Influence of Ni addition on structure and magnetic properties of FeCo-based Finemet-type alloys

Structure and magnetic properties of melt-spun Ni x (Fe 0.5 Co 0.5 ) 73.5−x Si 13.5 B 9 Nb 3 Cu alloys ( x =5,10,15,20,25,30) annealed at different temperatures were investigated. Microstructure has been analyzed from XRD patterns. Magnetic properties were mainly studied by the evolution of initial permeability from room temperature to 740°C. All the 500°C-annealed samples gave rise to an ultrafine structure composed of a residual amorphous matrix and α - FeCoNi(Si) crystals (tens of nanometers in size). It was found that Ni content had a significant impact on the Curie temperature of amorphous phase T c am and initial permeability μ i . There was a general reduced tendency of T c am with Ni content increasing. For higher Ni content alloys ( x ≥ 25 ) annealed at 500°C, the μ i can be enhanced to the order of 10 4 , which is much higher than that of Ni-free FeCo-based Finemet-type alloys. The reason for the enhancement in μ i was systematically analyzed in terms of the effective anisotropy model.