Shape‐induced superstructure formation in concentrated ferrofluids under applied magnetic fields.

The field‐induced ordering of concentrated ferrofluids based on spherical and cuboidal maghemite nanoparticles is studied using small‐angle neutron scattering, revealing a qualitative effect of the faceted shape on the interparticle interactions as shown in the structure factor and correlation lengths. Whereas a spatially disordered hard‐sphere interaction potential with a short correlation length is found for ∼9 nm spherical nanoparticles, nanocubes of a comparable particle size exhibit a more pronounced interparticle interaction and the formation of linear arrangements. Analysis of the anisotropic two‐dimensional pair distance correlation function gives insight into the real‐space arrangement of the nanoparticles. On the basis of the short interparticle distances found here, oriented attachment, i.e. a face‐to‐face arrangement of the nanocubes, is likely. The unusual field dependence of the interparticle correlations suggests a field‐induced structural rearrangement. [ABSTRACT FROM AUTHOR]