Structural Characterization of Epitaxial GMR Magnetic Multilayers and Spin Valves Grown by Sputter Deposition
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Structural Characterization of Epitaxial GMR Magnetic Multilayers and Spin Valves Grown by Sputter Deposition H. Geng, R. Loloee, J.W. Heckman, J. Bass*, W.P. Pratt, Jr.*, and M.A. Crimp Department of Materials Science and Mechanics, *Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-1226, U.S.A. ABSTRACT Epitaxial Cu/Py/FeMn and (Cu/Co)×20 GMR magnetic multilayers were grown on single crystal ( 1 1 0 ) Nb that was deposited on ( 11 2 0 ) Al2O3 substrates by dc magnetron sputtering. Electron backscatter patterns (EBSPs) revealed that the Cu films display two twin variants, corresponding to two stacking sequences of {111} planes in fcc. The epitaxial orientation relationship between the bcc Nb and both fcc Cu variants was the Nishiyama-Wasserman (N-W) relationship. Conventional TEM observations revealed epitaxial growth for both the Cu/Py/FeMn and (Cu/Co)×20 multilayers. High-resolution TEM confirmed epitaxial growth of close packed ( 1 1 0 ) Nb on ( 11 2 0 ) Al2O3 substrates with [111]Nb||[0001]Al2O3. Numerous small twins were observed in the Cu near the Cu-Nb interface of the Cu/Py/FeMn multilayer. In the Cu/Co multilayer, the growth planes of the Cu and Co were found to be {100} instead of the expected close-packed {111} planes of the fcc structure. INTRODUCTION Electron transport in giant magnetoresistance (GMR) materials is influenced by layer quality, interface structures, and film orientation [1]. Initial structural characterization of these devices has concentrated on polycrystalline materials grown via dc magnetron sputtering [2]. Most of these multilayers have incorporated Nb superconducting contacts, required for GMR measurements with the current perpendicular to the layer planes (CPP) geometry. Because of the difficulty of separating electron-scattering contributions due to interfaces from those caused by other defects in polycrystalline multilayers, it is of interest to grow epitaxial magnetic multilayers and spin valves to reduce structural defects associated with grain boundaries. Molecular beam epitaxy (MBE) is widely used for growing epitaxial films, but sputter deposition offers higher growth rates and lower cost. In the present study, dc magnetron sputtering was used to grow epitaxial films of Cu/Py/FeMn (Py = Permalloy (NiFe)) and (Cu/Co)×20. Three electron microscopy techniques were used to characterize these materials. Electron backscatter patterns (EBSPs) were used to determine crystal structures, orientations, and growth epitaxy. Conventional transmission electron microscopy (CTEM) was used to characterize microstructural features such as crystallographic epitaxy, layer quality, and gross interfacial roughness. The crystal structures and orientations were further characterized using highresolution TEM (HRTEM) with fast Fourier transform (FFT) image analysis. EXPERIMENTAL DETAILS The substrates for GMR multilayer fabrication were ( 11 2 0 ) oriented sapphire (Al2O3) single crystal wafers that had been pre-deposition annealed at 1200°C. The multilayers were
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