Nanocrystalline Hf and Ta containing FeCo based alloys for high frequency applications

► Nanocrystalline alloys with nominal compositions (Fe81Co19)84M9B7, where M = Hf, HfTa, and Ta, were produced by heat treating amorphous ribbons. ► Ta additions reduce eddy current loss by increasing the resistivity, enabling higher frequency operation. ► A reduction in grain size is observed by X-ray diffraction for the Ta containing alloys, leading to better soft magnetic properties. FeCo based nanocrystalline materials have excellent soft magnetic properties even at high temperature, but are limited to low frequency applications due to their relatively low electrical resistivities, ρ e , resulting in high eddy current losses. Amorphous alloys of (Fe 81Co 19) 84M 9B 7 where M = (Hf, HfTa, Ta) were prepared by meltspinning and annealed for increasing times at their respective crystallization temperatures. The nanocrystalline alloys had coercivities less than 0.4 Oe and saturation inductions greater than 1 T. The electrical resistivities of the amorphous ribbons were all similar with values of ρ e ≃ 180 μ Ω cm. After annealing at the crystallization temperature, the M = Ta alloy had the largest ρ e of 140 ± 3 μΩ cm. The Ta alloy also had the best high frequency properties, with an initial permeability of 822 at 1 MHz.