Structure analysis of Co/Re superlattice grown on an Al₂O ₃ (110) substrate

The structure of a Re(5 nm)/(Co(2 nm)/Re(3 nm))₁₉ sample grown via magnetron sputtering was thoroughly analyzed via x-ray reflectivity (XRR) and transmission electron microscopy (TEM). Cross-sectional TEM results indicate that the sample is epitaxial, with the in-plane c-axis of the superlattice pointing along the c-axis of the substrate. A quantitative analysis of XRR yielded an average thickness of each layer and an interface roughness ∼ 0.4 nm between the Co and Re layers. A careful analysis of the TEM data reveals that the in-plane lattice parameters for Co, Re and Al₂O₃ are approximately 0.24 nm, 0.43 nm; 0.26 nm, 0.44 nm; and 0.24 nm, 0.44 nm, respectively. The lattice spacings of Al₂O₃ correspond to a/2 and c/3, where a, and c are lattice parameters, respectively. Interestingly, both high resolution and Z-contrast images show a very uniform period thickness. Z-contrast images, however, show that the initial interface roughness between the Re buffer layer and the first Co layer is amplified as the layers get farther away from the substrate. A high magnification TEM analysis shows that the bottom Co-Re interface roughness is about 0.2 nm, whereas the top Re-Co interface roughness is about 0.7 nm. Previous anistropic magnetoresistance measurements are discussed in light of these new structural data.