Nanostructural considerations in giant magnetoresistive Co-Cu-based symmetric spin valves

The effect of surface modifying species, Pb, on the nanostructure and resulting giant magnetoresistive properties of symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO is investigated, using high-resolution and conventional transmission electron microscopy. When a monolayer of Pb is deposited on the first Co/Cu bilayer, the net ferromagnetic coupling between the middle Co layer and the NiO-pinned Co layers is reduced by an order of magnitude. Nanostructure investigations revealed that, in the absence of Pb, the Co and Cu layers grow coherently over each other within columnar grains. The coherent growth mode of the metal layers in the Pb-free samples gives rise to topographically correlated ferromagnetic films, a requisite if ferromagnetic coupling arises from Neel's ''orange-peel'' effect. In the presence of Pb, the deposition proceeds by the nucleation of fine, (apparently) randomly oriented nanocrystallites in successive metal layers, instead of a coherent growth mode, thereby disrupting topographic correlation between the adjacent ferromagnetic films, and reducing the net ferromagnetic coupling due to Neel's ''orange-peel'' effect. Due to well defined orientation relationships between the adjacent grains in the Pb-free samples, a faceted surface morphology develops, on which the top NiO film is deposited. This is in contrast to the Ph-containing samples which have an irregular, undulatorylike surface. Although not directly observable, the characteristics of Co-Cu interfaces can be inferred from the nature of the top Co surface.