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The optimum design of high-sensitivity Superconducting Quantum InterferenceDevices (SQUIDs) and other devices based on thin HTS films requires accurateinductance modeling. This needs the London penetration depth $\lambda$ to bewell defined, not only at 77 K, but also for any operating temperature, giventhe increasingly widespread use of miniature low-noise single-stagecryocoolers. Temperature significantly affects all inductances in any activesuperconducting device and cooling below 77 K can greatly improve deviceperformance, however accurate data for the temperature dependence of inductanceand $\lambda(T)$ for HTS devices is largely missing in the literature. Wereport here inductance measurements on a set of 20 different thin-film YBCOSQUIDs at 77 K with thickness $t = 220$ or 113 nm. By combining experimentaldata and inductance modeling we find an average penetration depth $\lambda(77)= 391$ nm at 77 K, which was independent of $t$. Using the same methods wederive an empirical expression for $\lambda(T)$ for a further three SQUIDsmeasured on a cryocooler from 50 to 79 K. Our measured value of $\lambda(77)$and our inductance extraction procedures were then used to estimate theinductances and the effective areas of directly coupled SQUID magnetometerswith large washer-style pick-up loops. The latter agree to better than 7% withexperimentally-measured values, validating our measured value of $\lambda(77)$and our inductance extraction methods.

Determining the temperature-dependent London penetration depth in HTS thin films and its effect on SQUID performance