Giant Magnetoresistance in Co/Ag Multilayers
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GIANT MAGNETORESISTANCE INCo/Ag MULTILAYERS M. TAN*, J. A.BARNARD*, M. R. PARKER**, and D.SEALE** The University Of of Electrical Alabama, Engineering, *Department Tuscaloosa, of Metallurgical and Materials Engineering, **Department AL 35487-0202
ABSTRACT The magnetoresistance (MR), magnetic properties, and crystal structure of dc magnetron sputtered Co/Ag periodic.multilayers have been investigated. The Co layer thickness was fixed at -30 A while the thickness of the Ag layer was systematically varied. 'Giant' magnetoresistance was observed. The MR ratio has been found to decline monotonically with increasing Ag thickness in the range 30 A to 107 A. Although the maximum room temperature MR ratio is a modest 4.78%, a more technologically significant measurement of field sensitivity (MR ratio/FWHM of the MR vs. H peak) is a promising 0.1%/Oe at its best. The effect of the number of bilayer units has also been examined and no substantial differences were noted between multilayers containing 8, 9, and 10 bilayer units. Coercivities as determined by both magnetometer and the splitting of the MR peaks are in agreement and increase from 25 to 38 Oe with increasing Ag thickness. Evidence for antiferromagnetic coupling is apparent inthe hysteresis loops. High angle X-ray diffractometry (HXRD) in the 0-20 mode revealed a strong Ag(1 11) texture in the film, with satellite peaks indicating a layered structure. Low angle XRD (LXRD) also yielded broad superlattice peaks in all samples at least to the second order. 1. INTRODUCTION Giant magnetoresistance (GMR) in multilayer magnetic thin films has been under intensive study since the first report on GMR behavior associated with anti-
ferromagnetic (AFM) coupling in the Fe/Cr single crystal multilayer system in 1988[1]. Multilayers composed of Fe or Co and one of a number of transition metals have been found to exhibit GMR that is potentially useful for the recording industry[2-7]. MR dependence on the Ag thickness in Co/Ag multilayers has been shown to be very different by different workers[5,6]. The MR ratio in sputtered samples was found to increase with Ag thickness and reach a maximum of 5.1% at room temperature for 60 A of Ag[5], while molecular beam epitaxy deposited samples showed an oscillatory MR dependence on Ag thickness varying from 4 to 35 A,with a maximum of 16% at 25 A[6]. The field required to saturate the sample in [5] was below 1 kOe and in [6] was of the order of 10 kOe. For the thickness range of Ag spacer layer (30 A and beyond) studied in this work, the MR ratio decreases with an increase in the Ag spacer thickness. Although the absolute value of the MR ratios in the films studied here are not large they are achieved in comparatively low fields (an important issue for technological applications). In this work Co/Ag multilayers were made by dc magnetron sputtering, with fixed Co layer thickness and changing Ag layer thickness. Structure characterization, MR measurement and magnetic property measurements were conducted. 2. EXPERIMENTAL METHODS AND MATERIAL
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