Magnetoresistance (MR) is the change in electric resistance of a conductor by a magnetic field. In nonmagnetic conductors, for example in metals like copper or gold, the MR is due to the Lorentz force that a magnetic field exerts on moving electrons; in general, it is relatively low. In magnetic conductors, for example, in ferromagnetic metals, the spin polarization of the electrons generates other contributions to the MR; for a review, see Campbell and Fert (1982).
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A first well-known property of ferromagnetic conductors induced by spin--orbit coupling, is the dependence of resistance on the angle between magnetization and electric current; when a magnetic field is applied, it modifies this angle and thereby the resistance; this is called anisotropic magnetoresistance or AMR. The AMR of ferromagnets does not exceed a few percents at room temperature; however, it is used in a number of applications because the magnetization of a soft ferromagnet can be easily rotated by very small fields.
Another type of MR in ferromagnets comes from the so-called spin disorder contribution to the resistivity. Spin disorder at the atomic scale increases the resistivity; by reducing the spin disorder with an applied field, one generates a negative and isotropic MR. This MR is generally low in ferromagnetic transition metals and alloys, but can be very high in other types of ferromagnetic materials, for example, in some mixed valence Mn oxides. Discovered in 1994, the MR of these Mn oxides has been called colossal magnetoresistance or CMR; for a review, see Coey et al. (1999).
The giant magnetoresistance or GMR is another type of MR, existing in heterostructures and discovered in 1988 in Fe/Cr multilayers (Baibich et al. 1988; Binasch et al. 1989). The GMR of multilayers is induced by the variation of the angle between the magnetizations of consecutive magnetic layers. The discovery of the GMR was preceded by the discovery of exchange coupling between magnetic layers across a nonmagnetic metal layer. This interlayer exchange coupling was found in 1986 in Fe/Cr/Fe trilayers (Griinberg et al. 1986), in Gd/Y (Majkrzak et al. 1986) and in Dy/Y (Salamon et al. 1986) multilayers. In Fe/Cr/Fe trilayers or Fe/Cr multilayers, there are ranges of thickness of the Cr layers where the coupling is antiferromagnetic and induces an antiparallel alignment of the magnetizations of adjacent Fe layers. When a magnetic field aligns all the magnetizations in parallel, the resistance of the multilayer decreases dramatically, this was called giant magnetoresistance or GMR.
The discovery of interlayer exchange and GMR has triggered extensive experimental and theoretical studies on multilayers and more general magnetic nanostructures. GMR effects have been observed not only in exchange coupled magnetic multilayers but also in uncoupled multilayers, spin valve structures, multilayered nanowires and granular systems.
In multilayers, GMR has been studied not only for current in the plane (CIP) butalso with current perpendicular to the plane of the layers (CPP-geometry). The CPP-GMR has revealed interesting spin accumulation effects which have formed the basis for further developments.
Today GMR is used in various types of device such as sensors, read heads and magnetic RAM. These devices generally operate at very small fields; the interest in multilayers is that only very small fields are required to rotate the magnetization of a soft ferromagnetic layer. While comparable low fields can activate AMR and GMR devices, the amplitude of GMR is larger. On the other hand, the amplitude of the CMR can be much larger, however, its drawback for applications comes from the relatively high field required to alter the magnetization of a ferromagnetic above its spontaneous value.Beyond GMR of magnetic multilayers and its applications, research on GMR has revealed a new class of magnetotransport phenomena which can be obtained in magnetic nanostructures by making use of the spin polarization of carriers. This is a new field ofresearch which is called "spin electronics". Examples of emerging directions of research with important potential applications in this area are: spin injection effects (Johnson 1993; Berger 1996; Slonczewski 1996; Vasko et al. 1997; Tsoi et al. 1998), spin dependent tunneling (Moodera et al. 1995) and magneto-Coulomb blockade (Barnas and Fert 1998).
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Add:Unit H, 4F Rihua Mansion, No. 8 Xinfeng 2nd road, Torch Hi-Tech Zone, Xiamen, China.
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The present review is focused on GMR in magnetic multilayers. In section 2, we present an experimental overview which gathers results on conventional multilayers, spin valves, multilayers on grooved substrates and multilayered nanowires. In section 3, we summarize the two-current model of conduction in ferromagnets and present a simple picture of GMR.
The physics of GMR is discussed in section 4. We review the theoretical models and make use of selected experimental data to discuss the current understanding of GMR. Section 5 is a guide to experimental data. The related problem of interlayer exchange is not in the scope of this chapter; the reader is referred to the review article of Griinberg et al. (1999) in this series, and also to Rhyne and Erwin (1995) for the special case of rare earth multilayers.
For the general problems of preparation and characterization of thin films and multilayers, and also for an earlier review of interlayer exchange and GMR, the reader is referred to Heinrich and Bland (1994).
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Xiamen Everbeen Magnet Electron Co.,Ltd. http://www.everbeenmagnet.com
Add:Unit H, 4F Rihua Mansion, No. 8 Xinfeng 2nd road, Torch Hi-Tech Zone, Xiamen, China.
Tel:0086-592-5781916
Fax:0086-592-5123653
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