AC susceptibility as a tool to probe the dipolar interaction in magnetic nanoparticles

TL;DR

This paper demonstrates how AC susceptibility measurements can be used to analyze the effects of dipolar interactions and clustering in magnetic nanoparticles, providing a model to interpret experimental data.

Contribution

The authors develop a comprehensive model incorporating both long-range dipolar interactions and particle clustering, enabling detailed analysis of susceptibility data.

Findings

Dipolar interactions significantly influence magnetic nanoparticle behavior.

The model successfully fits experimental data from different nanoparticle samples.

A simple technique assesses the impact of dipolar interactions based on susceptibility peak heights.

Abstract

The dipolar interaction is known to substantially affect the properties of magnetic nanoparticles. This is particularly important when the particles are kept in a fluid suspension or packed inside nano-carriers. In addition to its usual long-range nature, in these cases the dipolar interaction may also induce the formation of clusters of particles, thereby strongly modifying their magnetic anisotropies. In this paper we show how AC susceptibility may be used to obtain important information regarding the influence of the dipolar interaction in a sample. We develop a model which includes both aspects of the dipolar interaction and may be fitted directly to the susceptibility data. The usual long-range nature of the interaction is implemented using a mean-field solution, whereas the particle-particle aggregation is modeled using a distribution of anisotropy constants. The model is then applied to two samples studied at different concentrations. One consists of spherical magnetite nanoparticles dispersed in oil and the other of cubic magnetite nanoparticles embedded on PLGA nanospheres. We also introduce a simple technique to access the importance of the dipolar interaction in a given sample, based on the height of the AC susceptibility peaks for different driving frequencies. Our results help illustrate the important effect that the dipolar interaction has in most nanoparticle samples.