Font Size:
RESPONSE OF A MAGNETIC NANOPARTICLES SYSTEM UNDER AN EXTERNAL ROTATING MAGNETIC FIELD”
Building: Cero Infinito
Room: Posters hall
Date: 2024-12-12 02:00 PM – 04:00 PM
Last modified: 2024-11-19
Abstract
The magnetic response of a magnetic nanoparticles (MNP) system to a rotating external field (RMF) is studied via Monte Carlo simulations. The field of amplitude Ho and frequency w, was applied in the y-z plane rotating clockwise. The energy was modeled by the Stoner-Wolfharth (SW) 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 noninteracting 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 fixed case. Furthermore, the loop areas were estimated, and increased with w 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 =300 °K for all components, and the areas decreased with T but they are practically not zero at Tb.
For the model with dipolar interactions a new scenario emerges. In the low temperature regime, the blocked state is present for both My and Mz in all anisotropies, and extends beyond the interval of amplitudes Ho 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 SW model, 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, and becomes half-wave symmetric at higher 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 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 SW model have larger areas at low temperatures that vanish near the Tr, unlike the areas of the model with interactions due to the extension of the blocked state.
In the noninteracting 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 fixed case. Furthermore, the loop areas were estimated, and increased with w 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 =300 °K for all components, and the areas decreased with T but they are practically not zero at Tb.
For the model with dipolar interactions a new scenario emerges. In the low temperature regime, the blocked state is present for both My and Mz in all anisotropies, and extends beyond the interval of amplitudes Ho 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 SW model, 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, and becomes half-wave symmetric at higher 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 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 SW model have larger areas at low temperatures that vanish near the Tr, unlike the areas of the model with interactions due to the extension of the blocked state.