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Manganese and zinc ferrite magnetic nanoparticles (MNPs) were successfully synthesized<br />using the polyol method in ethylene glycol and were found to have high saturation magnetization<br />values (90&#8722;95 emu/g at 4 K) when formed by ~30-nm crystallites assembled in an ~80-nm multicore<br />structure. Hyperthermia data revealed a sigmoidal dependence of the specific absorption rate (SAR)<br />on the alternating magnetic field (AMF) amplitude, with remarkable saturation SAR values in water<br />of ~1200 W/g_Fe+Mn and ~800 W/g_Fe+Zn for the Mn and Zn ferrites, respectively. The immobilization<br />of the MNPs in a solid matrix reduced the maximum SAR values by ~300 W/g_{Fe+Mn, Zn} for both<br />ferrites. The alignment of the MNPs in a uniform static magnetic field, before their immobilization<br />in a solid matrix, significantly increased their heating performance. Toxicity assays performed in<br />four cell lines revealed a lower toxicity for the Mn ferrites, while in the case of the Zn ferrites, only<br />~50% of cells were viable upon their incubation for 24 h with 0.2 mg/mL of MNPs. Cellular uptake<br />experiments revealed that both MNPs entered the cells in a time-dependent manner, as they were<br />found initially in endosomes and later in the cytosol. All of the studied cell lines were more sensitive<br />to the ZnFe₂O₄ MNPs.