Growth and x-ray characterization of Co/Cu (111) superlattices

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Abstract
We have prepared Co/Cu(111) superlattices by molecular-beam-epitaxy methods on sapphire (112¯0) substrates with a Nb buffer layer. The structural properties with various superlattice periodicities and layer thicknesses have been thoroughly analyzed by high-resolution x-ray Bragg-scattering and surface-scattering methods. The characterization includes the structural coherence lengths and mosaic distributions parallel and perpendicular to the film plane, the epitaxial relation to the Nb buffer, the stacking sequence, and in particular the interfacial roughness. These parameters are crucial for the interpretation of magnetic data concerning the exchange coupling in the Co/Cu superlattice. For all the superlattices studied we find that they are coherently strained with complete in-plane matching and corresponding out-of-plane Poisson response. The stacking is always completely fcc-like, with no hcp contribution even for a Co/Cu thickness ratio of 2/1. The interfacial roughnesses as obtained from fits to low- and high-angle x-ray reflectivity data are 14 and 6 Å, respectively. These parameters are discussed within the framework of random and correlated roughness effects.