Nowadays, magnetic nanoparticles may be arranged into highly ordered, crystal-like structures, so-called mesocrystals. Compared to their bulk counterpart, the characteristics of mesocrystals are not only determined by the specific material employed, but also by the shape of its nanoscale constituents, their ordering on the meso- and on the macroscale and the resulting mutual interactions between them. We use ferromagnetic resonance (FMR) experiments and Brillouin light scattering (BLS) microscopy in combination with micromagnetic simulations to investigate and characterize dipolar interactions between magnetic nanoparticles (MNPs) within such mesocrystals. The MNPs investigated in this work consist of iron oxide (magnetite - Fe3O4 ) and are coated with non-magnetic polymers, forming highly ordered hexagonal monolayer crystals as shown in fig. 1 a. The magnetic response of the regularly arranged hexagonal mesocrystals can be tuned in a controlled way by varying the thickness of the non-magnetic polymer coating of the MNPs and thus the lattice constant of the mesocrystal. The spectral features reveal that the dipolar coupling strength between the MNPs decreases as the spacing between MNPs increases, as shown in fig. 2 a. Structuring the monolayer by employing electron beam lithography, well-defined, circularly shaped MNP assemblies can be fabricated as shown in fig. 1 b. Performing BLS experiments a main signal accompanied by a satellite signal can be observed, showing distinct dependencies on the externally applied field. 2D-BLS-mapping of the circular assembly reveals the associated active areas within the assembly for the two resonances. The experimental findings of the FMR and BLS experiments are fully corroborated by micromagnetic simulations.
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(a) Self-assembled, hexagonally arranged MNP monolayer. (b) Using electron beam lithography, well-defined MNP assemblies can be fabricated from monolayers as shown in (a).

(a) In FMR experiments on MNP monolayers (see fig. 1 (a)) of different spacing between the MNPs, but constant MNP size. (b) Investigations of structured MNP assemblies as shown in fig. 1 (b) by BLS reveal two signals, both showing distinct dependencies on the externally applied field.