Since its advent in 1955, the magnetic recording industry has constantly and dramatically increased the performance and capacity of hard disk drives to meet the computer industry's insatiable demand for more and better storage. The areal density storage capacity of hard drives has increased at a historic rate of roughly 27% per year - peaking in the 1990s to as much as 60% per year - with the result that by the end of the millennium disk drives were capable of storing information in the 600-700 Mbits/in2 range.
The read-write head technology that has sustained the hard disk drive industry through much of this period is based on the inductive voltage produced when a permanent magnet (the disk) moves past a wire-wrapped magnetic core (the head). Early recording heads were fabricated by wrapping wire around a laminated iron core analogous to the horseshoe-shaped electromagnets found in elementary school physics classes. Market acceptance of hard drives, coupled with increasing areal density requirements, fuelled a steady progression of inductive recording head advances. This progression culminated in advanced thin-film inductive (TFI) read-write heads capable of being fabricated in the necessary high volumes using semiconductor-style processors.
Although it was conceived in the 1960s, it was not until the late 1970s that TFI technology was actually deployed in commercially available product. The TFI read/write head - which essentially consists of wired, wrapped magnetic cores which produce a voltage when moved past a magnetic hard disk platter - went on to become the industry standard until the mid-1990s. By this time it became impractical to increase areal density in the conventional way - by increasing the sensitivity of the head to magnetic flux changes by adding turns to the TFI head's coil - because this increased the head's inductance to levels that limited its ability to write data.
The solution lay in the phenomenon discovered by Lord Kelvin in 1857 - that the resistance of ferromagnetic alloy changes as a function of an applied magnetic field - known as the anisotropic magnetoresistance (AMR) effect.
