The temperature and the dc magnetic field dependence of the effective surface impedance Zs of epitaxial YBa2Cu3O7−x thin films on [001] tilt SrTiO3 bicrystals with tilt angles of 36.8° and 24° have been investigated at 87 GHz. The effects of the grain boundaries become increasingly important with decreasing characteristic voltage IcRn and increasing unit areal normal resistance RnA. The boundaries can consistently be described with a resistively shunted Josephson transmission line model taking into account the effect of finite film thickness. Thermal or magnetic loading of the junction leads to additional losses, whereas the effective microwave penetration depth exhibits a specific extremal behavior. From the magnetic response of Zs at 4.2 K, the junction lower critical fields as well as the values of IcRn and RnA can be deduced. The data agree well with dc transport measurements.

Our aim has been to further improve the magnetic field resolution, BN, of single-layer rf SQUID magnetometers operating in liquid nitrogen. Following the approaches recently introduced in dc SQUIDs, we tested designs with direct-coupled pickup coils having an area Ap=0.6 to 0.8 cm2, and a single-layer thin-film flip-chip flux transformer with Ap=16 cm2. In conditions of still suboptimal coupling between SQUID and the 150 MHz tank circuit, we attained BN≂90 fT/Hz1/2 above 3–4 Hz at Ap=0.8 cm2, and BN≂24 fT/Hz1/2 above 0.5 Hz when Ap=16 cm2. For rf SQUID with lumped-element tank circuit, we project the lower BN limit to be ≤50 fT/Hz1/2 at Ap≤1 cm2. This might be attainable through further coupling optimization, and increase of tank frequency to the highest possible value of ≥500 MHz.

The effects of microwave current and dc magnetic field on quality factor and resonant frequency at 3 GHz of planar resonators, which were differently coupled to hysteretic microwave SQUIDs, are discussed. The dissipative response is analyzed in terms of a single-junction SQUID, and its coupling to a resonant tank circuit. The temperature dependences and field periodicities of all relevant parameters are discussed, and conditions for optimized operation are deduced. The results indicate potential for further improvement of the performance of microwave SQUID magnetometers.

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