Magnetic Relaxation in Two Dimensional Assembly of Dipolar Interacting Nanoparticles

TL;DR

This study uses kinetic Monte Carlo simulations to explore how dipolar interactions, aspect ratio, and temperature influence magnetic relaxation in two-dimensional nanoparticle arrays, revealing complex behaviors like antiferromagnetic coupling and relaxation time variations.

Contribution

It provides new insights into the effects of dipolar interactions and aspect ratio on magnetic relaxation dynamics in 2D nanoparticle assemblies, using extensive simulations.

Findings

Dipolar interactions induce antiferromagnetic coupling in square arrangements.

Relaxation time decreases with increasing dipolar strength in certain configurations.

Thermal fluctuations significantly affect relaxation properties.

Abstract

Using the two-level approximation of the energy barrier, we perform extensive kinetic Monte Carlo simulations to probe the relaxation characteristics in a two-dimensional ($L^{}_x\times L^{}_y$) array of magnetic nanoparticle as a function of dipolar interaction strength $h^{}_d$, aspect ratio $A^{}_r=L^{}_y/L^{}_x$, and temperature $T$. In the case of weak dipolar interaction ($h^{}_d\approx0$) and substantial temperature, the magnetic relaxation follows the N\'eel Brown model as expected. Interestingly, the dipolar interaction of enough strength is found to induce antiferromagnetic coupling in the square arrangement of MNPs ($A^{}_r=1.0$), resulting in the fastening of magnetic relaxation with $h^{}_d$. There is also a rapid increase in relaxation even with $A^{}_r<100$ above a particular dipolar interaction strength $h^{\star}_d$, which gets enhanced with $A^{}_r$. Remarkably, there is a slowing down of magnetic relaxation with $h^{}_d$ for the highly anisotropic system such as linear chain of MNPs. It is because the dipolar interaction induces ferromagnetic interaction in such a case. The thermal fluctuations also affect the relaxation properties drastically. In the case of weak dipolar limit, magnetization relaxes rapidly with $T$ because of enhancement in thermal fluctuations. The effect of dipolar interaction and aspect ratio on the magnetic relaxation is also clearly indicated in the variation of N\'eel relaxation time $\tau^{}_N$. In the presence of strong dipolar interaction ($h^{}_d>0.3$) and $A^{}_r=1.0$, $\tau^{}_N$ decreases with $h^{}_d$ for a given temperature. On the other hand, there is an increase in $\tau^{}_N$ with $h^{}_d$ for huge $A^{}_r$ $(>100)$. We believe that the concepts presented in this work are beneficial for the efficient use of self-assembled MNPs array in data storage and other related applications.