High-temperature superconducting YBCO dc SQUID gradiometers fabricated on STO bicrystal substrates

HTS do gradiometers have been fabricated with slots and flux dams in their SQUID washers. Using a single layer of YBCO, the gradient sensitivity is limited by the sensing magnetometer in the centre of the structure. The spatial response of such devices has been measured experimentally indicating that single layer devices, in terms of gradient sensitivity, have characteristics that deviate slightly from idealized first order gradiometers. The flux noise of these devices is discussed with particular emphasis on their unshielded low frequency noise properties. We also discuss the effect of different cooldown procedures on the flux noise.