Double relaxation oscillation superconducting quantum interference devices with gradiometric layout

Double relaxation oscillation superconducting quantum interference devices (DROSs) with a gradiometric signal SQUID and either a reference SQUID or a reference junction will be presented in this article. The devices are user friendly, particularly those with a reference junction. Because of the large flux-to-voltage transfer of partial derivative V/partial derivative Phi = 0.7-1 mV/Phi(0), the devices can be operated in a flux locked loop based on direct voltage readout without loss of sensitivity. The typical white flux noise of the DROSs amounts to root S-Phi=5-6 mu Phi(0)/root Hz, which corresponds to an energy resolution epsilon =S-Phi/2L(sq)similar or equal to 200 h. Coupled to an external planar first-order gradiometer, a white magnetic field sensitivity of root S-B<2fT/root Hz was measured inside a magnetically shielded room. (C) 1997 American Institute of Physics.