Self-assembly of magnetite nanorods into superchain-like ordered superstructures with antiparallel spin alignment

The self-assembly of anisotropic magnetite colloidal nanorods into ordered structures allows to obtain novel materials with new functionalities, stemming from the interaction and arrangement of the individual nanoparticles within the assembly and the resulting collective and direction-dependent properties. To succeed, high-quality, reproducible and tunable magnetite nanorod synthesis are needed, as well as meticulously designed assembly protocols to prevent random aggregation driven by the strong interparticle interactions. In this work, we present an improved and versatile method to obtain magnetite nanorods with different lengths but similar aspect ratios and use them as building blocks for the obtention of ordered assemblies. By using a liquid-liquid interface evaporation method and without the application of any external magnetic field, we achieve novel superstructures composed of superchains of magnetite nanorods aligned side-by-side. The superchains are hereby formed by antiparallel aligned dipoles and can be obtained with nanorods of different sizes by adjusting the assembly conditions. Without the need of a silica shell to obtain this alignment, and given the highly direction dependent magnetic properties, the close interparticle contact offers exciting perspectives for magnetoelectrical applications.