Non-Uniform Hysteresis in Small Clusters of Magnetic Nanoparticles

TL;DR

This study uses simulations to analyze how dipolar interactions influence local and average hysteresis in small clusters of magnetic nanoparticles, revealing size-dependent behaviors and implications for biomedical applications.

Contribution

It provides new insights into the role of dipolar interactions on hysteresis and organization in nanoparticle clusters, using first-principle calculations and kinetic Monte Carlo simulations.

Findings

Local hysteresis varies with nanoparticle position in small clusters.

Dipolar interactions increase hysteresis loop area and coercivity.

Hysteresis saturates for clusters with 20 or more particles.

Abstract

Using first-principle calculations and kinetic Monte Carlo simulation, we study the local and averaged hysteresis in tiny clusters of k magnetic nanoparticles (MNPs) or k-mers. We also analyze the variation of local dipolar field acting on the constituent nanoparticles as a function of the external magnetic field. The dipolar interaction is found to promote chain-like arrangement in such a cluster. Irrespective of cluster size, the local hysteresis response depends strongly on the corresponding dipolar field acted on a nanoparticle. In a small k-mer, there is a wide variation in local hysteresis as a function of nanoparticle position. On the other hand, the local hysteresis is more uniform for larger k-mer, except for MNPs at the boundary. In the case of superparamagnetic nanoparticle and weak dipolar interaction, the local hysteresis loop area Ai is minimal and depends weakly on the k-mer size. While for ferromagnetic counterpart, Ai is considerably large even for weakly interacting MNPs. The value of Ai is found to be directly proportional to the dipolar field acting on the nanoparticle. The dipolar interaction and k-mer size also enhances the coercivity and remanence. There is always an increase in Ai with clutser size and dipolar interaction strength. Similarly, the averaged hysteresis loop area A also depends strongly on the k-mer size, particle size and dipolar interaction strength. A and Ai always increase with k-mer size and dipolar interaction strength. Interestingly, the value of A saturates for k>=20 and considerable dipolar interaction irrespective of particle size. We believe that the present work would help understand the intricate role of dipolar interaction on hysteresis and organizational structure of MNPs and their usage in drug delivery and hyperthermia applications.