Emergence of New Collective Properties of Cobalt Nanocrystals Ordered in fcc Supracrystals:  II, Magnetic Investigation

It has been shown that by annealing three dimensional (3D) disordered and fcc supracrystal assemblies of 7.5 nm Co nanocrystals, a structural transition of the nanocrystals from a poorly crystallized fcc structure to a pure hcp phase occurs. The annealing process does not result in either oxidation or coalescence of the nanocrystals. Not only is the system highly stable, but also the thermal treatment actually improves the mesoscopic structural order in the fcc supracrystals. In the native state, an effect of the mesoscopic order on the magnetic properties of the 3D assemblies is observed which we attribute to the difference in distribution of dipolar interaction energies. In this paper, we extend this study to the same systems after annealing in order to investigate the effect of increased anisotropy and crystallinity on the magnetic behavior. We find a significant increase in both blocking temperature (to a near room-temperature value) and saturation magnetization. Surprisingly, the effects of mesoscopic order are no longer observed.