Nonmonotonic Evolution of the Blocking Temperature in Dispersions of Superparamagnetic Nanoparticles

TL;DR

This study uses Monte Carlo simulations to explore how dipolar interactions affect the blocking temperature in superparamagnetic nanoparticle dispersions, revealing a critical concentration that causes a nonmonotonic evolution of $T_B$.

Contribution

It identifies a critical concentration c* that separates two regimes of $T_B$ behavior, providing a unified explanation for conflicting experimental observations.

Findings

Below c*, $T_B$ remains constant, indicating non-interacting behavior.

Above c*, $T_B$ increases with concentration due to enhanced energy barriers.

Results align with classical models and experimental data, confirming the two-regime behavior.

Abstract

We use a Monte Carlo approach to simulate the influence of the dipolar interaction on assemblies of monodisperse superparamagnetic ${\gamma}-Fe_{2}O_{3}$ nanoparticles. We have identified a critical concentration c*, that marks the transition between two different regimes in the evolution of the blocking temperature ($T_{B}$) with interparticle interactions. At low concentrations (c < c*) magnetic particles behave as an ideal non-interacting system with a constant $T_{B}$. At concentrations c > c* the dipolar energy enhances the anisotropic energy barrier and $T_{B}$ increases with increasing c, so that a larger temperature is required to reach the superparamagnetic state. The fitting of our results with classical particle models and experiments supports the existence of two differentiated regimes. Our data could help to understand apparently contradictory results from the literature.