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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/AlOx , by varying the oxidation time tOx of the capping layer. From XRR simulations, we show that the nature of the stack is gradually defined according to the value of tOx. For low tOx values (< 40 s), a simple Pt/Co/Al/AlOx multilayer is appearing whereas a Pt/Co/CoO/AlOx architecture takes place for higher tOx. The oxygen-induced magnetic properties obtained by extraordinary Hall effects measurements are explained by the structural results. The increase of Co-O bondings with tOx is at the origin of the appearing of the perpendicular magnetic anisotropy (PMA).