Giant exchange bias by tuning Co/CoO core/shell structure

In this report, the exchange-bias effect of Co/CoO core/shell nanoparticles with the amorphous CoO shell has been investigated. At low temperatures, the developed spin-glass state with net magnetic moments in the CoO shell provides a unidirectional anisotropy in the Co core via exchange coupling, leading to a giant exchange bias field up to 3.6 kOe. By improving the crystallinity of CoO shell by performing surface oxidation at an elevated temperature, the spin-glass ordering is suppressed by the antiferromagnetic ordering, resulting in a significantly lower exchange-bias field (2.2 kOe) and a prominent training effect due to the existence of multiple easy axes in the multi-crystalline CoO shell. Theoretical studies confirm that the emerging spin-glass state in the CoO shell has a significant impact on the exchange bias of the system. These new findings provide a novel approach toward manipulating and fine-tuning the exchange bias effects in nanoscale systems for spintronics applications.