HRTEM and EELS Studies of Py/Al and Co/Al Multilayers
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0998-J09-04
HRTEM and EELS Studies of Py/Al and Co/Al Multilayers Jiaming Zhang1, Martin A. Crimp1, N. Theodoropoulou2, A. Sharma2, R. Loloee2, W. P. Pratt Jr. , and J. Bass2 1 Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI, 48824 2 Department of Physics and Astronomy, Michigan State University, East Lansing, MI, 48824 2
ABSTRACT Ferromagnetic/non-magnetic (F/N) metallic multilayers in the Current-Perpendicular-toPlane (CPP) geometry show giant Magnetoresistance (MR) and are promising candidates for potential use in high density storage devices. F/Al interfaces were recently shown to have large interface specific resistances that enhance the CPP-resistance. However, the CPP resistances showed instability over time at room temperature and also upon annealing to 453K. To help understand both the large interface specific resistances and their instabilities, we have undertaken cross-sectional High Resolution Transmission Electron Microscopy (HRTEM) and Electron Energy Loss Spectroscopy (EELS) studies of both as-sputtered and annealed Py/Al and Co/Al multilayers. We find well-layered, but rough structures with local F/Al interfaces being tilted up to ~15 from the plane perpendicular to the growth direction. HRTEM images appear to show diffuse interfaces, but a through-focus series of images suggests considerable grain overlap in the electron beam direction, thereby complicating interpretation. This combination of HRTEM imaging and EELS analysis suggests that any interfacial mixing is limited in scale, and shows no evidence of intermetallic compound formation. No obvious differences are seen between assputtered and annealed samples.
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INTRODUCTION We recently reported [1,2] that the current-perpendicular-to-plane (CPP) specific resistances (AR = area times resistance) of Ferromagnetic/Al (F/Al with F = Co, Fe, Co91Fe9, and Py = Ni1-xFeX with x ~ 0.2) ) interfaces were unusually large, the interface scattering asymmetries were small, and the values of R for F/Al multilayers increased by 5-10% after 6-11 months and then by an additional 2-7% upon annealing to ~ 453K. Both the interfacial properties, and the changes in MR, might be affected by the structures and changes therein of: (a) the individual layers and interfaces, and (b) interfacial roughness, and also by formation of intermediate phases, as a range of intermediate phases are known to form in such systems [3]. For example, relatively rapid Al9Co2 and B2-ordered CoAl phase formation has been observed in Co/Al bilayer atom probe tips annealed at 573K for 5min [4]. To examine the atomic scale structures of the layers and interfaces, and how they change upon annealing to 453K, we undertook High Resolution Transmission Electron Microscopy (HRTEM) studies with associated fast Fourier transforms (FFT) on separately prepared Co/Al and Py/Al multilayers. To characterize the compositions in the interfacial regions, we complemented these studies with spatially resolved electron energy loss spectroscopy (EELS) performe
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