Universal behavior of dense clusters of magnetic nanoparticles

A detailed numerical simulation of quasistatic hysteresis loops of dense clusters of interacting magnetic nanoparticles is carried out. Both clusters of magnetically soft and magnetically hard nanoparticles are considered. The clusters are characterized by an average particle diameter D, the cluster radius R-c, the particle saturation magnetization M-s, and the uniaxial anisotropy constant K. The number of particles in the cluster varies between N-p = 30 - 120. The particle centers are randomly distributed within the cluster, their easy anisotropy axes being randomly oriented. It is shown that a dilute assembly of identical random clusters of magnetic nanoparticles can be characterized by two dimensionless parameters: 1) the relative strength of magnetodipole interaction, K/M-s(2), and the average particle concentration within the cluster, eta = VNp/V-c. Here V is the nanoparticle volume, and V-c is the volume of the cluster, respectively. In the strong interaction limit, M-s eta/H-a >> 1, where H-a = 2K/M-s is the anisotropy field, the ultimate hysteresis loops of dilute assemblies of clusters have been constructed. In the variables (M/M-s, H/M-s) these hysteresis loops depend only on the particle volume fraction.. In the weak interaction limit, M-s eta/H-a << 1, the assembly hysteresis loops in the variables (M/M-s, H/H-a) are close to the standard Stoner-Wohlfarth hysteresis loop. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).