| Sumario: | In this work, we studied structural and magnetic properties of 18 nm sized Zn-substituted magnetite, 28 nm sized unsubstituted and 17 nm sized (Mn, Zn)-substituted iron oxide nanoparticles, synthesized by thermal decomposition method. Their features were examined by analyzing the x-ray diffraction data, 57Fe Mössbauer spectra and magnetization measurements by SQUID interferometer. The microstructure was inspected comparing the different size and strain broadening models incorporated into Fullprof software. In terms of crystallinity and size dispersion, applied synthesis protocol shows superiority over decomposition of iron oleate and the co-precipitation synthesis route. The saturation magnetization at T = 5 K was found to be within the MS = 91.2–98.6 A m2 kg−1 range, while at 300 K MS of pure and Zn-substituted Fe3O4 nanoparticles is 83.6 and 86.2 A m2 kg−1, respectively. Effective magnetic anisotropy constant Keff, estimated under slow measurements by SQUID, is below 20 kJ m−3 in all three samples. Some preliminary measurements of the magnetic hyperthermia performance, expressed via specific absorption rate value showed that the best heating performances were displayed by 18 nm sized oleic acid-coated Zn0.13Fe2.87O4 cubo-octahedrons with SAR ≅ 425 W/gFe at H0 = 20 kA m−1 and f = 228 kHz.
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