Influence of packaging on the surface oxidation and magnetic properties of cobalt nanocrystals
Résumé
One frequently encountered obstacle during both ambient fabrication and the use of metal nanoclusters is spontaneous oxidation, which hampers many technological applications. In this work, we studied the influence of packaging material on the surface oxidation and magnetic properties of Co nanocrystals. We demonstrate that 'epoxy-electronic-varnish' capping (60-80 nm) effectively preserves the magnetic properties of pristine Co nanocrystals (∼14 nm); without showing any exchange bias coupling over a period of >720 h, slower temperature variation of coercivity reminiscent of a Co single crystal and intercluster interaction dominated high blocking temperatures >300 K. Packaging by a silver (Ag) capping layer of similar thickness facilitates pronounced exchange bias compared to the even air exposed Co nanocrystals. Numerical fits M(H) = M(∞)[1 - (H*/H)1/2] to the high field part (10-50 kOe) of M-H loops yield a sharp rise in H* values at cryogenic temperatures (5-25 K) only in Ag capped Co nanocrystals, indicating a large induced anisotropy due to altered Co/Ag interfacial spin structures.