Many of the past improvements in disk-drive capacity have been a result of advances in the read-write head, which records data by altering the magnetic polarities of tiny areas, called domains (each domain representing one bit), in the storage medium.  To retrieve that information, the head is
positioned so that the magnetic states of the domains produce an electrical signal that can be interpreted as a string of 0’s and 1’s.

Early products used heads made of ferrite, but beginning in 1979 silicon chip-building technology enabled the precise fabrication of thin-film heads.  This new type of head was able to read and write bits in smaller domains.  In the early 1990s thin-film heads themselves were displaced with the
introduction of a revolutionary technology from IBM.  The innovation, based on the magnetoresistive effect (first observed by Lord Kelvin in 1857), led to a major breakthrough in storage density.

Rather than reading the varying magnetic field in a disk directly, a magnetoresistive head looks for minute changes in the electrical resistance of the overlying read element, which is influenced by that magnetic field.  The greater sensitivity that results allows data-storing domains to be shrunk further.  Although manufacturers continued to sell thin-film heads through 1996, magnetoresistive drives have come to dominate the market.

In 1997 IBM introduced another innovation–the giant-magnetoresistive (GMR) head–in which magnetic and nonmagnetic materials are layered in the read head, roughly doubling or tripling its sensitivity.  Layering materials with different quantum-mechanical properties enables developers to engineer a specific head with desired GMR capabilities.  Currie Munce, director of storage systems and technology at the IBM  Almaden Research Center in San Jose, says developments with this technology will enable disk drives to store data at a density exceeding 100 gigabits per square inch of
platter space.

Interestingly, as recently as 1998 some experts thought that the SPE limit was 30 gigabits per square inch.  Today no one seems to know for sure what the exact barrier is, but IBM’s achievement has made some assert that the “density demon” lives somewhere past 150 gigabits per square inch.

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