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• Medium viscosity influences high-frequency magnetic properties of nanobeads. • Nanobead's mechanical behaviors depend on their size and medium viscosity. • SAR measurements for biological applications should be done in a viscous medium. Magnetic nanoparticles placed in a high frequency alternating magnetic field release heat. This phenomenon can be used to heat and kill cancerous cells, the so-called magnetic hyperthermia treatment. Our study focuses on the influence of the medium viscosity on the heating power of magnetic nanobeads (MNBs) containing superparamagnetic nanoparticles. The specific absorption rate (SAR) of two types of MNBs differing by their size was determined by measuring the area of their high-frequency hysteresis loops. Three different behaviors are observed as viscosity rises: (i) SAR first drops to a minimum value for both MNB sizes; (ii) interestingly, and only for smaller MNBs, after going through a minimum, SAR increases again towards a steady value; (iii) for larger MNBs, SAR remains constant after reaching its minimum value. The physical origin of these different behaviors is interpreted, the key ingredient to explain them being the magnetic interactions, the possibility of the bead to form chains, and their possibility to physically rotate under the influence of the magnetic field. Finally these results lead to questioning the representativeness of SAR measurements in water for nanoparticles intended for biological applications, in which the medium viscosity is very different from the one of water. [ABSTRACT FROM AUTHOR]