A New Intrinsic Magnetic Property Due to the Mesoscopic Ordering of Cobalt Nanocrystals in Supra-Crystals

Recently, it has been shown that nanoparticles with a narrow size distribution can self-organize into highly or- dered fcc supra-crystals. This new generation of materials have been found to exhibit unique collective properties, which arise from the periodic ordering of the nanoparticles. Using the expertise we have to control the ordering of cobalt nanoparticles in 3D assemblies, we have been able to carry out a carefully controlled comparative investigation of the magnetic properties of a supracrystal and a disordered assembly. In this letter, we report for the first time a new intrinsic effect of supra-crystal order on the magnetic behavior which we attribute to a decrease in the distribution of energy barri- ers, arising from nanoparticle ordering. We also show that these assemblies can be annealed at 350°C to give a structural transition of the Co nanoparticles from an fcc to hcp phase. This annealing does not induce either coalescence or oxidation of the nanocrystals.