Directly coupled YBCO dc SQUID magnetometers

YBa2Cu3O7-x magnetometers have been made on 10 mm x 10 mm MgO substrates by directly coupling the magnetometer pick-up loop to a de SQUID with narrow strip lines. The de SQUIDs were made with YBa2Cu3O7-x step-edge Josephson j unctions. The layout of the magnetometer pick-up loop was chosen as a compr omise between maximizing the loop effective area and minimizing the loop in ductance. The SQUID was designed to have L-S similar to 100 pH in order to obtain beta(L) = 2I(0)L(S)/Phi(0) approximate to 1 with the single-junction critical current I-0 similar to 10 mu A. We have made magnetometers with w hite noise levels down to 55 fT Hz(-1/2) and a 1/f knee at 1 Hz (ac biased) . Noise measurements were made on a field-cooled magnetometer. The noise me asured at 1 Hz when cooled in 'zero field' was 175 fT Hz(-1/2). When cooled in magnetic fields of B = 50 mu T and B = 100 mu T we measured the noise a t 1 Hz to be 430 fT Hz(-1/2) and 1.3 pT Hz(-1/2), respectively.