Inorganics, Vol. 12, Pages 329: Local Energy Minima and Density of Energy Barriers in Dense Clusters of Magnetic Nanoparticles

Inorganics, Vol. 12, Pages 329: Local Energy Minima and Density of Energy Barriers in Dense Clusters of Magnetic Nanoparticles

Inorganics doi: 10.3390/inorganics12120329

Authors: Natalia L. Gorn Elena K. Semenova Dmitry Berkov

In this paper, we focus on the properties of local energy minima and energy barriers in immobilized dense clusters of magnetic nanoparticles. Understanding of these features is highly interesting both for the fundamental physics of disordered systems with long-range interparticle interaction and for numerous applications of modern ferrofluids consisting of such clusters. In particular, it is needed to predict the ac-susceptibility of these systems and their magnetization relaxation after a sudden change in the external field, because both processes occur via magnetization jumps over energy barriers that separate the energy minima. Due to the exponential increase in the corresponding jump time with barrier height (tsw∼exp(ΔE/kT)), direct Langevin dynamics simulations of this process are not feasible. For this reason, we have developed efficient numerical methods both for finding as many energy minima as possible and for the reliable evaluation of energy barriers between them. Our results for the distribution of overlaps between the local energy minima imply that there is no spin-glass state in such clusters even when they consist of particles with a small anisotropy. Further, we show that the distributions of energy barrier heights are qualitatively different for clusters of particles with small, intermediate, and large anisotropies, which has important consequences for the magnetization dynamics of these systems.