Hard disk drives

Magnetic systems have been used in recent years for the long-term storage of data in computers. The first hard disk came in 1956 from IBM inside their RAMAC (Random Access Method of Accounting and Control) computer, capable of storing 100,000 characters on each of fifty 24-inch disk platters and constructed from iron oxide and aluminium. These disks had a data, or areal, density of around 2 kilobits per square inch.

Seventeen years later IBM released the Winchester hard disk, containing the basic technologies used in modern hard disk drives: a very small read/write head capable of ``skiing'' around 1/18,000,000 of an inch above the surface of the disk. The Winchester had an areal density of 1.7 megabits per square inch.

Seagate Storage Technology developed the first hard disk for personal computers in 1980. Although this disk had a similar capacity to the RAMAC, the entire assembly fit into a 5.25 inch enclosure (form factor): the same width and double the height of a standard modern CD-ROM drive. Three years later, Rodime introduced a hard disk in a 3.5 inch form factor, and in 1985 Quantum attached this to a hard card which plugged directly into a personal computer's system board.

This form factor evolution continued throughout the late 1980s, until standard 3.5 inch hard drives with integrated electronics appeared. Introduced by Conner in 1988, these had the same physical dimensions as a standard desktop PC hard disk drive today. The same year saw the first 2.5 inch hard drive, now the de facto standard for laptop computers, though the 1.8 inch form factor is gaining popularity with slimline and sub-notebook sized laptops.

Currently the smallest hard disk drive with this configuration is the Hitachi Microdrive (figure 1.3), having a one inch form factor and a height of just five millimetres but with a capacity of four gigabytes.

Hard disk drive manufacturers are constantly looking for ways to improve areal density, as this equates to a greater storage capacity. Areal density is widely regarded as the crucial metric driving the hard disk industry. The highest areal density today is more than fifty million times greater than in the late 1950s: the present record is held by Toshiba Corporation at 133 gigabits per square inch and areal density is presently doubling every twelve months.

This trend cannot, however, continue indefinitely. Present methods of hard disk production are approaching physical limits, and the areal density will no longer be able to increase beyond these fundamental limits. To overcome these physical limitations, we can look to the behaviour of magnets at the microscopic scales used in hard disks to find potential solutions.

Figure 1.3: An exploded view of the Hitachi Microdrive. The disk platter and read/write head can be seen in the third layer from the top. The long edge of the disk housing is one inch (base image artwork credit: 2004 Griff Wason).