Ferromagnetic Tunnel Junctions with Barriers Oxidized in Oxygen Plasma

Cross-geometrical ferromagnetic tunnel junctions with a spin-valve-like structure of Ni-Fe/Co/Al-AlO/Co/Ni-Fe/Fe-Mn/Ni-Fe were fabricated by magnetron sputtering using metal masks. The Al layer was oxidized in an atmosphere of oxygen plasma by applying rf low power. The tunnel resistance increased with an increase of the plasma oxidization time. The oxidizing speed was much greater than that achieved by using natural oxidization. An MR ratio of 16% was observed in the as-deposited junction with the barrier oxidized for 35 seconds. By annealing at 300°C for one hour, the MR ratio increased to 24%. The annealing effect differs according to the plasma oxidization conditions, and we found that the MR ratios increased by annealing only for the junctions whose Al barrier was oxidized for under 45 seconds. This suggests that the remaining Al layer plays an important role in the annealing effects.