Magnetization Reversal in Two-dimensional Ensemble of Nanoparticles with Positional Defects

TL;DR

This study investigates how positional defects and dipolar interactions influence magnetization reversal and relaxation in two-dimensional nanoparticle ensembles, revealing conditions for slow relaxation, negative magnetization, and the effects of anisotropy.

Contribution

It provides a detailed analysis of the effects of anisotropy, disorder, and dipolar interactions on magnetization dynamics in 2D nanoparticle assemblies, highlighting new insights into relaxation behavior.

Findings

Magnetization decay is independent of aspect ratio and disorder strength at low dipolar interactions.

Strong dipolar interactions promote antiferromagnetic coupling and rapid magnetization decay.

Large shape anisotropy can halt magnetization relaxation and induce negative magnetization.

Abstract

We study relaxation behaviour in the two-dimensional assembly of magnetic nanoparticles (MNPs) with aligned anisotropy axes and positional defects. The anisotropy axes orientation and disorder strength is changed by varying $\alpha$ and $\Delta$, respectively. The magnetization decay does not depend on the aspect ratio $A^{}_r$ of the system and $\Delta$ for small dipolar interaction strength $h^{}_d=0.2$. Remarkably, the magnetization decays rapidly for considerable $h^{}_d$ with negligible $\Delta$ and $A_r=1.0$. The dipolar interaction of enough strength promotes antiferromagnetic coupling in square ensembles of MNPs. There is a prolonged magnetization decay for large $\Delta$ because of enhancement in ferromagnetic coupling. Notably, magnetization relaxes slowly for $\alpha<\alpha^\star$ even with moderate $h^{}_d$ and a significant $A^{}_r$. Interestingly, the slowing down of the magnetic relaxation shifts to a lower $\alpha^{\star}$ with $h^{}_d=1.0$. The magnetization ceases to relax for $\alpha\leq60^\circ$ and $h^{}_d\leq0.6$ due to large shape anisotropy with $A^{}_r=400.0$. Remarkably, a majority of the magnetic moment reverses its direction by $180^\circ$ for $\alpha>60^\circ$ and large $h^{}_d$, resulting in the negative magnetization. The effective N\'eel relaxation time $\tau^{}_N$ also depends strongly on these parameters. $\tau^{}_N$ depends weakly on $\alpha$ and $\Delta$ for $h^{}_d\leq0.2$, irrespective of $A^{}_r$. On the other hand, $\tau^{}_N$ decreases with $\alpha$ for significant $h^{}_d$ provided $\alpha$ is greater than $45^\circ$ because of antiferromagnetic coupling dominance. In a highly anisotropic system, there is an enhancement in $\tau^{}_N$ with $\alpha$ ($\leq30^\circ$) even with moderate $h^{}_d$. While for $\alpha>30^\circ$, $\tau^{}_N$ decreases with $\alpha$. These observations are useful in novel materials, spintronics based applications, etc.