Hysteresis loops of exchange-coupled magnetic nanoparticle clusters

Experimental evidence indicates that significant exchange coupling may exist between magnetic nanoparticles (MNPs) in dense MNP aggregates such as nanoflower clusters. Here, we examine the role of inter-particle exchange interactions in determining the magnetic properties of MNP clusters, in particular their athermal hysteresis in a low-frequency alternating field. We consider mechanically fixed close-packed clusters where each particle is modeled as a single macrospin coupled to the others by both dipolar interactions and nearest-neighbor exchange. Upon simulating the quasi-static hysteresis curves, we compute the loop area, remanent moment and coercive field, and we classify each curve by its shape. Computing curve types across parameter space reveals how their shape is determined by the interplay between exchange coupling, dipolar interactions, and uniaxial anisotropy. Strong exchange coupling produces fully saturated loops with coherent moment rotation. Moderate exchange and anisotropy result in magnetically soft clusters with high susceptibility. Finally, for complex clusters, weak to moderate exchange and strong anisotropy may produce highly irregular curves with several abrupt changes in magnetization. Our analysis demonstrates that exchange coupling between MNPs significantly increases the cluster energy product, thereby contributing to explain the exceptional heating power of nanoflowers.