Magnetisation reversal in two-dimensional ensemble of nanoparticles with positional defects

Abstract

We investigate the relaxation behaviour in the two-dimensional assembly of magnetic nanoparticles (MNPs) with aligned anisotropy axes and positional defects. The orientation of the anisotropy axes and the strength of disorder are changed by varying $\alpha$ and $\Delta$, respectively. The magnetisation 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 magnetisation decays rapidly for considerable $h_d$ with negligible $\Delta$ and $A_r=1.0$ because the dipolar interaction of enough strength promotes antiferromagnetic coupling in square ensembles of MNPs. On the other hand, there is a prolonged magnetisation decay for large $\Delta$ because of the enhancement in ferromagnetic coupling by the positional disorder. Notably, magnetisation relaxes slowly for $\alpha <\alpha ^\star$ even with moderate $h_d$ and significant $A_r$. Interestingly, the slowing down of the magnetic relaxation shifts to a lower $\alpha ^{\star }$ when $h_d=1.0$. Unusual magnetic relaxation behaviour is observed in the highly anisotropic system $A_r=400.0$. Even in a perfectly ordered system ($\Delta \approx 0$), the magnetisation ceases to relax for $\alpha \le 60^\circ$ and $h_d\le 0.6$ due to large shape anisotropy. Remarkably, the magnetisation decays rapidly for $\alpha >60^\circ$, independent of $\Delta$. In such cases, a majority of the magnetic moment reverses its direction by $180^\circ$, resulting in the negative averaged magnetisation. The effective Néel relaxation time $\tau _\textrm{N}$ also depends strongly on these parameters. $\tau _\textrm{N}$ depends weakly on $\alpha$ and $\Delta$ for $h_d\le 0.2$, irrespective of $A_r$. On the other hand, $\tau _\textrm{N}$ decreases with $\alpha$ for significant $h_d$ provided $\alpha$ is greater than $45^\circ$ because of the dominance of the antiferromagnetic coupling. In a highly anisotropic system, there is an enhancement in $\tau _\textrm{N}$ with $\alpha$ (${\le }30^\circ$) even with moderate $h_d$, while for $\alpha >30^\circ$, $\tau _\textrm{N}$ decreases with $\alpha$. The lowering of $\tau _\textrm{N}$ occurs relatively at smaller $\alpha$ with a large $h_d$. These observations are useful in diverse applications, such as novel materials, sensors, spintronics-based applications, etc.