Microstructural evolution, phase transformations and magnetic characteristics of Fe@Fe3O4@NaCl core-shell nanocomposites when varying the EB PVD synthesis temperature

The manuscript is devoted to a comprehensive study of the physicochemical, structural and magnetic properties of Fe-NaCl nanocomposite systems with an iron content of ~30 wt.%, synthesized by the electron beam physical vapor deposition (EB PVD) method at substrate temperatures (Ts) of 50 °C and 410 °C. Using SEM, STEM/HAADF, EDS, XRD, HRTEM, TGA and Raman spectroscopy, it was proven that the thermodynamic conditions of deposition critically determine the phase composition of nanoinclusions. The transition from completely oxidized, highly hydrated agglomerates of magnetite (Fe3O4, coherent scattering region size 17 nm) at 50 °C to the formation of isolated spherical nanoparticles (15.5 - 36 nm) of the "metal core - oxide shell" type at 410 °C was recorded (Fe@Fe3O4). The appearance of a highly crystalline α-Fe core is accompanied by an increase in remanent magnetization from 0.46 to 2.3 emu/g and the saturation magnetization from 16.0 to 24.0 emu/g. The structural perfection and thermal stability of the samples obtained at 410 °C make them promising for use in magnetic devices and ideal objects for studying the spatial distribution of magnetic fields by holographic electron microscopy (Holo-TEM).