From the Co Nanocrystals to Their Self-Organizations: Towards Ferromagnetism at Room Temperature

Here, we show that rather uniform 7 nm Co nanocrystals self-organize into long-range two- and three-dimensional superlattices. Due to this ability we have to control the mesoscopic ordering; unexpected intrinsic collective chemical and physical properties have been discovered. By annealing treatment, a crystallographic transition from a polycrystalline fcc- to single crystalline hcp-Co phase is obtained leading to a drastic change in the magnetic properties such as an increase in the blocking temperature that can reach a value close to room temperature. Neither coalescence between nanocrystals nor oxidation of the Co material is observed. In these artificial solids, when the individual nanocrystal anisotropy is low, super-spin glass behavior is observed.