Synthesis of nano-sized Fe–Ni powder by chemical process for magnetic applications

Synthesis of nano-sized Fe–Ni permalloy of the composition 20 wt% Fe and 80 wt% Ni took place by electroless chemical reduction method in alkaline tartarate bath using hypophosphite as a reducing agent. The powder was cold compacted at 600 MPa and then sintered at 1050 °C. Metallographic investigations were performed by optical microscope and SEM with EDAX analysis. Hot-stage XRD was performed for the investigated materials to follow the phase transformations in the material. Physical, magnetic and electrical properties were studied for the prepared powder and its related sintered compacts. FeNi powder prepared from the experiments has a 200 nm particle size with 2.4 wt% phosphorus content. The prepared powder has amorphous structure with a low saturation induction (B_s) but by raising the temperature to 500 °C, the FeNi₃ intermetallic appears first and then the cubic FeNi solid solution is formed at 1050 °C, which has the highest saturation induction value.After cold compaction and sintering, the electrical conductivity and the saturation induction increased by increasing the time of sintering but the coercive force decreased and the material becomes softer after sintering. Measurements of the magnetic permeability indicate that the optimum applying field for the investigated sintered material is between 40 and 100 Oe which gives the highest range of the magnetic permeability. From the magneto-resistance measurements, it is shown that the sintered material has a positive magneto-resistance in the field direction but a negative one in the direction perpendicular to the current and the field.