Structural and magnetic study of mechanically alloyed Fe-Ni

The Fe_xNi_100-x (x = 50, 65 and 80) alloys were synthesized in a conventional horizontal low energy ball mill. The X-ray diffraction was used to identify and characterise various phases during the milling process of the Fe₈₀Ni₂₀ alloy exhibiting a bcc structure whereas for x = 65 and 50 the fcc structures are found. The steady state grain size is about 10 nm. Magnetisation measurements after various milling periods allow to monitor a rate at which Ni atoms dissolve in the iron lattice. The room temperature values of the effective magnetic moment raise with the increasing milling period. All the alloys studied exhibit the ferromagnetic ordering. The magnitude of the magnetic interactions is moderately suppressed at prolonged milling as revealed by the Curie temperatures reduced down to 950 K. Such variations are caused by the deviations in the interatomic arrangements of atoms especially in the intergrain regions. The Moessbauer spectroscopy confirmed the ferromagnetic ordering and was used to calculate the distribution of hyperfine magnetic fields. The mean hyperfine fields are 33.8 T for Fe₈₀Ni₂₀ and correspond to the one to two Ni atoms in nearest neighbourhood. In the remaining alloys, at most, five Ni atoms are located in a neighbourhood of the Fe atom.