Figure 2.

Spin relaxation and interaction energies in in‐field self‐assembled iron oxide and cobalt ferrite superstructures of different aspect ratios (A = {2/5, 5/5, 8/5}). a) The macrospins (red arrows) in the iron oxide systems relax and interact as a dynamic system of fluctuating spins, leading to a significant increase in cohesive energy per NC relative to the nonmagnetic system (i.e., van der Waals + steric energies) for all the considered aspect ratios. Overall, these superparamagnetic superstructures are mechanically stabilized by attractive magnetic interactions. b) The blocked macrospins remain vertically aligned in the initial state in the cobalt ferrite systems owing to the strong MA favoring the 〈100〉‐directions, giving rise to MA‐induced “SFM.” The cohesive energy per NC increases for A > 1 as a result of dominating attractive magnetic dipolar interactions (shape anisotropy parallel to magnetization), and decreases for A < 1 owing to dominating repulsive magnetic dipolar interactions (shape anisotropy perpendicular to magnetization). Dark and yellow cubic boxes in the snapshots are the NC core and oleic acid surfactant layer, respectively.