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° bicrystal junctions with superior noise performance in dc and ac magnetic fields. These magnetometers are composed of a 100 $p\mathrm{H}$ SQUID loop with a linewidth of 4 μm and a pickup coil consisting of 16 parallel, 50-μm-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 μT or cooled in static magnetic fields above that of the Earth. The best noise at 1 Hz of such a device cooled in 64 μT 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 ${\text{1×10}}^{\text{−4}}\mathrm{/K}.$