magnetoresistance
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.
Magnetoresistive (MR)
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.
Giant MR
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.
Extraordinary MR
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.
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| GMR Heads for Reading |
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| Most modern disk drives use GMR (giant MR) heads for reading, but use inductive coils for writing. (Illustration assistance courtesy of Hitachi Global Storage Technologies.) |
magnetoresistance [mag¦nēd·ō·ri′zis·təns]
Magnetoresistance
The change of electrical resistance produced in a current-carrying conductor or semiconductor on application of a magnetic field H. Magnetoresistance is one of the galvanomagnetic effects. It is observed with H both parallel to and transverse to the current flow. The change of resistance usually is proportional to H2 for small fields, but at high fields it can rise faster than H2, increase linearly with H, or tend to a constant (that is, saturate), depending on the material. In most nonmagnetic solids the magnetoresistance is positive. See Galvanomagnetic effects
In semiconductors, the magnetoresistance is unusually large and is highly anisotropic with respect to the angle between the field direction and the current flow in single crystals. When the magnetoresistance is measured as a function of field, it is the basis for the Shubnikov–de Haas effect, much as the field dependence of the magnetization gives rise to the de Haas-van Alphen effect. Measurement of either effect as the field direction changes with respect to the crystal axes serves as a powerful probe of the Fermi surface. Magnetoresistance measurements also yield information about current carrier mobilities. Important to practical applications is the fact that the geometry of a semiconductor sample can generate very large magnetoresistance, as in the Corbino disk. See De Haas-van Alphen effect, Fermi surface, Semiconductor
Multilayered structures composed of alternating layers of magnetic and nonmagnetic metals, such as iron/chromium or cobalt/copper, can feature very large, negative values of magnetoresistance. This effect, called giant magnetoresistance, arises from the spin dependence of the electron scattering which causes resistance. When consecutive magnetic layers have their magnetizations antiparallel (antiferromagnetic alignment), the resistance of the structure is larger than when they are parallel (ferromagnetic alignment). Since the magnetic alignment can be changed with an applied magnetic field, the resistance of the structure is sensitive to the field. Giant magnetoresistance can also be observed in a simpler structure known as a spin valve, which consists of a nonmagnetic layer (for example, copper) sandwiched between two ferromagnetic layers (for example, cobalt). The magnetization direction in one of the ferromagnetic layers is fixed by an antiferromagnetic coating on the outside, while the magnetization direction in the other layer, and hence the resistance of the structure, can be changed by an external magnetic field. Films of nonmagnetic metals containing ferromagnetic granules, such as cobalt precipitates in copper, have been found to exhibit giant magnetoresistance as well. See Antiferromagnetism, Ferromagnetism, Magnetization
Magnetoresistors, especially those consisting of semiconductors such as indium antimonide or ferromagnets such as permalloy, are important to a variety of devices which detect magnetic fields. These include magnetic recording heads and position and speed sensors. See Magnetic materials
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