To provide a core that uses an Fe-base amorphous alloy thin band, thus concurrently satisfying a low-noise requirement through high BS.

The core uses an Fe-base amorphous alloy thin band in which the saturation magnetic flux density BSis 1.60 T or more. Heat treatment is performed onto the core with the conditions that a maximum temperature is 250 to 370°C, a time duration is 30 minutes or more, an applied magnetic field is 200 A/m or more, and the ratio of a magnetic flux density B80and the Fe-base amorphous alloy thin band's saturation magnetic flux density BSwhen the external magnetic field in the core is 80 A/m, i.e., B80/BSis 0.90 or more. This heat treatment step terminates the magnetic flux application when a stress relaxation degree of the Fe-base amorphous alloy thin band becomes 90% or more.

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