Néel Relaxation of Magnetic Nanoparticle Clusters

Understanding thermal relaxation effects in magnetic nanoparticle (MNP) systems is key to several imaging techniques and clinical applications. Here, we consider the Néel relaxation of compact MNP clusters, using Langevin dynamics simulations to compute the relaxation time as a function of magnetostatic coupling strength. By also analyzing individual thermal reversals, we establish connections between the magnetic structure of a cluster and its Néel relaxation time. In particular, faster relaxation and more exotic behavior are observed for 3-D clusters with several nearly degenerate states, as the magnetization intermittently jumps to metastable states, which can facilitate reversal. Conversely, aggregates with many moments in a single flux-closed loop exhibit fewer metastable states and are efficiently blocked by strong dipole coupling.