AUTOMATIC SYSTEM FOR THE DIRECT AND CONTINUOUS MEASUREMENT OF THE IRREVERSIBILITY LINE OF HIGH T-C SUPERCONDUCTORS

A system for the direct and continuous determination of the irreversib ility line in high T-c superconductors is described. The system, based on a standard ac susceptometer, operates by applying a controlled dc field to the sample that maximizes the imaginary (out-of-phase) compon ent of the ac susceptibility. This control is achieved using a proport ional-integral-derivative loop with the reference set to zero and with the magnetic field derivative of the imaginary component of the ac su sceptibility taken as the feedback sample (error signal). To obtain a dc voltage proportional to this derivative a double lock-in detection scheme is used. This apparatus is able to carry out a continuous measu rement of this line by sweeping the temperature in the range of intere st while the sample is maintained, by the controller, at the irreversi bility magnetic field at each temperature. Hence, the whole process is accomplished in the same time as a typical resistivity-versus-tempera ture measurement, which is faster than other methods. (C) 1998 America n Institute of Physics. [S0034-6748(98)02408-3].