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

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 α and Δ, respectively. The magnetisation decay does not depend on the aspect ratio A r of the system and Δ for small dipolar interaction strength h_d = 0.2. Remarkably, the magnetisation decays rapidly for considerable h d with negligible Δ 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 because of the enhancement in ferromagnetic coupling by the positional disorder. Notably, magnetisation relaxes slowly for α < α even with moderate h_d and significant A_r. Interestingly, the slowing down of the magnetic relaxation shifts to a lower α 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 (≈ 0), the magnetisation ceases to relax for α ≤ 60° and h_d ≤ 0.6 due to large shape anisotropy. Remarkably, the magnetisation decays rapidly for α > 60, independent of Δ. In such cases, a majority of the magnetic moment reverses its direction by 180°, resulting in the negative averaged magnetisation. The effective Néel relaxation time τN also depends strongly on these parameters. τN depends weakly on α and for h_d ≤ 0.2, irrespective of A r . On the other hand, τN decreases with α for significant h d provided α is greater than 45° because of the dominance of the antiferromagnetic coupling. In a highly anisotropic system, there is an enhancement in τN with α (≤30°) even with moderate h_d , while for α > 30°, τN decreases with α. The lowering of τN occurs relatively at smaller α with a large h_d . These observations are useful in diverse applications, such as novel materials, sensors, spintronics-based applications, etc. (author)