A comparative study of the structural and magnetic properties of γ-Fe_1-xNi_x alloys and their nitrides γ′-(Fe_1-xNi_x)₄N

X-ray diffraction and Mössbauer spectroscopy are performed to investigate γ-Fe_1-xNi_x alloys and their nitrides γ′-(Fe_1-xNi_x)₄N with corresponding composition. X-ray diffraction shows that both alloys and nitrides have face-centered cubic crystal structure with approximately 18 to 20% expansion in volume of the nitrides. Mössbauer spectra at room temperature show drastic changes before and after nitrogenation. The changes are also rather different for the samples with lower nickel concentrations and those with the higher ones. For x = 0.25 and 0.30, the alloys mainly exhibit paramagnetic singlets while the nitrides exhibit complex ferromagnetic sextets. For x = 0.75 and 0.78, however, the alloys show ferromagnetic sextets while the nitrides show paramagnetic doublets. The reason for this is carefully discussed in terms of the lattice parameter dependence of Curie temperatures, which is caused by the introduction of interstitial nitrogen atoms into the Fe-Ni alloy structure.