To elucidate in details any possible influence of the adjacent layers on perpendicular magnetic anisotropy in very thin ferromagnetic CoFeB electrodes of CoFeB/MgO based stacks, we grew by sputtering BL/CoFeB (1 nm) and BL/CoFeB (1 nm)/MgO (2 nm), being BL (buffer layer) = Ta (5 nm) or Ru (1 nm) multilayers, consisting of 30 repetitions of the bi- or tri-layers. Specular X-ray reflectivity (XRR) has been performed on both as grown and annealed (300 degrees C in vacuum) multilayers. From XRR results, good reproducibility of each layer thickness is achieved, indicating a well-controlled film growth. Further, CoFeB/MgO interface is found to be quite smooth and stable with annealing, as shown by the limited interface width, while BL/CoFeB interface widens upon annealing, in particular when BL = Ta is used. We discuss the above findings, also considering the role of possible layer crystallinity in affecting the interface width, and tentatively relate them to the magnetic anisotropic behavior of the stacks. (C) 2012 Elsevier B.V. All rights reserved.