Relaxation in Ordered Assembly of Magnetic Nanoparticles

Abstract

We study the relaxation characteristics in the two-dimensional (l x × l y ) array of magnetic nanoparticles (MNPs) as a function of aspect ratio A r = l y /l x , dipolar interaction strength h d and anisotropy axis orientation using computer simulation. The anisotropy axes of all the MNPs are assumed to have the same direction, α being the orientational angle. Irrespective of α and A r , the functional form of the magnetization-decay curve is perfectly exponentially decaying with h d ≤ 0.2. There exists a transition in relaxation behaviour at h d ≈ 0.4; magnetization relaxes slowly for α ≤ 45 • ; it relaxes rapildy with α > 45 • . Interestingly, it decays rapidly for h d > 0.6, irrespective of α. It is because the dipolar interaction promotes antiferromagnetic coupling in such cases. There is a strong effect of α on the magnetic relaxation in the highly anisotropic system (A r ≥ 25). Interesting physics unfolds in the case of a huge aspect ratio A r = 400. There is a rapid decay of magnetization with α, even for weakly interacting MNPs. Remarkably, magnetization does not relax even with a moderate value of h d = 0.4 and α = 0 • because of ferromagnetic coupling dominance. Surprisingly, there is a complete magnetization reversal from saturation (+1) to −1 state with α > 60 • . The dipolar field and anisotropy axis tend to get aligned antiparallel to each other in such a case. The effective Néel relaxation time τ N depends weakly on α for small h d and A r ≤ 25.0. For large A r , there is a rapid fall in τ N as α is incremented from 0 to 90 • . These results benefit applications in data and energy storages where such controlled magnetization alignment and desired structural anisotropy are desirable.

Magnetization Reversal in Two-dimensional Ensemble of Nanoparticles with Positional Defects

“…In a recent work, we have shown the relaxation time can be modulated by just varying the angle between the anisotropy axis and chain axis of MNPs [44]. In a another study, we have shown the Néel relaxation time can be enhanced or decreased by varying the anisotropy axes in the prefectly ordered system [45].…”

Anisotropic Effect of Dipolar Interaction in Ordered Ensembles of Nanoparticles

“…Therefore, these ordered nanoparticles ensembles provide a rich theoretical framework to study the role of dipolar interaction and magnetic anisotropy on magnetic response. Much research efforts ranging from experimental to theoretical modelling have been devoted to unearthing the magnetic properties in such versatile systems [33][34][35][36][37][38][39]. Deng et al presented a detailed and very informative overview regarding forming an anisotropic assembly of MNPs [33].…”

“…In recent work, we studied the magnetic relaxation characteristics in the ordered assembly of nanoparticles using kinetic Monte Carlo simulation (kMC) with aligned anisotropy axes [39]. There is a fastening or slowing down of relaxation depending on the orientation of anisotropy axes with sufficient dipolar coupling strength.…”