A disk drive records information in a pattern of magnetized regions on the disk surface. The most obvious encoding would represent binary 0s and 1s by regions magnetized in opposite directions, but that's not the way it's done in practice. Instead a 1 is represented by a transition between opposite states of magnetization, and a 0 is the absence of such a flux reversal. Each spot where a transition might or might not be found is called a bit cell. Boosting the areal density of the disk is a matter of making the bit cells smaller and packing them closer together.
Small bit cells require small read and write heads. (You can't make tiny marks with a fat crayon.) Equally important, the heads must be brought very close to the disk surface, so that the magnetic fields cannot spread out in space. The heads of the RAMAC drive hovered 25 micrometers above the disk on a layer of compressed air, jetting from nozzles on the flat surface of the heads. The next generation of drives dispensed with the air compressor: The underside of the head was shaped so that it would fly on the stream of air entrained by the spinning disk. All modern heads rely on this aerodynamic principle, and they fly very low indeed, buzzing the terrain at a height of 10 or 15 nanometers. At this scale, a bacterial cell adhering to the disk would be a boulder-like obstacle. For comparison, the gate length of the smallest silicon transistors is about 20 nanometers.
Achieving such low-altitude flight calls for special attention to the disk as well as the heads. Obviously the surface must be flat and smooth. As a magnetic coating material, bridge paint gave way some time ago to electroplated and vacuum-sputtered layers of metallic alloys, made up of cobalt, platinum, chromium and boron. The aluminum substrate has lately been replaced by glass, which is stiffer and easier to polish to the required tolerances. The mirror-bright recording surface is protected by a diamondlike overcoat of carbon and a film of lubricant so finely dispersed that the average thickness is less than one molecule.
Much of the progress in disk data density can be attributed to simple scaling: making everything smaller, and then adjusting related variables such as velocities and voltages to suit. But there have also been a few pivotal discontinuities in the evolution of the disk drive. Originally, a single head was used for both writing and reading. This dual-function head was an inductive device, with a coil of wire wrapped around a toroidal armature. In write mode, an electric current in the coil produced a magnetic field; in read mode, flux transitions in the recorded track induced a current in the coil. Today, inductive heads are still used for writing, but read heads are separate, and they operate on a totally different physical principle.
With an inductive read head, the magnitude of the induced current dwindles away as the bit cell is made smaller. By the late 1980s, this effect was limiting data density. The solution was the magnetoresistive head, based on materials whose electrical resistance changes in the presence of a magnetic field. IBM announced the first disk drive equipped with a magnetoresistive head in 1991 and then in 1997 introduced an even more sensitive head, based on the "giant magnetoresistive" effect, which exploits a quantum mechanical interaction between the magnetic field and an electron's spin.
On a graph charting the growth of disk density over time, these two events appear as conspicuous inflection points. Throughout the 1970s and '80s, bit density increased at a compounded rate of about 25 percent per year (which implies a doubling time of roughly three years). After 1991 the annual growth rate jumped to 60 percent (an 18-month doubling time), and after 1997 to 100 percent (a one-year doubling time). If the earlier growth rate had persisted, a state-of-the-art disk drive today would hold just 1 gigabyte instead of more than 100.
The rise in density has been mirrored by an equally dramatic fall in price. Storing a megabyte of data in the 1956 RAMAC cost about $10,000. By the early 1980s the cost had fallen to $100, and then in the mid-1990s reached $1. The trend got steeper after that, and today the price of disk storage is headed down toward a tenth of a penny per megabyte, or equivalently a dollar a gigabyte. It is now well below the cost of paper.