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 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 hd 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 fora < 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 approximate to 0), the magnetisation ceases to relax for alpha <= 60 degrees and h(d) <= 0.6 due to large shape anisotropy. Remarkably, the magnetisation decays rapidly for alpha > 60 degrees, independent of Delta. In such cases, a majority of the magnetic moment reverses its direction by 180 degrees, resulting in the negative averaged magnetisation. The effective Neel relaxation time tau(N) also depends strongly on these parameters. tau(N) depends weakly on a and Delta for h(d) <= 0.2, irrespective of A(r). On the other hand, tau(N) decreases with alpha for significant h(d) provided alpha is greater than 45 degrees because of the dominance of the antiferromagnetic coupling. In a highly anisotropic system, there is an enhancement in tau(N) with alpha (<= 30 degrees) even with moderate h(d), while for alpha > 30 degrees, tau(N) decreases with alpha. The lowering of tN 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.

Automated summary

This study investigates the relaxation behavior of two-dimensional assemblies of magnetic nanoparticles with aligned anisotropy axes and positional defects. The study finds that the magnetization decay rate is affected by parameters such as alpha, Delta, and h(d). Strong dipolar interactions result in rapid magnetization decay due to antiferromagnetic coupling, while positional disorder leads to prolonged magnetization decay. The effective Neel relaxation time also depends on these parameters, with stronger antiferromagnetic coupling leading to shorter relaxation times. These findings have important implications for various applications such as novel materials and spintronics-based devices.