Plan-view microstructures of Co/Ru bilayers
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Plan-view microstructures of two Co/Ru bilayers with a composition of [COI2ARU45A]2 and [CO4OARU35A]2 have been studied by conventional and high resolution electron microscopy. Large differences in electron diffraction and image contrast between the two bilayers were observed, which are recognized as the microstructural variations during the relaxation of large coherent planar strains when the Co layers wet coherently or semicoherently the Ru layers. For the [CO12ARU45A]2 bilayer, the Co layers are unrelaxed from the Ru layers; only one set of electron diffraction patterns was observed, and the image consists of three types of contrasts which are closely related with either the generation and movement of misfit dislocations or large coherent strains. For the [CO4OARU35A]2 bilayer, the Co layers are relaxed basically from the Ru layers; two sets of electron diffraction patterns with double diffraction spots were observed, and the image consists of small irregular areas with moire fringe dots.
I. INTRODUCTION Great attention has been attached to the study of artificially fabricated magnetic metallic ultrathin multilayers due to their particular magnetic properties. l'A The magnetic properties, especially the unusual perpendicular magnetic anisotropy exhibited by some of these multilayers, are particularly interesting for magnetic fundamental studies and for applications to recording media in relation to high density magnetic recording.3 It can be understood that the magnetic properties of a particular multilayer system are closely associated with its crystallography and microstructural characteristics, such as quality of buffer layer, thickness of individual layers, structural variation of epitaxial films, interfacial states between individual layers, chemical interaction between the different species, and distribution of stressstrain fields, etc. Many techniques have been applied to achieve the structural characterization of multilayers,4 for example, small or large angle x-ray scattering and reflection low or high energy electron diffraction. Electron microscopy, as a powerful tool for structural determination, has been applied to study the structure of multilayer films, 512 including conventional electron microscopy (TEM) and high resolution electron microscopy (HREM). Based on the combination of both plan-view and crosssection observations, it has given a lot of direct atomic scale information about the structure of metallic multilayers, such as the direct observation of column growth mechanism, 67 the distribution of misfit dislocations and
coherent strains,6 as well as the thickness of individual layers and the roughness of multilayers.8'9 Up to now, however, almost all of the electron microscopy observations were mainly focused on the cross-sectional view of multilayers. One first reason for this is the difficulty in the explanation of planview electron micrographs that show the total interaction effect between the incident electron beam and the different layers: one often does not know what kind of interact
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