Equilibrium magnetization and magnetization relaxation of multicore magnetic nanoparticles

Abstract

Multi-core magnetic nanoparticles show promising features for biomedical applications. Their magnetic properties, however, are not well-understood to date, so that several ad hoc assumptions are often needed to interpret experimental results. Here, we present a comprehensive computer simulation study on the effect of dipolar interactions and magnetic anisotropy on the equilibrium magnetization and magnetization relaxation dynamics of monodisperse multi-core magnetic nanoparticles in viscous solvents. We include thermal fluctuations of the internal N\'eel relaxation via the stochastic Landau-Lifshitz-Gilbert equation coupled to rotational Brownian motion of the cluster. We find that the effective magnetic moment of the cluster is reduced compared to the non--interacting case due to frustrated dipole-dipole interactions. Furthermore, the magnetization relaxation is found to proceed in a two--step fashion with a fast initial decay being followed by a long-time relaxation. For moderate dipolar interaction strengths, the latter can be approximated quite well by an exponential decay with rate given by the sum of the relaxation rates in the immobilized state and the Brownian rotation. These findings can be helpful for a better interpretation of experimental data obtained from magnetization relaxation measurements