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

Abstract

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.