Effects of annealing on the microstructure and giant magnetoresistance of Co-Cu-based spin valves
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TRODUCTION
THE study of magnetic multilayers composed of alternating magnetic and nonmagnetic metal layers has recently attracted great interest. In these systems, the interplay between electron transport properties and magnetic behavior results in a variety of fascinating phenomena, in particular, in the giant magnetoresistance (GMR) effect.[1,2] Since the GMR effect was discovered, research has been directed toward increasing the size of the GMR effect, decreasing the size of the magnetic field required to produce the effect, and improving the ability of GMR materials to withstand high-temperature annealing. Technological applications of great economic importance are likely to result from such efforts to achieve large GMR at low fields. While GMR values as large as 110 pct have been reported in superlattices at room temperature,[3] and saturation fields as low as 0.2 mT (2 Oe) have been reported for GMR spin valves,[4] nothing even close to 110 pct GMR at 0.2 mT has ever been found. Instead, the 110 pct GMR occurred at ⬃3 T, and the 0.2 mT result gave a GMR of only 3 pct. However, there does not appear to be any fundamental impediment to achieving large values of GMR at extremely low saturation fields. If samples could be tailor-made with atomic perfection, it should be possible to eliminate the sources of the large saturation fields in samples exhibiting large GMR values. The spin-dependent electron transport properties of magnetic multilayers exhibiting GMR are critically dependent on growth conditions and on the resulting microstructure. An understanding of the influence of defects (point, line, and planar), growth morphology (grain size, distribution, and texture), and interfacial characteristics (roughness and intermixing) on the electron transport properties would enhance the ability to control the growth of films with desired sets of properties. In particular, interdiffusion M.A. MANGAN, Postdoctoral Researcher, formerly with the Naval Research Laboratory, Washington, DC, is with the Institute for Defense Analysis, Alexandria, VA. G. SPANOS, Metallurgist, is with the Physical Metallurgy Branch, Naval Research Laboratory, Washington, DC 20375. R.D. McMICHAEL, P.J. CHEN, and W.F. EGELHOFF, Jr. are Materials Scientists with the National Institute of Standards and Technology, Gaithersburg, MD 20899-3460. Manuscript submitted January 21, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
and roughness are thought to have a significant effect on GMR. For example, in Fe/Cr multilayers, some studies have concluded that the magnetoresistance is increased by interfacial roughening.[5,6] The role of compositional mixing at NiFe/Cu interfaces in NiFe/Cu/NiFe/FeMn spin valves has been examined by increasing the thickness of the intermixed regions through annealing.[7,8,9] In contrast to Fe/Cr, interfacial roughening was observed to decrease the magnetoresistance due to the presence of high resistivity intermixed layers at the NiFe/Cu interfaces. In Co/Cu multilayers, however, the examination of the effect of intermixing
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