Theoretical Investigation of Nanoparticle Relaxation for Magnetic Hyperthermia Applications: Effects of Size, Anisotropy, and Interparticle Distance Distributions

Theoretical models on the relaxation of magnetic nanoparticles are essential for designing optimized systems for biomedical applications like Magnetic Hyperthermia. However, existing models often assume uniform, ideal magnetic nanoparticles in noninteracting systems or simple, equidistant magnetic nanoparticle assemblies, which do not accurately reflect experimental complexities. Parameter distributions are expected to modify the AC hysteresis loop of the system and, consequently, the power absorption from the magnetic field, as it is proportional to its enclosed area. However, studies on this impact remain limited. In this article, we investigate the magnetic relaxation in Magnetic Hyperthermia conditions of noninteracting magnetic nanoparticle systems with typical experimental size and anisotropy-constant distributions for two easy-axis configurations: oriented parallel to the applied field and randomly oriented. Additionally, we analyze magnetic nanoparticles arranged into chains with a dispersion in interparticle distances. Our findings reveal that the irreversibility of hysteresis loops increases with the coefficient of variation of all distributions considered when easy-axes are aligned with the external magnetic field, but this effect is diminished by random orientation. Furthermore, we correlate internal structures formed by the dispersion of interparticle distances in chains with said increase of irreversibility. Coercivity is modified by size distributions, while susceptibility is modified by anisotropy constant distributions. As irreversibility, coercivity and susceptibility impact on the hysteresis loop area, studying their change with realistic parameter distributions is critical to improve and understand Magnetic Hyperthermia performance, filling the gap between theory and experiment.