This work was financially supported by the University of Buenos Aires (UBACyT; TX 117 and X093), ANPCyT (Agencia nacional de Promoción de Ciencia y Tecnología) PICT 10621, the International Program for Physical Science of Uppsala University, Sweden (IPPS), the Research Institute of Universidad Nacional de Ingeniería, the CONCYTEC (Peruvian Research Council), and CYTED network VIII-G.

Sonochemical synthesis of monodisperse nickel nanoparticles (Ni-NPs) by reduction of Ni acetylacetonate in the presence of polyvinylpyrrolidone stabilizer is reported. The Ni-NPs size is readily controlled to 5 nanometer diameter with a standard deviation of less than 5%. The as-prepared Ni-NPs sample was dispersed in acetone, for 4 weeks. For structural analysis was not applied to a magnetic field or heat treatment as key methods to direct the assembly. The transition from separate Ni-NPs into self-organization of three dimensions (3D) superstructures was studied by electron microscopy. Experimental analysis suggests that the translation and rotation movement of the Ni-NPs are governed by magnetic frustration which promotes the formation of different geometric arrangements in two dimensions (2D). The formation of 3D superstructures is confirmed from scanning electron microscopy revealin...