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

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 $\alpha \le 60^{\circ }$ and $h_d^{}\le 0.6$ 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 Néel relaxation time ${\tau }_N^{}$ also depends strongly on these parameters. ${\tau }_N^{}$ depends weakly on $\alpha$ and $\Delta$ for $h_d^{}\le 0.2$, 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 $\alpha$ ($\le 30^{\circ }$) 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.

22 pages, 8 figures

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