Ž . Physica C 330 2000 155–159 www.elsevier.nlrlocaterphysc Peculiarities of rf SQUID response in finite magnetic fields E. Il’ichev a, ) , V. Schultze a , R.P.J. IJsselsteijn a , R. Stolz a , V. Zakosarenko a , H.E. Hoenig a , H.-G. Meyer a , M. Siegel b a Department of Cryoelectronics, Institute for Physical High Technology, P.O. Box 100239, D-07702 Jena, Germany b Institut fur Schicht und Ionentechnik, Forschungszentrum Julich, D-52425 Julich, Germany ¨ ¨ ¨ Received 12 November 1999; accepted 16 November 1999 Abstract Single-layer washer-type high-T YBa Cu O rf SQUIDs with grain-boundary Josephson junctions, as well as low-T c 2 3 7yx c Nb rf SQUIDs with Nb–Al O –Nb tunnel junctions, have been investigated in finite magnetic fields. It was shown 2 3 experimentally that the suppression of the critical current of the Josephson junction due to the magnetic field leads to a modulation of the amplitude of the SQUID output signal. The role of the ‘‘unwanted’’ junction in high-T rf SQUIDs, which c is formed by the grain boundary running through the washer of the SQUIDs on bicrystal substrates, has also been clarified. The drop of the SQUID signal at a finite magnetic field is originated by the penetration of the magnetic field into the unwanted junction. Based on these results, a direct radio-frequency method for the determination of the first critical field H for long Josephson junctions has been developed. q 2000 Elsevier Science B.V. All rights reserved. c1 PACS: 74.50 qr; 85.25 Cp Keywords: SQUID; Josephson junction The stable operation in finite magnetic fields H is one of the crucial requirements for real applications of SQUID-based devices. The magnetic field focus- ing effects depend strongly on the design of the SQUID. For example, in the simplest case SQUID magnetometers are designed as so-called washer type SQUIDs. Due to the large demagnetization factor of Ž. a washer even small external magnetic fields can i promote the nucleation and subsequent displacement Ž. of vortices near the edges of the washer, and ii suppress the critical current I of the junction fol- c Ž . lowing a Fraunhofer-like dependence I H . Both c ) Corresponding author. Tel.: q 49-3641-206-121; fax: q 49- 3641-206-199. Ž . E-mail address: ilichev@ipht-jena.de E. Il’ichev . effects have been experimentally investigated for dc SQUIDs. It was shown that these effects lead to w x magnetic hysteresis 1,2 , to random telegraph noise w x 3,4 , and to a depression of the amplitude of the Ž voltage–fluxon characteristic see for example, Fig. wx. 8 in Ref. 5 . For rf SQUIDs the situation is not so clear. To Ž . our knowledge, the influence of I H on the rf c SQUID output signal was investigated neither exper- imentally nor theoretically. Moreover, single layer high-T rf SQUIDs as often used for applications, c contain additional ‘‘weak places’’ in the main quan- tization circuit. When bicrystal substrates are used for rf SQUID fabrication, an additional long junction Ž . crosses the washer see Fig. 1 . For SQUIDs based on step-edge junctions, this ‘‘weak place’’ is pro- duced by the second grain boundary on the step-edge 0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0921-4534 99 00599-7