Interplay between structural and magnetic ordering in model colloidal clusters of cubic particles

A minimal coarse-grained model for cubic nanoparticles bound by magnetic dipolar forces is introduced, with limited site-site steric interactions. Static phase diagrams as a double function of the magnetic dipolar strength and the short-range attraction are computationally investigated for selected numbers of cubic particles. Despite some generic trends, the competition between directional magnetic interactions and the short-range anisotropic attraction with cubatic symmetry gives rise to very different structures showing linear, more spherical, or even columnar arrangements. The finite-temperature behavior explored using parallel tempering Monte Carlo simulations further suggests that the cubatic order is usually maximum at low temperature, in contrast with magnetic ordering which requires some significant thermal activation. Such a competition is also manifested in the multiple phase changes predicted to occur for specific system sizes as the result of cluster finite-size effects.