Engineering the Magnetic Dipolar Interactions in 3D Binary Supracrystals Via Mesoscale Alloying

Inspired by metallic alloys in atomic solids, two distinct metallic nanoparticles are used, considered as artificial metal atoms, to engineer ordered binary nanoparticle alloys at the mesoscale, called binary supracrystals. Here, ferromagnetic 7.2 nm Co nanoparticles are used as large A site particles, while either ferromagnetic 4.6 nm Co or nonmagnetic 4.0 nm Ag nanoparticles are used as small B site particles to fabricate long-range ordered binary supracrystals with a stoichiometry of AB(2) and AB(13). The interparticle distances between 7.2 nm Co nanoparticles within the Co/Ag binary supracrystals can be tuned by a control of crystal structure from AB(2) (CoAg2) to AB(13) (CoAg13). A decrease of magnetic coupling between Co nanoparticles is observed as the Co-Co interparticle distance increases. Furthermore, by alloying 7.2 and 4.6 nm Co nanoparticles to form AB(2) (CoCo2) binary supracrystals, a collective magnetic behavior of these two particle types, due to the dipolar interaction, is evidenced by observing a single peak in the zero-field-cooled magnetization curve. Compared with the CoAg2 binary supracrystals, a spin orientation effect in sublattice that reduces the dipolar interactions in the supracrystals is uncovered in CoCo2 binary supracrystals.