Directly coupled direct current superconducting quantum interference device magnetometers based on ramp-edge Ag:YBa2Cu3O7-x/PrBa2Cu3O7-x/Ag:YBa2Cu3O7-x junctions

Directly coupled de superconducting quantum interference device (SQUID) magnetometers on LaAlO3 substrates were fabricated using ramp-edge superconductor/normal-metal/superconductor junctions, where Ag-doped YBa2Cu3O7-x was used for the electrode and PrBa2Cu3O7-x for the normal-metal barrier. A Aux noise of 8 X 10(-6) Phi(0) Hz(-1/2) at 10 kHz measured with a de bias current was achieved at 75 K, which corresponded to a field sensitivity of 400 fTHz(-1/2) for a magnetometer with a Dick-up loop area of 8.5 mmX7.5 mm. Most significantly, the noise floor increased at lower frequencies with a frequency dependence slightly less than 1/f. The field noise of the SQUID magnetometers increased by only 25% after cycling the devices from zero field to 500 mG. In a static earth's magnetic field background, the field noise of the SQUID magnetometers increased by less than a factor of 2. (C) 1998 American Institute of Physics. [S0003-6951(98)03 123-4].