Optimization of exchange bias in Co/CoO magnetic nanocaps by tuning deposition parameters

In the present work, we report exchange bias tuning by varying thin film deposition parameters such as synthesis method and underlying layer patterning. The patterned substrates for this study were prepared by self-assembly of polystyrene (PS) latex spheres (~530 nm) on Si (100) substrate. The desired magnetic nanocaps composed of CoO/Co bilayer film on these patterned substrates were prepared by molecular beam epitaxy technique under ultra-high vacuum conditions. For this, a Co layer of ~10 nm thickness was deposited on the substrates and then oxidized in-situ to form CoO/Co/PS in-situ oxidized film or ex-situ in ambiance which also gives CoO/Co/PS naturally oxidized film. Simultaneously, reference thin films of Co (~10 nm) were also prepared on plane Si substrate and similar oxidation treatments were performed on them respectively. The magnetic properties studied using SQUID technique revealed higher exchange bias (~1736 Oe) in the in-situ oxidized Co/PS film as compared to that in naturally oxidized Co/PS film (~1544 Oe) and also compared to the reference film. The observed variations in the magnetic properties are explained in terms of surface patterning induced structural changes of the deposited films and different oxidation methods.