Unambiguous signature of exchange interactions between nanoparticles in a collective system

TL;DR

This paper introduces a theoretical method to detect exchange interactions in nanoparticle systems by analyzing magnetic susceptibility, validated through experiments on magnetic nanopowders.

Contribution

It develops a mean field theory to identify unambiguous signatures of exchange interactions via magnetic susceptibility measurements.

Findings

Discovered a fundamental inequality for magnetic susceptibility.

Violation of this inequality indicates exchange interactions.

Experimental data confirms the theoretical predictions.

Abstract

We report a simple, efficient manner to assess magnetic interactions, more specifically the exchange interaction, in systems as blocked magnetic nanoparticles. Specifically, we investigate what is the theoretical limit for the external magnetic susceptibility in a system described by the plain old Stoner-Wohlfarth model. We go beyond and introduce a general mean field theory for interacting systems, thus estimating how the magnetic susceptibility is affected due to the dipolar and exchange interactions inside the system. We disclose a fundamental inequality for the magnetic susceptibility and show its violation is an unambiguous signature of the existence of exchange interactions between nanoparticles in a collective system. To test the robustness of our theoretical achievements, we examine magnetization measurements and Henkel plots for blocked magnetic nanopowders. The agreement between experiment and theory provides evidence to confirm the validity of our findings.