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

Abstract

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 plane rotating clockwise. The energy was modeled by the Stoner–Wohlfarth scheme for fixed or random orientations of the anisotropy, and is in contact with a thermal bath at a temperature T. Interparticle dipolar interactions were also considered. 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 y component (My) for the random 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 areas were estimated. The values of TB were close from the room temperature TR=300K 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, the blocked state is present for both My and Mz for all anisotropy orientations, and extends beyond the interval of amplitudes H0 estimated theoretically for the model without interactions and 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 larger range 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 particle produces an effective field – the sum of both dipolar and external field – that is asymmetric with respect to the zero field line at low temperatures, becomes half-wave symmetric at higher temperatures, so the the centered character of the loops is restored. In addition, the loop areas show a peak for all orientations of the anisotropy axes in an intermediate range of temperatures. This result can be associated with a dominance of the anisotropy induced by 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.