Structure and magnetic properties of magnesium ferrite fine powders

Fine powders of magnesium ferrite, MgFe₂O₄, were produced through the sol-gel supercritical drying method, with two portions then being calcined at 773 K and 1073 K. The powder structural and magnetic properties were determined from transmission electron microscope micrographs, x-ray diffraction, Mössbauer effect spectroscopy and magnetometry measurements. The powder structure matched the MgFe₂O₄ spinel phase, with small amounts of α-Fe₂O₃ being observed in heated samples. As-produced powders were superparamagnetic at room temperature, with single magnetic domain particle behavior being observed at low temperatures, and for the 1073 K heated sample. The particle size distribution for the as-produced powder was evaluated separately from the micrographs, by fitting the magnetization data to a weighted Langevin function, and by fitting Mössbauer spectra taken at temperatures from 25 K to 298 K. Very similar particle size distributions were found from all three methods. The average particle diameter was 11 nm for the as-produced powder, and increased for heated samples. The saturation magnetization and magnetocrystalline anisotropy energy density values were both consistent with bulk values, in contrast to the large differences between particle and bulk values described for other fine particle systems.