibly thin.

With two magnetic layers oriented in opposite directions, the head is fooled: Instead of reading the magnetic thickness of the top layer or the combined magnetic thickness of the two layers, it reads the magnetic thickness of the top layer minus the thickness of the bottom layer.

The apparently smaller magnetic thickness combined with smaller magnetic grain diameters allows much more data to be written on the disk. However, the actual thickness of the coating — two layers rather than one — allows all the high-density data to remain stable in heat and cold.

The coating can be made with existing equipment at little or no extra cost. The greatest challenges were to find a way to fabricate the layers so their grains pointed in the proper plane, and to modify the tools to uniformly deposit a layer of ruthenium just 3 atoms thick over the disk surface.

IBM already is using the new "antiferromagnetically-coupled" media in its Travelstar notebook hard drives, which boast data densities of up to 25.7 gigabits per square inch. By 2003, the new coatings may permit hard disk drives to store 100 gigabits per square inch.

"Were making the media look thinner, but keeping the volumes large," says IBM physicist Eric Fullerton. "That way you get more energy, but the magnetic grains seem smaller so the bits can be packed closer together." Fullerton and his colleagues are so enamored of the wee but powerful ruthenium that theyve dubbed it "pixie dust."