Disorder-Induced Complex Magnetization Dynamics in Planar Ensembles of Nanoparticles

Abstract

The magnetic relaxation characteristics are investigated in the two-dimensional (l x ×l y ) assembly of nanoparticles as a function of out-of-plane positional disorder strength ∆(%) using numerical simulations. Such defects are redundantly observed in experimentally fabricated nanostructures, resulting in unusual magnetization dynamics. The magnetization decays exponentially for small and negligible dipolar interaction strength h d ≤ 0.2. In such a case, the magnetization relaxation does not depend on ∆ and aspect ratio A r = l y /l x , as expected. In square-like MNPs ensembles and perfectly ordered system (∆(%) = 0), the magnetization relaxes rapidly with an increase in h d . Consequently, the effective Néel relaxation time τ N decreases with h d . The dipolar interaction of sufficient strength promotes antiferromagnetic coupling in such a system, resulting in rapid magnetization decay. Remarkably, the out-of-plane disorder instigates the magnetic moment to interact ferromagnetically in the presence of large h d , even in the square-like assembly of MNPs.As a result, magnetization relaxation slows down, resulting in a monotonous increase of τ N with an increase in ∆ and h d in such cases. Notably, there is a prolonged magnetization decay in the highly anisotropic system with large h d . The dipolar interaction induces ferromagnetic coupling along the long axis of the system in such cases. Therefore, the magnetization ceases to relax as a function of time for large h d , irrespective of disorder strength ∆(%). The present work could provide a concrete theoretical basis to explain the unexpected relaxation behaviour observed in experiments.These results are also beneficial in digital data storage and spintronics based applications where such nanostructures are extensively used.