Effective shielding for low-level magnetic fields

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Abstract
An effective method of shielding slowly varying low-level magnetic fields is presented. The key of this method is the use of soft magnetic materials with a highly rectangular B-H loop and the application of the shaking field to them. Metglas 2705M amorphous ribbon of about 22 μm in thickness was used in the experiments since it has almost vertical flanks in the dc B-H loop in as-prepared state and small coercivity Hc∼0.5 A/m. A shielding factor of the cylindrical case with a copper pipe inside (diameter : 60 mm; length : 100 mm) and with two crossed layers of the ribbon outside was measured against low-level disturbing fields of 10 Hz applied transversely to the case. Although the shielding factor was as small as 3.5 without shaking, it became higher than 150 under the shaking field of 1 kHz, 2.9 A/m(rms). This value is far better than that with Mumetal or 4-79 Mo-Permalloy. A shaking frequency as high as 1 kHz or higher makes it easy to suppress the leakage of the shaking field into the shielded space.

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