Low-frequency noise in high-transition-temperature superconducting multilayer magnetometers in ambient magnetic fields

Each magnetometer consisted of a dc superconducting quantum interference de vice (SQUID), with the YBa2Cu3O7-x washer patterned into 4 mum lines, coupl ed to a multiturn, multilayer flux transformer containing flux dams. The no ise at 1 Hz did not increase when bare SQUIDs or magnetometers were cooled and operated in fields up to values well above the magnetic field of the ea rth. When the magnetic field was changed, the noise in a bare SQUID was con stant up to a threshold field of 12 muT. The addition of a flux transformer containing flux dams increased the magnetic field sensitivity by a factor of 43, while reducing the threshold field only moderately, to 5 muT. (C) 20 01 American Institute of Physics.