Ferromagnetic tunnel junctions with plasma-oxidized Al barriers and their annealing effects

Abstract

Cross-geometrical Ni–Fe/Co/Al–AlO/Co/Ni–Fe/Fe–Mn/Ni–Fe tunnel junctions were fabricated by magnetron sputtering. To form the insulating layer, an Al layer was reverse sputtered in an atmosphere of either oxygen or oxygen–argon mixture at low power after deposition. The oxidization time necessary to form an AlO barrier was much shorter than that by natural oxidization, lasting for only a half to a few minutes. By adding argon to oxygen, the oxidization was slowed down and high MR ratios were obtained for a wide range of time. A magnetoresistance (MR) ratio of 16% was observed in the as-deposited junction when the barrier was oxidized in oxygen plasma for 35 s. In addition, the MR ratio increased to 24% by annealing at 300 $°\mathrm{C}.$ In as-deposited junctions, the tunnel resistances were increased by increasing the plasma oxidization time, but the MR ratios gradually decreased. The estimated tunnel barrier width increased and the barrier height decreased with the plasma oxidization time. After annealing, the MR ratio increased only for those junctions oxidized for short times. This suggests that the remaining Al between the AlO and the Co surface plays an important role in the effects of annealing.