Use of a phase-sensitive detector for measuring losses in superconductors as a function of frequency

We describe the design of a system that automatically measures losses in superconductors as a function of frequency. The method follows a suggestion by Goemoery who notes that, for a symmetric hysteresis loop, one can trace out the loop by measuring the output voltage of a phase-sensitive detector as a function of the phase angle between the applied field and the detector setting. We have measured the losses at 77 degree K in aligned YBCO for frequencies from 10 Hz to 40 kHz with amplitudes of applied field up to 100 Oe. The usual method of measuring the in-phase component of dB/dt to determine the losses was also employed and we found that the two methods gave, within experimental error, identical results over the frequency range we have covered. copyright 1995 American Institute of Physics