Collective magnetic properties of cobalt nanocrystals self-assembled in a hexagonal network: Theoretical model supported by experiments

Numerical calculations of magnetization curves versus applied field based o n a simple model taking into account dipolar interactions were performed fo r cobalt nanocrystals deposited on a substrate and organized in a hexagonal network. A random distribution of the easy axes orientations of the nanocr ystals is considered. The study is focused on the effect of the applied fie ld orientation relative to the substrate surface. Two orientations were cho sen: parallel and perpendicular to the surface. The corresponding hysteresi s loops are compared to that of a volumic random distribution of nanocrysta ls at vanishing concentration. The calculation results are compared to expe rimental data for spherical cobalt nanocrystals coated by lauric acid (C12H 25COO-). The particles are either dispersed in hexane (considered as random ly distributed) or deposited in a hexagonal network on a highly oriented py rolithic graphite substrate. The changes in the magnetization curves with t he applied field orientation on the one hand and when going from dispersed to deposited particles on the other hand were calculated and measured. Qual itative agreement is obtained.