Collective dipolar interactions in self-assembled magnetic binary nanocrystal superlattice membranes.

Co-assembly of two types of nanocrystals (NCs) into binary NC superlattices (BNSLs) provides a solution-based, inexpensive way to create novel metamaterials with rationally designed properties. The fundamental challenge is to probe and understand the nature and extent of complex interparticle interactions present in BNSLs, which can lead to collective properties that differ from their dispersed constituents or phase-separated counterparts. Here, we report the growth and magnetic characterization of large-area (∼1 cm(2)) BNSL membranes self-assembled from distinct magnetic NCs at the liquid-air interface. The resulting BNSL membranes exhibit a single-phase-like magnetization alignment process, which is not observed in the phase-separated NC mixtures having the same stoichiometry. This single-phase-like magnetic behavior is attributed to the collective interparticle dipolar interactions between two NC components in BNSLs, corroborated by calculation of the random dipolar fields as well as Monte Carlo simulation. The collective magnetic properties are demonstrated in magnetic BNSL membranes having different structures (stoichiometry) and different NC combinations.