Due to their maintained nanoscale features, the presented robust nanocomposite materials offer an exciting opportunity to develop functional materials. Cubic iron oxides (γ-Fe 2 O 3 or Fe 3 O 4 ) are ferrimagnetic materials with a high saturation magnetization, typically between 60 and 80 emu g -1 . The superparamagnetic nature of the starting sub-20 nm building blocks used in this study allows to achieve a large magnetic response that is easily switchable without the need to surpass a large coercive magnetic field, as in the case of bulk ferrimagnets. Retaining such a nano-characteristic property in bulk materials has been so far challenging, especially when it comes to guaranteeing the final material's mechanical integrity. The magnetic hystereses of the composite materials were obtained via vibratingsample magnetometry (VSM). As can be seen in Fig. (black line), the pure supraparticles can achieve a high ultimate magnetization, M S , of 42.9 emu g -1 at an applied field of 10 kOe and a low magnetic coercivity, H C , of 3.0 Oe. Embedding the SP in the epoxy matrix allowed to preserve the superparamagnetic behavior (Fig. (green line)) and resulted only in a small decrease of the maximal magnetization, due to the very high filling content of inorganic material consistent with the dilution effects expected in a ferrimagnetic-diamagnetic composite system. Crosslinking the NC-NC interface via heat-treatment again increased the M S to 50.6 emu g -1 due to a reduction of the organic content in the first hierarchical level, as discussed before (also see Fig. ). Interestingly, the H C also increased to 39 Oe which is typically attributed to magnetic domains above 20 nm. However, SAXS confirmed that the nanostructure was preserved after heat-treatment (see Fig. ) and wide-angle X-ray scattering only shows a slight narrowing of the diffraction peaks due to a small increase in grain size from 7 nm to 13 nm according to the Scherrer equation. In fact, previous studies on heat-treated, bulk supercrystalline materials at the used conditions also showed a slight increase in grain size, even though no changes in the magnetic coercivity were observed, even when the annealing temperature was increased to cause an onset of necking due to sintering. We attribute the higher sensitivity to the annealing step to the changed microstructure of the supraparticles. In contrast to the previously used bulk, polycrystalline material, 5 the µm-sized SPs have a higher surface to volume ratio and can therefore be prone to structural changes (e.g. oxidation or sintering of NCs) on the supercrystalline surface due to the applied heat. Furthermore, when the outer SP surface was functionalized with amine ligands (Fig. (purple line)), the effect of increased H C of 71 Oe becomes more predominant accompanied with a slight reduction in M S to 47.9 emu g -1 . This effect again indicates structural changes occurring for NCs located at the outer supraparticle surfaces, which seem to be further promoted by the ligand exchange and could support the above mentioned hypothesis of partial decomposition of the amine motifs during the annealing step. This phenomenon highlights the arising challenge of tailoring interfaces at multiple hierarchy levels. Further studies should be carried out to gain a better understanding on the ongoing processes and to improve interface design strategies; e.g. by reducing the crosslinking temperature to 150 °C as demonstrated recently by our group. Nevertheless, the obtained coercivities are yet noticeably lower than earlier attempts to obtain bulk, nanograined iron oxide materials,