Monte Carlo simulation of a magnetic nanoparticles system under an external rotating magnetic field

SARACCO, GUSTAVO P.; BAB, MARISA A.

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS

ELSEVIER SCIENCE BV

Año: 2023 vol. 583 p. 1710141 - 17101412

0304-8853

The magnetic response of an identical magnetic nanoparticles (MNP) system to a rotatingexternal field (RMF) is studied via Monte Carlo simulations. The field of amplitude H0 andfrequency ω, was applied in the y − z plane rotating clockwise. The energy was modeled bythe Stoner-Wolfharth scheme for fixed or random orientations of the anisotropy, and is incontact with a thermal bath at a temperature T. Interparticle dipolar interactions were alsoconsidered. In the non interacting system and for low temperature, hysteresis is observed in the z magnetization component Mz for both orientations of the anisotropy axis and only in the ycomponent (My ) for the fixed case. Furthermore, the loop areas were estimated, and increased with ω for all orientations and (My , Mz ) components. At higher temperatures the superparamagnetic state is observed, so both the blocking temperatures TB and loop areaswere estimated. The values of TB were close from the room temperature TR = 300 ◦K forall components, and the areas decreased with T but they are practically not zero at TB.When dipolar interactions are included a new scenario is revealed. In the low temperatureregime, the blocked state is present for both My and Mz in all anisotropies, and extends beyond the interval of amplitudes H0 estimated theoretically for the model without interactionsand fixed anisotropy. The loops are displaced with respect to the origin of the magnetization-external field plane. When the temperature is raised, the blocked state extends for a largerrange than the model without interactions, and the loop displacement decreases with T.These behaviors could be explained by observing that the average dipolar field per particleproduces an effective field –the sum of both dipolar and external field– that is asymmetricwith respect to the zero field line at low temperatures, and becomes half-wave symmetric athigher temperatures. This behavior restores the centered character of the loops. In addition,the loop areas show a peak for all orientations of the anisotropy axes in an intermediate rangeof temperatures. This result can be associated with a dominance of the anisotropy inducedby the dipolar field. Finally, by comparing the areas of the loops of the models with and without interactions, it The magnetic response of an identical magnetic nanoparticles (MNP) system to a rotatingexternal field (RMF) is studied via Monte Carlo simulations. The field of amplitude H0 andfrequency ω, was applied in the y − z plane rotating clockwise. The energy was modeled bythe Stoner-Wolfharth scheme for fixed or random orientations of the anisotropy, and is incontact with a thermal bath at a temperature T. Interparticle dipolar interactions were alsoconsidered.In the non interacting system and for low temperature, hysteresis is observed in the z mag-netization component Mz for both orientations of the anisotropy axis and only in the ycomponent (My ) for the fixed case. Furthermore, the loop areas were estimated, and in-creased with ω for all orientations and (My , Mz ) components. At higher temperatures thesuperparamagnetic state is observed, so both the blocking temperatures TB and loop areaswere estimated. The values of TB were close from the room temperature TR = 300 ◦K forall components, and the areas decreased with T but they are practically not zero at TB .When dipolar interactions are included a new scenario is revealed. In the low temperatureregime, the blocked state is present for both My and Mz in all anisotropies, and extends be-yond the interval of amplitudes H0 estimated theoretically for the model without interactionsand fixed anisotropy. The loops are displaced with respect to the origin of the magnetization-external field plane. When the temperature is raised, the blocked state extends for a largerrange than the model without interactions, and the loop displacement decreases with T.These behaviors could be explained by observing that the average dipolar field per particleproduces an effective field –the sum of both dipolar and external field– that is asymmetricwith respect to the zero field line at low temperatures, and becomes half-wave symmetric athigher temperatures. This behavior restores the centered character of the loops. In addition,the loop areas show a peak for all orientations of the anisotropy axes in an intermediate rangeof temperatures. This result can be associated with a dominance of the anisotropy inducedby the dipolar field. Finally, by comparing the areas of the loops of the models with and without interactions, itThe magnetic response of an identical magnetic nanoparticles (MNP) system to a rotatingexternal field (RMF) is studied via Monte Carlo simulations. The field of amplitude H0 andfrequency ω, was applied in the y − z plane rotating clockwise. The energy was modeled bythe Stoner-Wolfharth scheme for fixed or random orientations of the anisotropy, and is incontact with a thermal bath at a temperature T. Interparticle dipolar interactions were alsoconsidered.In the non interacting system and for low temperature, hysteresis is observed in the z mag-netization component Mz for both orientations of the anisotropy axis and only in the ycomponent (My ) for the fixed case. Furthermore, the loop areas were estimated, and in-creased with ω for all orientations and (My , Mz ) components. At higher temperatures thesuperparamagnetic state is observed, so both the blocking temperatures TB and loop areaswere estimated. The values of TB were close from the room temperature TR = 300 ◦K forall components, and the areas decreased with T but they are practically not zero at TB .When dipolar interactions are included a new scenario is revealed. In the low temperatureregime, the blocked state is present for both My and Mz in all anisotropies, and extends be-yond the interval of amplitudes H0 estimated theoretically for the model without interactionsand fixed anisotropy. The loops are displaced with respect to the origin of the magnetization-external field plane. When the temperature is raised, the blocked state extends for a largerrange than the model without interactions, and the loop displacement decreases with T.These behaviors could be explained by observing that the average dipolar field per particleproduces an effective field –the sum of both dipolar and external field– that is asymmetricwith respect to the zero field line at low temperatures, and becomes half-wave symmetric athigher temperatures. This behavior restores the centered character of the loops. In addition,the loop areas show a peak for all orientations of the anisotropy axes in an intermediate rangeof temperatures. This result can be associated with a dominance of the anisotropy inducedby the dipolar field. Finally, by comparing the areas of the loops of the models with and without interactions, it was found that the non interacting model have larger areas at low temperatures that vanish near the room temperature, unlike the areas of the model with interactions due to the extension of the blocked state.