The melt-spun Fe _83.3-84.3 Si ₄ B ₈ P _3-4 Cu _0.7 (at%) alloys have heterogeneous amorphous structures including a large amount of alpha-Fe clusters with less than 3 nm in size after rapid solidification, due to the unusual effect of the simultaneous addition of the small amounts of P and Cu. A homogeneous nanocrystalline structure composed of alpha-Fe grains with a size of about less than 20 nm can be realized by crystallizing the heteroamorphous alloys. The Fe _83.3-84.3 Si ₄ B ₈ P _3-4 Cu _0.7 nanocrystalline alloys show significantly high saturation magnetic flux density of 1.88-1.94 T almost comparable to the commercial silicon steel. The Fe _83.3-84.3 Si ₄ B ₈ P _3-4 Cu _0.7 alloys exhibit excellent magnetic softness; low coercivity of 7-10 A/m, relatively high effective permeability of 16 000-25 000 at 1 kHz due to the simultaneous realization of the uniform nanostructure composed by fine alpha-Fe grains and a small magnetostriction of 2-3 times 10 ^-6 . The nanocrystalline alloys also exhibit much smaller W than the silicon steel over the B _m range up to 1.7 T due to significantly higher B _s and excellent magnetic softness. Therefore, the Fe-Si-B-P-Cu alloys have a great advantage for engineering and industry, and, thus, should make a contribution to energy saving and conservation of earth's resources and environment.

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