COMPARISON OF VARIOUS METHODS TO DETERMINE EXPERIMENTALLY THE IRREVERSIBILITY LINE IN SUPERCONDUCTORS

An attempt has been made to determine the irreversibility temperatures [T(r)(H)] via the merger of zero-field-cooled and field-cooled magnetizations, via the vanishing of the isothermal hysteresis, and via the appearance of the differential paramagnetic effect in specimens of lead and niobium. The first method appears to define a lower limit on T(r)(H) and does not necessarily indicate a true thermodynamic region near T(c)(H). The differential paramagnetic effect, however, is a qualitative feature whose presence is sufficient to imply reversibility.