Ordering at Various Scales: Magnetic Nanocrystals

Here, it is shown that the internal crystallinity called nanocrystallinity of rather uniform Co nanoparticles can be improved by annealing. This induces marked changes in the magnetic properties such as an increase in the blocking temperature that can reach a value close to room temperature. It is shown that the acoustic breathing modes remain quite unchanged by changing the nanocrystallinity. Co nanocrystals with low size distribution are able to self-assemble either in fcc colloidal crystals called supracrystals or in films with voids. Collective intrinsic properties (chemical and physical) due to magnetic nanocrystal ordering in 2D and 3D superlattices are presented. Furthermore, when the nanocrystals are aligned, the magnetic properties of the assemblies are improved. By using magnetostatic bacteria, it is demonstrated that the magnetic anisotropy is mainly due to induced dipolar interactions with a low contribution of the influence of the orientation of the nanocrystal easy axes.