Beating Superparamagnetism

Musicians have an insatiable appetite for digital storage capacity; anything that allows more data to be stored in a given amount of physical space is

Musicians have an insatiable appetite for digital storage capacity; anything that allows more data to be stored in a given amount of physical space is welcome. Unfortunately, the laws of physics limit the density with which bits can be stuffed into that space.

Consider hard disks: individual bits of data are encoded in the orientation of tiny particles, or grains, that are laid out in concentric tracks on the surface of a circular platter. If a grain's magnetic field is oriented one way, it represents a zero; oriented the other way, it represents a one. A write head records data by orienting the grains with a variable magnetic field as the disk spins under the head. To recover the data, a read head senses the magnetic orientations of the grains.

With today's hard disks, the grains' magnetic fields are parallel to the disk's surface; this is called longitudinal recording (see Fig. 1a). The density of longitudinally recorded data has reached more than 100 gigabits per square inch (Gb/in2), but this technology is fast approaching its limits. Within two product generations, many researchers believe that longitudinal recording will hit a wall called superparamagnetism. As the grains shrink below a certain size, they weaken and cannot resist the various factors that demagnetize them, such as stray magnetic fields and heat. As a result, over time the data they encode become corrupted, making the hard disk useless.

One solution to this problem is perpendicular recording, in which the grains are perpendicular to the disk surface (see Fig. 1b). In this case, the magnetic field of each grain is oriented up or down rather than right or left. Because the grains are vertical, many more can be squeezed into a given area before superparamagnetism begins to take its toll. Interestingly, perpendicular recording is more than 100 years old, with roots in the work of late 19th-century Danish scientist Valdemar Poulsen, who was the first person to record sound magnetically using this technique.

Recently, Hitachi Global Storage Technologies ( has begun field tests of hard disks that use perpendicular recording. In addition, laboratory experiments have achieved the industry's highest data density of 230 Gb/in2. As well as more closely packed, vertically aligned grains, the distance between the read/write head and disk surface must be decreased to accurately record and recover the data; in the Hitachi tests, that distance is 10 nanometers.

Testers worldwide have been using perpendicular-recording hard drives as part of their daily routine since December 2004 to evaluate the long-term performance and reliability of the technology. Among the testers is Professor Shun-ichi Iwasaki, president and chief director of Japan's Tohoku Institute of Technology, who is considered to be the father of modern perpendicular recording. “Around 1975, I began to feel that the vertical direction was the right way to go to attain high-density recording,” he says, “and I began leading the activities to make perpendicular recording a practical technology. I am glad to see that the technology will come into use soon.”

Hitachi expects commercially available hard disks with perpendicular recording to achieve a data density of 230 Gb/in2 by 2007, which would allow 1-inch microdrives to offer capacities of 20 GB and 3.5-inch drives to provide 1 terabyte of storage. The company anticipates a tenfold increase in density over longitudinal recording in the next five to seven years, which would increase the capacity of 1-inch microdrives to 60 GB and 3.5-inch drives to 3 TB. That ought to keep electronic musicians happy for a while.