Estimation of Critical Current of HTS RF-SQUID

The operation of the RF SQUID is restricted by the condition that the inductance parameter beta(L) must be in the range of 1-3. However, since both ends of the Josephson junction (JJ) of RF-SQUID are shorted, it is difficult to non-destructively estimate the critical current (I-C). Thus, we proposed a technique for the non-destructive measurement of the I-C of a high-temperature superconducting (HTS) RF-SQUID ring by evaluating the behaviour of the flux in superconducting thin films using a SQUID magnetometer. A superconducting ring sample with JJ was placed below the HTS SQUID magnetometer and cooled down to 77 K. The change in the SQUID output was monitored on application of the magnetic field. When increasing the field, the waveforms indicated that the screening current of the ring sample exceeded the I-C of the JJ, and the JJ became a normal-conducting state. As a result, we estimated the I-C of the JJ of this sample a s 134 mu A using the values of mutual inductance and the coupling coefficient alpha between the coil and the sample.