Interface width evaluation in thin layered CoFeB/MgO multilayers including Ru or Ta buffer layer by X-ray reflectivity

Abstract

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 (1nm) and BL/CoFeB (1nm)/MgO (2nm), being BL (buffer layer)=Ta (5nm) or Ru (1nm) 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 width, and tentatively relate them to the magnetic anisotropic behavior of the stacks.