Dipolar Interaction and Sample Shape Effects on the Hysteresis Properties of 2d Array of Magnetic Nanoparticles

TL;DR

This study uses micromagnetic simulations to explore how dipolar interactions and sample shape influence the magnetic hysteresis and ground states of 2D arrays of magnetic nanoparticles, revealing shape-dependent magnetic properties.

Contribution

It provides new insights into the effects of dipolar strength and sample shape on magnetic behavior in nanoparticle arrays, highlighting the role of shape anisotropy and interaction strength.

Findings

Strong dipolar interactions induce in-plane magnetic order.

Weak interactions lead to randomly oriented magnetic moments.

Dipolar interactions reduce coercive field and shape affects remanence.

Abstract

We study the ground state and magnetic hysteresis properties of 2$d$ arrays ($L^{}_x\times L^{}_y$) of dipolar interacting magnetic nanoparticles (MNPs) by performing micromagnetic simulations. Our primary interest is to understand the effect of sample shape, $\Theta$- the ratio of the dipolar strength to the anisotropy strength, and the direction of the applied field $\vec{H} = H_{o}\hat{e}^{}_H$ on the ground state and the magnetic hysteresis in an array of MNPs. To study the effect of shape of the sample, we have varied the aspect ratio $A^{}_r=L^{}_y/L^{}_x$ which in turn, is found to induce shape anisotropy in the system. Our main observations are: (a) When the dipolar interaction is strong $(\Theta>1)$, the ground state morphology has in-plane ordering of magnetic moments. (b) The ground state morphology has randomly oriented magnetic moments which is robust with respect to system sizes and $A^{}_r$ for weakly interacting MNPs ($\Theta<1$). (c) Micromagnetic simulations suggests that the dipolar interaction decreases the coercive field $H^{}_c$. (d) The remanence magnetization $M^{}_r$ is found to be strongly dependent not only on the strength of dipolar interaction but also on the shape of the sample. (e) Due to anisotropic nature of dipolar interaction, a strong effect of shape anisotropy is observed when the field is applied along longer axis of the sample. The dipolar interaction in such a case induces an effective ferromagnetic coupling when the aspect ratio is very large. These results are of vital importance in high-density recording systems, magneto-impedance sensors, etc.