Hysteresis in two dimensional arrays of magnetic nanoparticles

Anand, Manish

doi:10.1016/j.jmmm.2021.168461

Cited by 19 publications

(8 citation statements)

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“…In an assembly of MNPs, the single particle energy barrier gets drastically modified because of the dipolar interaction. The corresponding interaction energy E dipolar can be evaluated using the following expression [54,55]…”

Section: Theoretical Frameworkmentioning

confidence: 99%

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

Anand¹

2021

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We study relaxation behaviour in the two-dimensional assembly of magnetic nanoparticles (MNPs) with aligned anisotropy axes and positional defects. The anisotropy axes orientation and disorder strength is changed by varying α and ∆, respectively. The magnetization decay does not depend on the aspect ratio A r of the system and ∆ for small dipolar interaction strength h d = 0.2. Remarkably, the magnetization decays rapidly for considerable h d with negligible ∆ and A r = 1.0. The dipolar interaction of enough strength promotes antiferromagnetic coupling in square ensembles of MNPs. There is a prolonged magnetization decay for large ∆ because of enhancement in ferromagnetic coupling. Notably, magnetization relaxes slowly for α < α even with moderate h d and a significant A r . Interestingly, the slowing down of the magnetic relaxation shifts to a lower α with h d = 1.0. The magnetization ceases to relax for α ≤ 60 • and h d ≤ 0.6 due to large shape anisotropy with A r = 400.0. Remarkably, a majority of the magnetic moment reverses its direction by 180 • for α > 60 • and large h d , resulting in the negative magnetization. The effective Néel relaxation time τ N also depends strongly on these parameters. τ N depends weakly on α and ∆ for h d ≤ 0.2, irrespective of A r . On the other hand, τ N decreases with α for significant h d provided α is greater than 45 • because of antiferromagnetic coupling dominance. In a highly anisotropic system, there is an enhancement in τ N with α (≤ 30 • ) even with moderate h d . While for α > 30 • , τ N decreases with α. These observations are useful in novel materials, spintronics based applications, etc.

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Section: Theoretical Frameworkmentioning

confidence: 99%

“…Therefore, we do not reiterate it here to avoid duplication. We have also described the implemented kMC algoirthm in our recent works [54,60]. We now describe the protocol used for investigation of the magnetic relaxation.…”

Section: Theoretical Frameworkmentioning

confidence: 99%

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

Anand¹

2021

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“…The kMC algorithm implemented in the present work is the same as that of Tan et al and Anand et al [28,42]. It is also described in greater detail in our recent work [45,46]. Therefore, we do not rewrite it here to avoid repetition.…”

Section: Theoretical Frameworkmentioning

confidence: 99%

Disorder-Induced Complex Magnetization Dynamics in Planar Ensembles of Nanoparticles

Anand¹

2021

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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.

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“…The dipolar interaction is found to induce antiferromagnetic coupling at low temperatures. In recent work, we investigated the hysteresis properties in ordered arrays of MNPs as a function of dipolar interaction strength, aspect ratio, external magnetic field directions using the kinetic Monte Carlo (kMC) algorithm [23].…”

Section: Introductionmentioning

confidence: 99%

Controlling Dipolar Interaction Effect in Two-Dimensional Magnetic Nanostructures

Anand

2021

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We investigate the dependence of magnetic properties on the out-of-plane disorder strength ∆, dipolar interaction strength h d in two-dimensional (l x × l y ) ensembles of nanoparticles using numerical simulations. Such positional defects are redundantly observed in experiments. The superparamagnetic character is dominant with negligible and weak interaction strength h d , irrespective of ∆ and aspect ratio of the system A r = l y /l x . The double-loop hysteresis curve, characteristics of antiferromagnetic coupling dominance, emerges with large h d and ∆(%) ≤ 5 in the square-like nanoparticles' assays. Remarkably, the dipolar interaction of sufficient strength drives the magnetic order from antiferromagnetic to ferromagnetic with large ∆ and A r ≤ 4.0, resulting in an enhancement in the hysteresis loop area. On the other hand, the ferromagnetic coupling gets increased with h d in systems with huge A r . Consequently, the hysteresis loop is enormous, even with moderate h d . The variation of the coercive field µ o H c , remanence M r , and amount of heat released E H (due to the hysteresis) with these parameters also suggests the transformation of nature dipolar interaction. They are significant even with large h d and smaller A r , indicating the antiferromagnetic coupling dominance. Interestingly, there is an enhancement in these with ∆ and large h d due to ferromagnetic interaction. Notably, they are very significant even with moderate h d in the highly anisotropic system and external field along the long axis of the sample. These results could help the experimentalist in explaining the unusual hysteresis characteristics observed in such systems and should also be beneficial in diverse applications such as data storage, magnetic hyperthermia, etc.

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Hysteresis in two dimensional arrays of magnetic nanoparticles

Cited by 19 publications

References 50 publications

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

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

Disorder-Induced Complex Magnetization Dynamics in Planar Ensembles of Nanoparticles

Controlling Dipolar Interaction Effect in Two-Dimensional Magnetic Nanostructures

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