Storing information

Of all computer components, none has improved faster than the magnetic hard-disk drive. From 1991 to 1997, the storage density on hard disks–the number of bits per square inch of surface areas–doubled every 18 months, and in the past two years the pace has quickened, with a doubling every 12 months or so. In just 15 years, PC hard drives have swelled from 20Mb to almost 40Gb.

Much of the progress in magnetic recording has come from
engineering–improving the heads that write and read data, for example, and flying them closer to the disk surface, where they can read and write smaller areas on the disk. But that evolution is coming to an end, says Hal Rosen, Manager, Advanced Disk Technology, IBM Research Center,
Almaden. If progress is to continue, new magnetic materials will be needed.

Each bit on a hard disk is created by magnetizing a small section of the surface of the disk. Looked at under a microscope, such a section can be seen to consist of a thousand or so individual grains, each crystal formed by atoms arranged in a closely packed pattern. Each grain behaves like a tiny bar magnet. At first the magnetic field of each grain points in one direction. Magnetic fields pointing right might indicate a 0, say, and a field pointing left a 1.

To cram more 0s and 1s onto a hard disk, one must shrink the surface area covered by an individual bit. If the number of grains in a bit dropped much below its current value, noise would make it difficult to distinguish a 0 from a 1. “So we have to make smaller and smaller grains,” says Rosen.

But this course, too, has its limits. The grains are now approaching a size where the normal jiggling around of atoms at room temperature is enough to flip the magnetization of a grain spontaneously from one direction to the other. This phenomenon, known as the superparamagnetic effect, will make it physically impossible to store information in grains smaller than about 10 Nm using the current magnetic material. With further materials engineering and improved signal processing, Rosen predicts that a storage density of 100 gigabits per square inch will be reachable, making possible a one terabyte–one trillion byte–hard drive.

Beyond that, significantly different technologies will be needed. To evaluate different approaches, researchers are using computer simulations that take into account every part of the magnetic disk system, from the write and read heads to the magnetic recording material and the magnetic interactions among the grains–all of which affect the ultimate performance of the disk.

One approach under investigation is perpendicular recording, in which the tiny bar magnets of the grains point up or down–that is, out of or into the disk–instead of right or left. Perpendicular recording uses thicker magnetic recording layers. Because a grain’s resistance to flipping is proportional to its volume, bits with smaller surface areas would then be feasible. It would also be possible to work with hard-to-magnetize materials that, once magnetized, better resist superparamagnetic flipping. But taking advantage of these properties will demand the development of special read-write systems and materials optimized for perpendicular recording.

Rosen also foresees replacing the magnetic recording layer on the disk with a completely new type of material, such as an iron-platinum alloy or a cobalt-iron-oxygen alloy, whose resistance to spontaneous flipping is inherently much higher than the alloys now in use. 

Complicating all these possibilities is the fact that, on the tiniest scales, a material’s behavior depends on its size–a 5 Nm grain of platinum, for example, has different properties from a 10 Nm grain because the behavior of electrons is influenced by how tightly they are confined.

To get nanometer-scale particles, tiny crystals are grown in solution, carefully controlling their nucleation and growth to produce a batch of particles very similar in size and shape. The solution is then purified, keeping only those of the right dimensions. With this technique, collections of 10 Nm particles can be obtained that vary in size by no more than plus or minus 5%.

A process has been developed whereby a single layer of these particles self-assembles on a substrate, creating a thin film composed of grains almost identical in size and shape. By studying its properties, researchers can discover the strengths and weaknesses of the types of magnetic materials that may be put to work on hard disks in the future. 

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