Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

TL;DR

This study explores the dipolar interactions and moment correlations in clusters of magnetic nanoparticles, revealing anticorrelations at the core level through experimental and simulation methods.

Contribution

It provides new insights into the directional magnetic correlations within nanoparticle clusters using polarized neutron scattering and Monte Carlo simulations.

Findings

Dipolar interactions induce anticorrelations at low temperatures.

Polarized neutron scattering reveals mixed positive and negative correlations.

Monte Carlo simulations support experimental observations.

Abstract

Here, we investigate the nature of the moment coupling between 10-nm DMSA-coated magnetic nanoparticles, in both colloidal dispersion and in powder form. The individual iron oxide cores were composed of > 95% maghemite and agglomerated to clusters. At room temperature the ensemble behaved as a superparamagnet according to M\"ossbauer and magnetization measurements, however, with clear signs of dipolar interactions at low temperatures. Analysis of temperature-dependent AC susceptibility data in the superparamagnetic regime indicates a tendency for dipolar coupled anticorrelations of the core moments within the clusters. To resolve the directional correlations between the particle moments we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross-section of the powder in low magnetic field at 300 K. We extract the underlying pair distance distribution function of the magnetization vector field by an indirect Fourier transform of the cross-section, and which suggests positive as well as negative correlations between nearest neighbor moments, with anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo simulations of such core-clusters.