Structural evolution and magnetic properties of nanocrystalline 50 Permalloy powders prepared by mechanical alloying

• Nanocrystalline 50 Permalloy powders were prepared by mechanical alloying.
• In addition to the nonuniform lattice strain, a uniform lattice strain was observed.
• A good combination of Ms (∼150 emu/g) and Hc (∼0.13 Oe) was obtained after 60 h of milling.
• Curie temperature was estimated to be ∼600 °C.

FeNi-based alloys commonly called Permalloys are interesting in their applications as soft magnetic materials with low coercivity and high permeability. In this study, nanocrystalline Fe–50 wt.% Ni alloy powders were prepared using a planetary ball mill at different milling times. The evolution of the microstructure and magnetic properties during the milling process were studied by the X-ray diffraction technique, the scanning electron microscope, the transmission electron microscope and the vibrating sample magnetometer. The results indicate that in the course of ball milling the Fe and Ni mixture, nanocrystalline FCC γ-(Fe, Ni) phase with the average crystallite size of 15 nm, particle size of 39 μm, nonuniform lattice strain of 0.45% and lattice parameter of 0.36062 nm formed after 24 h milling time. Although the crystallite size of the as-synthesized Permalloy powder is smaller than the magnetic exchange length, a low coercivity as expected from Herzer’s random anisotropy model is not observed. Among the different reasons, residual stress, γ-(Fe, Ni) phase formation and contamination are suggested as possible causes, which affect both coercivity and saturation magnetization.