The magnetic properties of Fe-Si-B-M (M: additives) alloys prepared by annealing amorphous alloys made by the single roller method over their crystallization temperature have been investigated for development of new Fe-based soft magnetic alloys. Excellent soft magnetic properties were obtained by adding the two elements Cu and Nb to Fe-Si-B alloys. It was found that these new alloys, called ‘‘FINEMET,’’ have an ultrafine grain structure composed of bcc Fe solid solution. They are suitable for many kinds of magnetic components such as saturable reactors, choke coils, and transformers, because they have superior soft magnetic properties and a high saturation flux density, and because different types of B-H hysteresis loops are obtained by magnetic field annealing.

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The Si content dependences of the magnetic properties of the Fe82.65Cu1.35SixB16−x alloys prepared by melt spinning were studied. The maximum Bs and minimum Hc were observed at around x=2 for the annealed Fe82.65Cu1.35SixB16−x alloys. The Bs and Hc of the annealed Fe82.65Cu1.35Si2B14 alloy were 1.84T and 6.5A∕m, respectively. The average grain size of the bcc Fe nanocrystal in the annealed alloys with x⩽2 was about 20nm, it increased with x at x>2. The annealing temperature range without the precipitation of o-Fe3B expanded with Si content.

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The magnetic properties and microstructures of newly developed Fe-Cu-B and Fe-Cu-Si-B alloys produced by meltspinning have been studied. The annealed alloys consist of nanoscale grains. The annealed alloys show a high-saturation magnetic flux density B s of more than 1.8 T and excellent soft magnetic properties such as a low core loss of about 0.3 W/kg at 50 Hz and 1.5 T (P 15=50 ). For the present alloys, the addition of Cu is effective for the formation of nanoscale grains and the improvement of the soft magnetic properties.

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