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Giant magnetoresistance

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Giant magnetoresistance (GMR) was discovered in 1988 by Baibich et al. (1988) on Fe/Cr (0 01) superlattices and by Binasch et al. (1989) on Fe/Cr/Fe (0 01) trilayers, in both cases on samples grown by MBE. In fig. 1, we show the variation of the resistance as a function of the magnetic field for Fe/Cr superlattices at 4,2 K. The resistance drops as the configuration of the magnetizations in neighbor Fe layers goes from antiparallel to parallel. Arrows indicate the saturation field Hs, that is the field required to overcome the antiferromagnetic interlayer coupling between the Fe layers and align the magnetizations of consecutive layers. It turns out that aligning the magnetizations reduces significantly the resistivity of the Fe/Cr superlattices, this is called GMR. The magnetoresistance ratio, defined as the ratio of resistivity change to the resistivity in the parallel configuration, reaches 79% at 4.2 K, for the sample with 9 ,~ thick Cr layers shown in fig. 1 (and still 20% at room temperature); note in fig. 1, an alternate definition of the MR is used where the denominator is the resistance at zero field. A record MR ratio of 220% has been obtained in 1994 by Schad et al. (1994a, 1994b) again on Fe/Cr multilayers.

The results of Baibich et al. (1988) and Binasch et al. (1989) were obtained on samples grown epitaxially by MBE. In 1990, Parkin et al. succeeded in reproducing similar GMR effects with Fe/Cr, Co/Ru and Co/Cr multilayers deposited by sputtering, that is with polycrystalline samples. In the perspective of applications, this was of definite interest because sputtering is a simple and fast deposition technique. In addition, Parkin et al. (1990) could explore very broad thickness ranges and find the oscillatory variation of the magnet resistance which reflects the oscillations of the interlayer exchange coupling as a function of the spacer thickness. GMR effects exist in the thickness ranges where the coupling is antiferr magnetic (AF) and vanish when the coupling is ferromagnetic, as shown in fig. 2 for Fe/Cr.

We also provide neodymium magnet ceramic magnet, alnico magnet, SmCo magnet, rubber magnetand magnetic products. These products are widely used in micro-motor, motor, computer, instrument, meter, automobile, motorcycle, horologe, office equipment, toy, magnetic therapy device and daily life industries.

Xiamen Everbeen Magnet Electron Co.,Ltd. http://www.china-magnet.net

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

E-mail:info@china-magnet.net

A more extensively studied system presenting GMR oscillations, found in 1991 (Mosca et al. 1991a; Parkin et al. 1991a), is Co/Cu. As shown in fig. 3, the variation of the MR ratio as a function of the thickness of Cu exhibits three well defined maxima associated with three ranges of antiferromagnetic coupling. The height of the maxima is a decreasing function of the Cu thickness. For thicknesses larger than about 45/k, the oscillatory behavior disappears and the GMR ratio decreases continuously with the thickness of Cu. In this thickness range, the exchange coupling is weaker than the coercive forces, the magnetic arrangement at a low field is approximately random and the GMR is due to the crossover of the magnetic configuration from random to parallel.GMR oscillations have been found in a large number of systems but the typical variation as a function of the spacer thickness illustrated in fig. 2 and fig. 3 is not always observed. It frequently occurs that, at small thicknesses of the spacer layer, the AF exchange coupling is reduced (or even reversed) by ferromagnetic coupling by pinholes or defects; Therefore, in some systems, the first GMR peak has a reduced height, as illustrated in fig. 4, or even disappears. Co/Ag is a typical example of a system in which bridging by pinholes suppresses GMR at small thickness of Ag. However, the GMR can be restored when the Co layers are broken into small islands to reduce the deleterious effects of the coupling by pinholes; This can be obtained by depositing only very small thicknesses of Co (Arakial. 1991; Loloee et al. 1995), depositing the Co at low temperature (Rodmacq et al. 1993) or annealing the multilayers (Hylton et al. 1993).

We also provide neodymium magnet ceramic magnet, alnico magnet, SmCo magnet, rubber magnetand magnetic products. These products are widely used in micro-motor, motor, computer, instrument, meter, automobile, motorcycle, horologe, office equipment, toy, magnetic therapy device and daily life industries.

Xiamen Everbeen Magnet Electron Co.,Ltd. http://www.china-magnet.net

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

E-mail:info@china-magnet.net

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