A Mössbauer study of the thermally induced evolution of the crystalline and magnetic structure of magnesium ferrite nanoparticles MgFe ₂ O ₄, synthesized via the solution combustion with subsequent stepwise annealing in the temperature range of 400–800 ^ C, has been carried out. The as-prepared product is an X-ray amorphous substance with a stoichiometric composition, which makes it possible to exclude the influence of initial crystallinity on the characteristics of the thermal products. The combined use of Mössbauer spectroscopy on ^57 Fe nuclei (at 300 and 80 K) and X-ray diffraction made it possible to trace the formation of the MgFe ₂ O ₄ phase and magnetic state transitions in the nanoparticle ensembles from superparamagnetic to relaxation. It was established that up to a temperature of 600 ^ C, the particles retain superparamagnetic behavior associated with the small crystallite sizes (less than 16 nm). Upon annealing in the range of 600–700 ^ C, a sharp growth in crystallite size (up to ~24 nm) occurs, along with the emergence of magnetically ordered domains and relaxation-dominated Mössbauer spectra. With further increase in annealing temperature (up to 800 ^ C), stabilization of the crystalline structure and a pronounced reduction in spin relaxation rates are observed, as evidenced by narrowing hyperfine lines and size-related shifts in hyperfine parameters. The findings underscore the importance of thermal treatment as a tool for controlling the magnetic state and crystalline ordering of MgFe ₂ O ₄ nanoparticles.
Kiseleva et al. (Mon,) studied this question.