A CDE Definition
A change in electrical resistance in metal or a semiconductor when it is subjected to a magnetic field. The property of magnetoresistance is used in reading the bits on magnetic tape and disk. Although used in earlier analog tape recorders, in 1991, IBM was the first to use a magnetoresistive (MR) read head in a computer disk drive.
As storage capacity increases, the bit gets smaller and its magnetic field becomes weaker. MR heads are more sensitive to weaker fields than earlier inductive read coils, in which the bit on the medium induced the current across a gap. The MR mechanism is an active element with current flowing through it. The magnetic orientation of the bit increases the resistance in a thin-film nickel-iron layer, and the difference in current is detected by the read electronics. MR heads use the traditional inductive coil for writing.
In 1998, IBM introduced drives with giant MR (GMR) heads, which are sensitive to even weaker fields. GMR heads use additional thin film layers in the sensing element to boost the change in resistance, and "giant" refers to this larger change. Almost all modern drives use GMR read heads.
Discovered in 1995 at the NEC Research Institute in Princeton, NJ, extraordinary MR (EMR) provides an even greater change in resistance. Quite unique in that the material is non-magnetic, EMR is expected to provide bit densities of a terabit per square inch some day. See superparamagnetic limit.
GMR Heads for Reading
The maximum number of bits per square inch that is commercially feasible on a magnetic storage device. As the magnetic bits get smaller, at some point they no longer hold their charge. Thermal fluctuations reduce the signal strength and render the bits unstable.
However, this ultimate areal density keeps changing as researchers find new techniques for recording and sensing the bit. Years ago the limit was thought to be 20 gigabits per square inch. Today, the limit is several hundred gigabits per square inch, and more than a terabit is expected in the years to come. See areal density, magnetoresistance and AFC.
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