A Mössbauer spectral study of some iron nitride-based nanocomposites prepared by ball milling

Powder mixtures of (Fe_yN)_x and (Al₂O ₃)_1-x or (SiO₂)_{1- x} with x=0.2 and 0.6 and y=3.8 have been ball milled for 4, 8, 16, 32, 64, and 128 h and their magnetic properties and Mössbauer spectra have been measured. The 5 and 295 K saturation magnetizations decrease with increasing milling time as a result of decreasing particle sizes. Fits with the Langevin function of the field dependence of the magnetization yield particle sizes that are slightly smaller than those determined from a Scherrer analysis of the broadening of the X-ray diffraction peaks. The Mössbauer spectra have been fit with a distribution of hyperfine fields between 0 and 40 T and the peaks in the distribution assigned to the different iron nitride phases present in the nanocomposites. This analysis of the Mössbauer spectra indicates that the γ′-Fe₄N phase present in Fe_yN is transformed into ε-Fe_3+xN and α-iron by the milling process. The weighted average field decreases and the weighted average isomer shift increases with milling time, as expected for the simultaneous effects of size reduction, the above phase transformation, and mixing with the oxides. A peak in the hyperfine field distribution at 4 T indicates the presence of small superparamagnetic particles, a presence which is also confirmed by the temperature dependence of the Mössbauer spectra and the failure of the magnetization to saturate even at applied fields of 5.5 T.