Structural Characterization of Thin Co/Cu Superlattices
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STRUCTURAL CHARACTERIZATION OF THIN Co/Cu SUPERLATTICES DAVID J. SMITH, A.R. MODAK, AND S.S.P. PARKIN* Center for Solid State Science, Arizona State University, Tempe, AZ 85287 *IBM Almaden Research Center, 650 Harry Rd, San Jose, CA 95120 ABSTRACT High-resolution electron microscopy has been used to characterize the microstructure of thin Co/Cu metallic multilayers grown on Si(100) substrates by dc magnetron sputtering. The nature of the buffer layer between the silicon wafer and the multilayer had a significant effect on the structural integrity of the multilayer. An increase in grain size was observed as a function of Cu layer thickness, with grain sizes typically being at least 3-4 times the bilayer period. A high degree of structural ordering was observed in superlattices comprised of thicker Cu layers with large grains often spanning the complete multilayer stack. Complementary magnetic measurements enabled the microstructure to be related to the oscillatory exchange coupling and large magnetoresistance exhibited by these thin magnetic films. INTRODUCTION Metallic multilayer structures consisting of alternating magnetic and non-magnetic thin films exhibit long-range oscillatory coupling and giant magnetoresistance (GMR) behavior that is intimately related to the occurrence of antiferromagnetic coupling between layers. The GMR effect was first reported in epitaxial, single-crystal Fe/Cr multilayers [1] but it was later also discovered in sputtered, polycrystalline Fe/Cr films [2], and eventually established as a widespread property common to many transition metals [3]. Various theoretical models have been proposed to explain the GMR effect as it relates to these artificially layered structures: better differentiation between (some of) the models is simplified with assistance from electron micrographs that compare microstructural features of the respective thin films. The high-resolution electron microscope (HREM) is nowadays capable of providing atomic-level structural information [4]: it thus represents an excellent tool for characterizing the morphology of magnetic multilayers where layer dimensions and other features of interest approach this scale. In this paper, we describe our recent HREM
observations of a variety of Co/Cu metallic superlattices grown on Cu, Fe and Ru buffer
layers. Particular attention is given to the relationship between microstructure and magnetic properties, especially the crucial role played by the buffer layers deposited upon the silicon substrate before growth of the multilayer. EXPERIMENTAL DETAILS
The Co/Cu multilayer samples were deposited on polished and chemically etched Si(100)
wafers using a computer-controlled dc magnetron sputtering system having a base pressure of -2x 10-9 Torr. Samples for electron microscopy were prepared in cross-section in the standard manner [5] by mechanical polishing, dimpling and ionmilling to perforation using 5 keV argon ions with the sample in a liquid-nitrogen milling stage. Electron microscope observations utilized a JEM-4000EX HREM oper
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