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 on a simple model taking into account dipolar interactions were performed for cobalt nanocrystals deposited on a substrate and organized in a hexagonal network. A random distribution of the easy axes orientations of the nanocrystals is considered. The study is focused on the effect of the applied field orientation relative to the substrate surface. Two orientations were chosen: parallel and perpendicular to the surface. The corresponding hysteresis loops are compared to that of a volumic random distribution of nanocrystals at vanishing concentration. The calculation results are compared to experimental data for spherical cobalt nanocrystals coated by lauric acid $\left({\mathrm{C}}_{12}{\mathrm{H}}_{25}{\mathrm{COO}}^{\mathrm{-}}\right).\mathrm{}$ The particles are either dispersed in hexane (considered as randomly distributed) or deposited in a hexagonal network on a highly oriented pyrolithic graphite substrate. The changes in the magnetization curves with the applied field orientation on the one hand and when going from dispersed to deposited particles on the other hand were calculated and measured. Qualitative agreement is obtained.