Dipolar interaction and sample shape effects on the hysteresis properties of 2d array of magnetic nanoparticles

Abstract

We study the ground-state and magnetic hysteresis properties of 2d 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 $\vec {H} = H_{0}{\hat{e}}^{}_H$ on the ground state and the magnetic hysteresis in an array of MNPs. To study the effect of the 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 an in-plane ordering of magnetic moments, (b) the ground-state morphology has randomly oriented magnetic moments that are robust regarding system sizes and $A^{}_r$ for weakly interacting MNPs ($\Theta <1$), (c) micromagnetic simulations suggest that the dipolar interaction decreases the coercive field $H^{}_c$, (d) the remanence magnetisation $M^{}_r$ is found to be strongly dependent not only on the strength of dipolar interaction but also on the shape of the sample and (e) due to the anisotropic nature of dipolar interaction, a strong shape anisotropy effect is observed when the field is applied along the long axis of the sample. In such a case, the dipolar interaction induces an effective ferromagnetic coupling when the aspect ratio is enormous. These results are of vital importance in high-density recording systems, magneto-impedance sensors, etc.