Self-Assembled Superparamagnetic Binary Nanoparticle Superlattices

We report binary nanoparticle superlattices (BNSLs) composed of two different types of superparamagnetic nanoparticles (NPs). Since the magnetic properties of these NPs depend both on size and composition, two strategies are used to form BNSLs. First, we use different sizes of the same material (e.g.-10.5 nm and 5.6 nm diameter Fe$_{3}$O$_{4}$ NPs). Second, we use different materials, such as 14.2 nm Fe$_{3}$O$_{4}$ NPs and 6 nm FePt NPs, or 14.2 nm Fe$_{3}$O$_{4}$ NPs and 7.1 nm CoPt$_{3}$ NPs. We observe the formation of large scale BNSLs (up to several $\mu $m) due to the high uniformity of these nanoparticles. Using a serial tilting capability of our TEM tomography holder we confirm that the BNSLs are icosohedral NaZn$_{13}$ and AlB$_{2}$ type structures, which are thermodynamically stable due to their high packing density. We further measured the magnetic properties of these BNSLs samples, and single component samples, by SQUID magnetometry. Dipolar and/or exchange coupling between two components is studied.