Directly coupled direct current superconducting quantum interference device magnetometers based on ramp-edge Ag:YBa₂Cu₃O_7-x/PrBa₂Cu ₃O_7-x/Ag:YBa₂Cu₃O_7-x junctions

Directly coupled dc superconducting quantum interference device (SQUID) magnetometers on LaAlO₃ substrates were fabricated using ramp-edge superconductor/normal-metal/superconductor junctions, where Ag-doped YBa₂Cu₃O_7-x was used for the electrode and PrBa₂Cu₃O_7-x for the normal-metal barrier. A flux noise of 8×10^-6₀Hz^-1/2 at 10 kHz measured with a dc bias current was achieved at 75 K, which corresponded to a field sensitivity of 400fTHz^-1/2 for a magnetometer with a pick-up loop area of 8.5mm×7.5mm. 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.