Low-frequency noise of improved direct-coupled high-Tc superconducting quantum interference device magnetometers in ac and dc magnetic fields

We have developed direct-coupled high-T-c superconducting quantum interference device (SQUID) magnetometers based on 30 degrees bicrystal junctions with superior noise performance in dc and ac magnetic fields. These magnetometers are composed of a 100 pH SQUID loop with a linewidth of 4 mu m and a pickup coil consisting of 16 parallel, 50-mu m-wide loops. The magnetic field noise down to 1 Hz did not increase when the magnetometers were exposed to ac fields with peak-to-peak amplitudes of up to 54 mu T or cooled in static magnetic fields above that of the Earth. The best noise at 1 Hz of such a device cooled in 64 mu T was 65 fT Hz(1/2). The increased noise below 1 Hz when the devices were cooled in a static magnetic field can be quantitatively described by the measured temperature fluctuations assuming a temperature coefficient of the pickup loop area of about 1x10(-4)/K. (C) 1999 American Institute of Physics. [S0003-6951(99)01644-7].