Investigation of Metallic/Oxide Interfaces in Pt/Co/AlO$_{\rm x}$ Trilayers by Hard X-Ray Reflectivity

X-ray reflectivity (XRR) is used to determine the oxidation front at the nanometer scale in sputtered perpendicular semi tunnel junctions, as the form Pt/Co/AlO$_{\rm x}$ , by varying the oxidation time ${\rm t}_{\bf Ox}$ of the capping layer. From XRR simulations, we show that the nature of the stack is gradually defined according to the value of ${\rm t}_{\bf Ox}$. For low ${\rm t}_{\bf Ox}$ values ($≪ 40$ s), a simple Pt/Co/Al/AlO $_{\rm x}$ multilayer is appearing whereas a Pt/Co/CoO/AlO$_{\rm x}$ architecture takes place for higher ${\rm t}_{\bf Ox}$. The oxygen-induced magnetic properties obtained by extraordinary Hall effects measurements are explained by the structural results. The increase of Co-O bondings with ${\rm t}_{\bf Ox}$ is at the origin of the appearing of the perpendicular magnetic anisotropy (PMA).