Null input current SQUID magnetometer for the measurement of the transition temperature of high-T c superconducting samples

In this article we describe a SQUID-based system used to measure the superconducting transition temperature in the range 4.2-300 K. The apparatus has been tested using a high-critical-temperature single crystal of YBa2 Cu3 O7-x (? 10-1 mm3 volume). The system is based on mutual inductance variations measurements, performed in a low-frequency ac magnetic field of 2×10-5 T. A feedback loop is closed on the input circuit in order to null out the current flowing in the pickup coil. This is achieved using an electronic scheme which does not involve any modification of the commercial SQUID electronics. Our experiment is performed using a copper wire pickup coil, but the scheme can also be used with a superconducting input circuit, thus allowing measurements in a dc regime. The obtained sensitivity is 5×10-5/?Hz. This figure can be further improved by optimizing the circuit parameters.