Integrated YBa2Cu3O7 magnetometer with flux transformer and multiloop pick-up coil

We report on a novel integrated magnetometer concept that combines inductance matching by a flux transformer with the low inductance of a multiloop pick-up coil. A 90 pH de superconducting quantum interference device together with a five turn flux transformer is coupled to a multiloop pick-up coil resulting in an effective magnetometer area of 1.4 mm(2) on a 10 x 10 mm(2) substrate. Using ramp-type Josephson junctions with PrBa2Cu3O7 barriers, a white noise level of root S-B(1 kHz) = 44 fT/root Hz is measured. The 1/f-flux noise of the integrated magnetometer is suppressed successfully by operation in a flux-locked loop mode with bias current modulation. At 77 K we find a flux density noise of root S-B(1 Hz) = 100 fT root Hz. (C) 1996 American Institute of Physics.