Exchange bias in Co/CoO thin films deposited onto self-assembled nanosphere arrays

•Antiferromagnetic/ferromagnetic nanocaps were synthesized by nanosphere lithography.•Exchange bias on CoO/Co/PS and CoO/Co/Si ultra-thin and thick films were studied.•These ultra-thin and thick films were prepared by in-situ oxidation.•Exchange bias values show drastic modifications on magnetic nanocaps.•Results are correlated with curvature induced variations in structural properties. Looking at the importance of exchange bias (EB) studies in novel magnetic systems, magnetic nanocaps consisting of antiferromagnetic (AF)/ferromagnetic (FM) layers were synthesized using nanosphere lithography, followed by ion beam sputtering. For this, CoO/Co/Si and CoO/Co/polystyrene nanosphere (PS) (800 nm diameter) ultra-thin and thick films were prepared by in-situ oxidation. Room temperature magnetization measurements exhibit a clear distinction between the films on PS and those on plane base Si substrate (reference films). Low temperature field cooled measurements on CoO/Co(5 nm)/PS film show a negative EB in both original (−37.4 mT) and trained hysteresis loops (−31.4 mT). However, corresponding reference films show larger EB values in comparison with those of PS substrate counterparts. Due to a higher Co layer thickness, the CoO/Co(100 nm)/PS film shows a lower EB value (−18.2 mT) in comparison with its corresponding CoO/Co(5 nm)/PS ultra-thin film (−37.4 mT). However, for the corresponding reference films, the ultra–thin film shows a higher EB (−68.5 mT) than the thick film (−12.6 mT), which is a generally observed behavior. The overall results are discussed in terms of the curvature induced modifications in the microstructural properties, which cause drastic changes in the magnetic properties of such nanostructures.