In 1991, IBM's work on AMR technology led to the development of MR (magnetoresistive) heads capable of the areal densities required to sustain the disk drive industry's continued growth in capacity and performance. These circumvented the fundamental limitation of TFI heads - fact that their recording had alternately to perform conflicting task writing data on as well retrieving previously-written by adopting a design which read write elements were separate, allowing each be optimised for its specific function.
In an MR head, the write element is a conventional TFI head, while the read element is composed of a thin stripe of magnetic material. The stripe's resistance changes in the presence of a magnetic field, producing a strong signal with low noise amplification and permitting significant increases in areal densities. As the disk passes by the read element, the disk drive circuitry senses and decodes changes in electrical resistance caused by the reversing magnetic polarities. The MR read element's greater sensitivity provides a higher signal output per unit of recording track width on the disk surface. Not only does magnetoresistive technology permit more data to be placed on disks, but it also uses fewer components than other head technologies to achieve a given capacity point.
The MR read element is smaller than the TFI write element. In fact, the MR read element can be made smaller than the data track so that if the head were slightly off-track or misaligned, it would still remain over the track and able to read the written data on the track. Its small element size also precludes the MR read element from picking up interference from outside the data track, which accounts for the MR head's desirable high signal-to-noise ratio.
Manufacturing MR heads can present difficulties. MR thin film elements are extremely sensitive to electrostatic discharge, which means special care and precautions must be taken when handling these heads. They are also sensitive to contamination and, because of the materials used in its design, subject to corrosion.
MR heads also introduced a new challenge not present with TFI heads: thermal asperities, the instantaneous temperature rise that causes the data signal to spike and momentarily disrupt the recovery of data from the drive. Thermal asperities are transient electrical events, usually associated with a particle, and normally do not result in mechanical damage to the head. Although they can lead to misreading data in a large portion of a sector, new design features can detect these events. A thermal asperity detector determines when the read input signal exceeds a predetermined threshold, discounts that data value and signals the controller to re-read the sector.
The various improvements offered by MR technology amount to an ability to read from areal densities about four times denser than TFI heads at higher flying heights. In practice this means that the technology is capable of supporting areal densities of at least 3 Gbits/in2. The technology's sensitivity limitations stem from the fact that the degree of change in resistance in an MR head's magnetic film is itself limited. It wasn't long before a logical progression from MR technology was under development, in the shape of Giant Magneto-Resistive (GMR) technology.
