Curie-Weiss behavior and the "interaction" temperature of magnetic nanoparticle ensembles: local structure strongly affects the magnetic behavior

TL;DR

This paper theoretically investigates how local structure and particle clustering influence the Curie-Weiss behavior and apparent interaction temperatures in magnetic nanoparticle ensembles, highlighting the importance of nanoscale arrangements.

Contribution

It demonstrates that local clustering and anisotropy alignments can produce apparent interaction temperatures even without true magnetic interactions.

Findings

Clustering affects the observed interaction temperature.

Both positive and negative interaction temperatures are possible.

Anisotropy axis alignment can mimic interaction effects.

Abstract

In this article, the Curie-Weiss type behavior and the appearance of an "interaction" or "ordering" temperature for a collection of magnetic nanoparticles is explored theoretically. We show that some systems where an interaction temperature is reported are too dilute for dipolar interactions to play a role unless at least some of the particles are clumped together. We then show using the most simple type of clumps (particle pairs) that positive and negative interaction temperatures are possible due to dipolar interactions. The clump orientation dramatically changes this result. Finally, we show that an apparent interaction temperature can be measured in magnetic nanoparticle systems that have no interactions between particles, due to some alignment of anisotropy easy axes. These results show that nanoscale physical structures affect the measured magnetic response of nanoparticles.