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

Each magnetometer consisted of a dc superconducting quantum interference device (SQUID), with the YBa2Cu3O7−x washer patterned into 4 μm lines, coupled to a multiturn, multilayer flux transformer containing flux dams. The noise 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 earth. When the magnetic field was changed, the noise in a bare SQUID was constant up to a threshold field of 12 μT. 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 μT.