magnetoresistance


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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

Magnetoresistance

 

a change in the electric resistance of a solid conductor caused by an external magnetic field. A distinction is made between transverse magnetoresistance, in which the electric current flows at right angles to the magnetic field, and longitudinal magnetoresistance, in which the current flows parallel to the magnetic field.

Magnetoresistance results from distortion of the trajectories of the current carriers in a magnetic field. In semiconductors the relative change in resistance Δp/p is 100-10,000 times greater than in metals and can reach hundreds of percent. Magnetoresistance is a galvanomagnetic phenomenon. It is used to study the electron energy spectrum and the mechanism of scattering of current carriers in a crystal lattice, and also to measure magnetic fields.

REFERENCES

Lifshits, I. M., M. Ia. Azbel’, and M. I. Kaganov. Elektronnaia teoriia metallov. Moscow, 1971.
Blatt, F. Fizika elektronnoi provodimosti v tverdykh telakh. Moscow, 1971. (Translated from English.)
Ansel’m, A. I. Vvedenie v teoriiu poluprovodnikov. Moscow-Leningrad, 1962.

E. M. EPSHTEIN

magnetoresistance

[mag¦nēd·ō·ri′zis·təns]
(electronics)
The change in the electrical resistance of a material when it is subjected to an applied magnetic field, this property has widespread application in sensors and magnetic read heads.
(electromagnetism)
The change in electrical resistance produced in a current-carrying conductor or semiconductor on application of a magnetic field.

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.


GMR Heads for Reading
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.)
References in periodicals archive ?
Spasov, "Temperature dependence of low-field magnetoresistance in perovskite manganites thin films," Vacuum, vol.
In fact, the giant magnetoresistance (GMR) effect was discovered using magnetic multilayers.
Quantum size effect and tunneling magnetoresistance in ferromagnetic semiconductor quantum heterostructures.
The additional magnetoresistance curves at different temperatures, though not needed in traditional extrapolations, have allowed for a potential new measurement procedure derived by generalizing an empirical rule developed by Kohler (2) to describe the behavior of many non-superconducting polycrystalline metals.
Mani, "Microwave-induced electron heating in the regime of radiation-induced magnetoresistance oscillations," Phys.
Investigations of magnetoresistance of manganite thin films in pulsed magnetic fields were some of them [4].
The spin polarization and the corresponding tunneling magnetoresistance (TMR) for a hybrid ferromagnetic/superconductor junction are calculated.
Semenchinskii, "Relationship between the Components of the Magnetoresistance Tensor under Conditions of the Quantum Hall Effect," JETP Lett.
2001--Stuart Parkin, IBM Almaden Research Center, San Jose, California Developer of Giant Magnetoresistance (Computer disk drive architecture
By contrast, the improvements described in back-to-back reports in the December Nature Materials boost the components' magnetoresistance to more than 200 percent.
Giant magnetoresistance, as the effect is called, could be used to read and write information in and out of computer storage devices or in magnetoresistive sensors.
He worked out on many fields including shallow impurities and excitons, electronic, optical, and transport properties of semiconductors and nanostructures, electronic and optical properties of semiconductor surfaces and interfaces, phonons and electron-phonon couplings in semiconductors and nanostructures, non-linear optical properties, many-body effects in semiconductors, exciton condensation, magnetic multilayers and giant magnetoresistance (GMR), femtosecond pump-and-probe phenomena, photonic crystals, metrology of semiconductor thin films and gratings, infrared and radiation detectors, semiconductor lasers and modulators, resonant tunneling diodes, quantum transport properties, single-photon generators, spintronics, quantum computing, optical metrology, and nano plasmonics.