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.
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Anand, M. Magnetisation reversal in two-dimensional ensemble of nanoparticles with positional defects. Pramana - J Phys 97, 186 (2023). https://doi.org/10.1007/s12043-023-02669-z
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DOI: https://doi.org/10.1007/s12043-023-02669-z