Collective effect on magnetic properties of 2D superlattices of nanosized cobalt particles

We report fabrication, structural and magnetic properties of cobalt nanocry stals. They are synthesized in colloidal assemblies. The coated cobalt part icles are air stable. Due to the narrow size distribution of these 8 nm par ticles, they self-assembled after deposition on a flat substrate. The magne tic response depends on the organization. When they are dispersed in a solu tion, the saturation is not reached at 2T even at 3 K, they are still a sup erparamagnetic part in the signal. When particles are organized in 2D super lattices, saturation is reached at lower applied field, compared to isolate d particles. This is attributed to an increase of dipolar interaction. Howe ver, the hysteresis loop markedly differs with the sample orientation in th e applied field. This confirms the dipolar magnetic interactions between co balt nanoparticles. (C) 2000 Elsevier Science B.V. All rights reserved.