Abstract
We describe here how the exchange bias effect in Co/CoO nanoparticles depends on the size
focusing and temperature treatment of precursor Co nanoparticles before oxidation at ambient conditions.
By appealing to magnetization, microscopy, neutron and synchrotron x-ray measurements
we found that as-synthesized Co nanoparticles readily oxidize in air only after 20 days. The highest
exchange bias field of 814 Oe is observed at T = 2K. When the same nanoparticles are centrifuged
and annealed at 70 C in vacuum prior to oxidation, the exchange bias field is increased to 2570 Oe.
Annealing of Co nanoparticles in vacuum improves their crystallinity and prevents complete oxidation,
so that Co-core/CoO-shell structure is preserved even after 120 days. The crystal structure
of CoO shell in both samples is different from its bulk counterpart. Implications of such distorted
CoO shells on exchange bias are discussed. Coating of Co nanoparticles with amorphous silica shell
makes them resistant to oxidation, but ultimately modifies the crystal structure of both Co core
and SiO2 shell.
