HTS SQUID magnetometer with SQUID vector reference for operation in unshielded environment

Using a HTS rf SQUID vector reference, we demonstrated magnetically unshielded operation of an HTS rf SQUID magnetometer. The reference consists of three magnetometer SQUIDS oriented in x, y and z directions. The sensing SQUID magnetometer is z-oriented and positioned at (baseline) distance of 8 cm from the z-reference. A balance (common mode rejection) better than 4000 was achieved by electronic adjustment of the subtraction coefficients of the reference rf SQUID channels in the homogeneous field of large Helmholtz coils. With 3.5 mm diameter washer rf SQUIDs, the magnetic field sensitivity of the magnetometer was 220 fT//spl radic/Hz at frequencies greater than 100 Hz in unshielded environment. A frequent task in nondestructive evaluation of materials is the determination of the magnitude and size of ferrous inclusions in non-magnetic metal alloys. As a preliminary test, we used the magnetometer for the detection of ferrous particles in unshielded environment. A magnetized 10 mg particle was identified from a distance of 80 mm by a scan of its remanent field. Magnetocardiograms (MCG) measurements are also performed.