Mössbauer studies of core-shell FeO/Fe$_{3}$O$_{4}$ nanoparticles

FeO/Fe$_{3}$O$_{4}$ nanoparticles were synthesized by thermal decomposition. Electron microscopy revealed that these nanoparticles were of the core-shell type and had a spherical shape with an average size of $\sim$20 nm. It was found that the obtained FeO/Fe$_{3}$O$_{4}$ nanoparticles had exchange coupling. The effect of anisotropy on the efficiency of heating (hyperthermic effect) of FeO/Fe$_{3}$O$_{4}$ nanoparticles by an external alternating magnetic field was examined. The specific absorption rate (SAR) of the studied nanoparticles was 135 W/g in the experiment with an external alternating magnetic field with a strength of 600 Oe and a frequency of 310 kHz. These data led to an important insight: the saturation magnetization is not the only factor governing the SAR, and the efficiency of heating of magnetic FeO/Fe$_{3}$O$_{4}$ nanoparticles may be increased by enhancing the effective anisotropy. Mössbauer spectroscopy of the phase composition of the synthesized nanoparticles clearly revealed the simultaneous presence of three phases: magnetite FeO/Fe$_{3}$O$_{4}$, maghemite $\gamma$-Fe$_{2}$O$_{3}$, and wustite FeO.