Controlling Dipolar Interaction Effect in Two-Dimensional Magnetic Nanostructures

TL;DR

This study uses numerical simulations to explore how dipolar interactions and positional disorder influence the magnetic properties of two-dimensional nanoparticle ensembles, revealing conditions that switch magnetic coupling from antiferromagnetic to ferromagnetic.

Contribution

It provides new insights into controlling magnetic interactions in 2D nanoparticle systems by tuning disorder and dipolar strength, aiding experimental understanding and applications.

Findings

Superparamagnetic behavior dominates at weak dipolar interactions.

Large dipolar strength can induce ferromagnetic order.

Hysteresis characteristics are significantly affected by disorder and interaction strength.

Abstract

We investigate the dependence of magnetic properties on the out-of-plane disorder strength $\Delta$, dipolar interaction strength $h^{}_d$ in two-dimensional ($l^{}_x\times l^{}_y$) ensembles of nanoparticles using numerical simulations. Such positional defects are redundantly observed in experiments. The superparamagnetic character is dominant with negligible and weak interaction strength $h^{}_d$, irrespective of $\Delta$ and aspect ratio of the system $A^{}_r=l^{}_y/l^{}_x$. The double-loop hysteresis curve, characteristics of antiferromagnetic coupling dominance, emerges with large $h^{}_d$ and $\Delta(\%)\leq5$ in the square-like nanoparticles' assays. Remarkably, the dipolar interaction of sufficient strength drives the magnetic order from antiferromagnetic to ferromagnetic with large $\Delta$ and $A^{}_r\leq4.0$, resulting in an enhancement in the hysteresis loop area. On the other hand, the ferromagnetic coupling gets increased with $h^{}_d$ in systems with huge $A^{}_r$. Consequently, the hysteresis loop is enormous, even with moderate $h^{}_d$. The variation of the coercive field $\mu^{}_oH^{}_c$, remanence $M^{}_r$, and amount of heat released $E^{}_H$ (due to the hysteresis) with these parameters also suggests the transformation of nature dipolar interaction. They are significant even with large $h^{}_d$ and smaller $A^{}_r$, indicating the antiferromagnetic coupling dominance. Interestingly, there is an enhancement in these with $\Delta$ and large $h^{}_d$ due to ferromagnetic interaction. Notably, they are very significant even with moderate $h^{}_d$ in the highly anisotropic system and external field along the long axis of the sample. These results could help the experimentalist in explaining the unusual hysteresis characteristics observed in such systems and should also be beneficial in diverse applications such as data storage, magnetic hyperthermia, etc.