  Micromagnetic simulation results on dynamic hysteresis loops of clusters of
iron oxide nanoparticles (NPs) with internal structure composed of nanorods are
compared with the widely used macrospin approximation. Such calculations
allowing for nanorod-composed NPs is facilitated by a previously developed
coarse-graining method based on the renormalization group approach. With a
focus on applications to magnetic hyperthermia, we show that magnetostatic
interactions improve the heating performance of NPs in chains and triangles,
and reduce heating performance in fcc arrangements. Hysteresis loops of
triangular and fcc systems of complex NPs are not recovered within the
macrospin approximation, especially at smaller interparticle distances. For
triangular arrangements, the macrospin approximation predicts that
magnetostatic interactions reduce loop area, in contrast to the complex NP
case. An investigation of the local hysteresis loops of individual NPs and
macrospins in clusters reveals the impact of the geometry of their neighbours
on individual versus collective magnetic response, inhomogenous heating within
clusters, and further differences between simulating NPs with internal
structure and the use of the macrospin approximation. Capturing the internal
physical and magnetic structure of NPs is thus important for some applications.
