Comparison of various methods to determine experimentally the irreversibility line in superconductors.

Abstract

An attempt has been made to determine the irreversibility temperatures [${\mathit{T}}_{\mathit{r}}$(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 ${\mathit{T}}_{\mathit{r}}$(H) and does not necessarily indicate a true thermodynamic region near ${\mathit{T}}_{\mathit{c}}$(H). The differential paramagnetic effect, however, is a qualitative feature whose presence is sufficient to imply reversibility.