Core Size And Interface Impact On The Exchange Bias Of Cobalt/Cobalt Oxide Nanostructures

Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is approximate to 100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is approximate to 10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic-antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores.