This paper reviews our results on the development of a new type of soft magnetic material with high saturation magnetic flux density (B_s) combined with excellent soft magnetic properties. A mostly single bcc structure composed of bcc grains with about 10–20 nm in size surrounded by a small amount of intergranular amorphous layers was obtained by crystallization of amorphous phases prepared by melt-spinning and sputtering technique in Fe-rich regions of Fe–M–B (M=Zr, Nb, Hf) ternary systems. The typical nanocrystalline bcc Fe₉₀Zr₇B₃, Fe₈₉Hf₇B₄ and Fe₈₄Nb₇B₉ alloys subjected to the optimum annealing exhibit high B_s above 1.5 T as well as high effective permeability (μ_e) at 1 kHz above 20000.
The high B_s for the Fe–M–B alloys is resulting from the high Fe concentrations owing to high glass-forming ability of M(Zr, Hf, Nb) and B. The origin of the good soft magnetic properties for the alloys are listed as follows. (1) The apparent anisotropy is decreased by the combined effects of the formation of the nanoscale bcc structure and the achievement of rather strong magnetic coupling between the bcc grains through the intergranular ferromagnetic amorphous phase. (2) The small saturation magnetostriction (λ_s) results from the nonequilibrium bcc phase. The solute-rich interglanular amorphous phase with high Curie temperature (T_c) and high thermal stability has an important role in the achievement of the good soft magnetic properties through the formation of the nanoscale bcc structure and the attainment of the rather strong magnetic coupling between the bcc grains.
The soft magnetic properties of the nanocrystalline Fe–M–B alloys were improved through the decrease in the bcc grain size and the increase in T_c of the intergranular amorphous phase by optimizing heating rate in the crystallization process and adding small amounts of elements. For example, the improved Fe₈₄Zr_3.5Nb_3.5B₈Cu₁ alloy shows the high μ_e of 100000 combined with the high B_s of 1.53 T. This excellent μ_e is comparable to those of nanocrystalline Fe_73.5Si_13.5B₉Nb₃Cu₁ and the zero-magnetostrictive Co based amorphous alloys, and the high B_s is comparable to those of the Fe based amorphous alloys with good soft magnetic properties. The Fe–M–B based alloys also have very low core losses, the sufficient thermal stability and the low stress-sensibility of the soft magnetic properties. Therefore, the nanocrystalline Fe–M–B alloys are expected as practical magnetic materials for magnetic transformers, inductors, and other devices and parts.