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

摘要

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