Monte Carlo Simulation of a Magnetic Nanoparticles System under an external Rotating Magnetic Field

G. P. SARACCO; M. A. BAB

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2023 vol. 583 p. 1 - 12

0304-8853

The magnetic response of an identical magnetic nanoparticles (MNP) system to a rotating external field (RMF)is studied via Monte Carlo simulations. The field of amplitude 𝐻0 and frequency 𝜔, was applied in the 𝑦−𝑧 planerotating clockwise. The energy was modeled by the Stoner–Wohlfarth scheme for fixed or random orientationsof the anisotropy, and is in contact with a thermal bath at a temperature T. Interparticle dipolar interactionswere also considered.In the non-interacting system and for low temperature, hysteresis is observed in the 𝑧 magnetizationcomponent 𝑀𝑧 for both orientations of the anisotropy axis and only in the 𝑦 component (𝑀𝑦) for therandom 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 blockingtemperatures 𝑇𝐵 and loop areas were estimated. The values of 𝑇𝐵 were close from the room temperature𝑇𝑅 = 300 K for all 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 temperature regime, theblocked state is present for both 𝑀𝑦 and 𝑀𝑧 for all anisotropy orientations, and extends beyond the intervalof amplitudes 𝐻0 estimated theoretically for the model without interactions and fixed anisotropy. The loops aredisplaced with respect to the origin of the magnetization-external field plane. When the temperature is raised,the blocked state extends for a larger range than the model without interactions, and the loop displacementdecreases 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 asymmetric with respectto the zero field line at low temperatures, becomes half-wave symmetric at higher temperatures, so the thecentered character of the loops is restored . In addition, the loop areas show a peak for all orientations of theanisotropy axes in an intermediate range of temperatures. This result can be associated with a dominance ofthe anisotropy induced by the dipolar field.Finally, by comparing the areas of the loops of the models with and without interactions, it was foundthat 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