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

I. Lisiecki Laboratoire LM2N, UMR CNRS 7070, Université P. et M. Curie, Bât F, 4 Pl. Jussieu, 75005 Paris, France

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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.

DOI: 10.12693/APhysPolA.121.426 PACS numbers: 75.50.Tt, 75.70.Cn