Growth temperature dependence of magnetoresistance in Co/Cu(111) wedged superlattices

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Abstract
The relationship of the growth and structure of highly oriented (111) Co/Cu superlattices to their magnetotransport properties is discussed. To eliminate changes in growth parameters between different samples, wedged superlattice structures were prepared in which uniform Co layers are separated by Cu layers whose thickness is varied by more than a factor of 2 across the substrate. Smooth variations in magnetoresistance (MR) as a function of Cu-layer thickness were obtained. No evidence for oscillations in MR was found with the exception of a peak in MR for thin Cu layers centered at about 10 Å. The magnitude of the MR at this peak was found to increase substantially for growth at 150 °C relative to growth at 0 °C. The increased MR is correlated with smoother Co/Cu interfaces as determined from in situ x-ray-photoelectron-diffraction studies and ex situ x-ray-diffraction experiments. At 150 °C it is found that Cu has a tendency to surface segregate at all stages during growth. Studies of the growth of Cu on Co show the presence of Co near (but excluded from) the surface even after the growth of relatively thick Cu layers. At both 0 °C and 150 °C, the superlattices showed a relatively high degree of surface roughness, when compared with the Cu and Co layer thicknesses. Either of these results suggest a mechanism for ferromagnetic bridging of the Co layers, previously conjectured to play an important role in (111) Co/Cu superlattices.