Soft-magnetic properties of Cu-free, Fe-based nanocrystalline alloys

The objective of this paper is to review our recent results on the development of a new soft magnetic material with high saturation magnetization (B_s) in simple Fe-based alloy systems. A nanoscale bcc phase with an average grain size of 15 nm surrounded by a small amount of an amorphous phase was found to form by annealing amorphous Fe-Zr-B, Fe-Hf-B and Fe-Nb-B alloys. The high permeability above 10000 at 1 kHz combined with high Bs above 1.5 T was obtained for the bcc alloys. Magnetostriction (λ_s) decreases rapidly by the transition from amorphous to bcc phase and is of the order -1×10^-6 for the bcc alloys. The small λ_s as well as the small grain size is concluded to be the reason for the good soft magnetic properties. The bcc Fe₈₉Hf₇B₄ alloy also shows the low core loss of 0.066 W/kg at 1 T and 50 Hz, which is much smaller than those of amorphous Fe-Si-B alloys in practical uses as core materials in transformer. The newly developed Fe-based alloys are highly expected to be used as various kinds of soft magnetic materials.