HTc dc SQUID magnetometers

High sensitivity HTc dc-SQUID magnetometers, of bicrystal junction type, have been designed, fabricated and characterized. The magnetometers are characterized by a direct-coupling planar design and their parameters and geometrical dimensions have been optimized in order to achieve the lowest possible noise level. The IV characteristics are RSJ-like and show no excess current. The measured SQUID normalized inductances and magnetic field modulation periods are in good agreement with the expected values. The magnetic field noise spectral densities, measured with a flux locked loop electronic setup, exhibit typical white noise levels of about 150 fT/Hz 1/2 at frequencies above 2KHz. At lower frequencies a 1/f noise is observed. By employing a bias-reversal electronic readout scheme, the 1/f corner frequency can be reduced to values less than 100 Hz.