The Dependence of the Magnetothermal Properties of Zn_xMn_{1 – x}Fe₂O₄ Nanoparticles on the Magnetic Field Near Physiological Brezovich Limit

The skin effect imposes significant restrictions on the possible frequency and, consequently, the weight and size characteristics of resonant sources of electromagnetic (EM) field that are promising for use in the magnetic hyperthermia procedure. On the one hand, the heat release of magnetic nanoparticles (MNPs) increases linearly with an increase in the frequency of the EM field, which, on the other hand, simultaneously exacerbates the side effect of EM radiation on healthy tissues and ultimately leads to the need to solve a complex multi-parameter optimization problem of EM field parameters. This paper is devoted to a detailed study of the dependence of the thermal response of magnetic Zn_xMn_1–xFe₂O₄ nanoparticles on the frequency and amplitude of the EM field and to the determination of the optimal frequency range, from physical and from technical and physiological viewpoints, which can be most effectively used for magnetic hyperthermia. Nontoxic and biocompatible MNPs of ZnMn ferrites were obtained by chemical coprecipitation. The results of this study show that the MNP size and the Zn content significantly affect the Specific Absortpion Rate (SAR). MNPs of ZnMn ferrite with a zinc content of x = 0.15 and 0.2 have higher specific absorption rates (SARs) at low frequencies while MNPs with a high zinc content (x = 0.25 and 0.3) have significant SARs at higher frequencies. At the same time, the frequency dependence of the imaginary part of the magnetic susceptibility for MNPs of ZnMn ferrite found in this study demonstrates a maximum at 15 kHz. This indicates the significance of this frequency range, which is located significantly lower than that traditionally used in the practice of magnetic hyperthermia (100 kHz and higher), as well as the need to find the optimal combination of physical mechanisms of heat release in the range of 15–100 kHz with sufficient heat release and a minimum concentration of injected MNPs, which allow successful implementation of the magnetic hyperthermia procedure.