Size Dependence of Alternating Current Magnetically-Induced Heating Characteristics of Ferrimagnetic MgFe₂O₄ Nanoparticles in Powder and Fluidic States

The alternating current (AC) magnetically-induced heating characteristics of MgFe₂O₄ ferrimagnetic nanoparticles (FMNPs) are investigated in both powder and ferrofluidic states for efficient hyperthermia agent applications. To obtain MgFe₂O₄ FMNPs with different sizes and size distributions, a ball milling process (ball diameters: 3 mm and 5 mm) is carried out within the framework of a modified sol–gel method. The MgFe₂O₄ FMNPs formed with 3 mm-ball milling process possess the smallest particle size and narrowest size distribution. These FMNPs provide the lowest AC magnetically induced heating temperature (T _AC,mag, ΔT = 65 °C) in powder state. However, they generate the highest T _AC,mag(ΔT = 5.1 °C) and specific loss power (SLP, 525 W/g) in fluidic state at a low frequency (f _appl = 110 kHz) and a magnetic field (H _appl = 140 Oe). It is also observed that the hysteresis loss and relaxation loss are crucial parameters that dictate the heating mechanism of FMNPs in ferrofluids. These losses depend on the coating conditions and dipole interaction, which in turn are closely related to the particle's size and its distribution. In addition, it is determined that the weaker magnetic dipole interaction in the ferrofluid FMNPs enhance the relaxation loss and hysteresis loss, leading to an improvement of the AC heat generation characteristics.