Abstract
We have measured the temperature dependent change ∂Φ/∂T in the flux signal of a superconducting quantum interference device (SQUID) operating in a flux locked loop. Thin film niobium dc SQUIDs were used for these investigations. One and the same device could be operated either as dc or rf SQUID allowing one to compare the effect of temperature variation in both modes of operation. We have found that relaxation of trapped flux appears to be the dominant result of temperature variation. The asymmetry of the two critical junction currents in dc SQUID operation could be seen, as well as an influence of an external magnetic field on ∂Φ/∂T. This influence was an order of magnitude smaller when the device was operated as an rf SQUID, compared to dc SQUID operation. By the use of a high frequency demagnetization process, ∂Φ/∂T could be reduced from typical values of about 0.3 Φ0/K to less than 0.05Φ0/K in certain cases.
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