O2 plasma oxidation effect on magnetic properties of spin valves

The use of nano-oxide to improve the performance of spin valves has been extensively studied. But most of the investigations so far have been carried out on different samples. This may make some of the conclusions drawn from the experiments inconsistent because of the fluctuation in deposition conditions and device structures. In this work, the effect of nano-oxide on the properties of spin valves has been investigated through post-growth oxidation of the same sample in oxygen plasma for different rf powers and durations. The sample investigated was a bottom spin valve with the structure Si/SiO₂/Ta/NiFe/IrMn/CoFe/Cu/CoFe/Ta. A relative increase of 20% and 12% was obtained in the giant magnetoresistance (GMR) ratio of as-deposited and annealed samples, respectively. It was found that, at a fixed rf power, there is a peak of the GMR ratio as the oxidation time increases. A higher peak value of the GMR ratio was obtained for lower rf power, although the required oxidation time is longer. This result can be well understood by considering both the enhanced specularity at the insulator/metal interface and the loss of magnetic effective thickness of the free layer by the oxidation. Magnetic parameters such as the interlayer coupling field (H₀) and the coercivity of the free layer (H_cf) were also greatly influenced by the oxidation process. When only the Ta layer was oxidized, H₀ increases very slightly, and H_cf increases with the oxidation time. However, when the CoFe free layer was oxidized, a significant increase was found for H₀, and H_cf changes to decreasing. These results can be explained based on the Néel and RKKY coupling models. Received: 25 October 2001 / Accepted: 21 December 2001 / Published online: 3 June 2002