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 °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 …