Relaxation in Ordered Assembly of Magnetic Nanoparticles

TL;DR

This study uses computer simulations to analyze how aspect ratio, dipolar interactions, and anisotropy orientation affect the magnetic relaxation behavior of two-dimensional arrays of magnetic nanoparticles, revealing complex dependencies and potential applications.

Contribution

It provides new insights into the relaxation dynamics of magnetic nanoparticle arrays considering aspect ratio, interaction strength, and anisotropy orientation, highlighting phenomena like magnetization reversal and relaxation time variations.

Findings

Magnetization decays exponentially with low dipolar interaction strength.

Transition in relaxation behavior occurs at a specific dipolar interaction threshold.

High aspect ratio systems exhibit rapid magnetization decay and reversal phenomena.

Abstract

We study the relaxation characteristics in the two-dimensional ($l^{}_x \times l^{}_y$) array of magnetic nanoparticles (MNPs) as a function of aspect ratio $A^{}_r=l^{}_y/l^{}_x$, dipolar interaction strength $h^{}_d$ and anisotropy axis orientation using computer simulation. The anisotropy axes of all the MNPs are assumed to have the same direction, $\alpha$ being the orientational angle. Irrespective of $\alpha$ and $A^{}_r$, the functional form of the magnetization-decay curve is perfectly exponentially decaying with $h^{}_d\leq0.2$. There exists a transition in relaxation behaviour at $h^{}_d\approx0.4$; magnetization relaxes slowly for $\alpha\leq45^\circ$; it relaxes rapildy with $\alpha>45^\circ$. Interestingly, it decays rapidly for $h^{}_d>0.6$, irrespective of $\alpha$. It is because the dipolar interaction promotes antiferromagnetic coupling in such cases. There is a strong effect of $\alpha$ on the magnetic relaxation in the highly anisotropic system ($A^{}_r\geq25$). Interesting physics unfolds in the case of a huge aspect ratio $A^{}_r=400$. There is a rapid decay of magnetization with $\alpha$, even for weakly interacting MNPs. Remarkably, magnetization does not relax even with a moderate value of $h^{}_d=0.4$ and $\alpha=0^\circ$ because of ferromagnetic coupling dominance. Surprisingly, there is a complete magnetization reversal from saturation (+1) to $-1$ state with $\alpha>60^\circ$. The dipolar field and anisotropy axis tend to get aligned antiparallel to each other in such a case. The effective N\'eel relaxation time $\tau^{}_N$ depends weakly on $\alpha$ for small $h^{}_d$ and $A^{}_r\leq25.0$. For large $A^{}_r$, there is a rapid fall in $\tau^{}_N$ as $\alpha$ is incremented from 0 to $90^\circ$. These results benefit applications in data and energy storages where such controlled magnetization alignment and desired structural anisotropy are desirable.