A Study of Reversible and Irreversible Magnetization Behaviour in Conventional Superconductors

Abstract

The complete expulsion of magnetic flux on cooling a superconductor in a dc magnetic field (Meissner-Oschenfeld effect) ensures that the magnetic response of a superconductor is reversible and that it is a thermodynamic property. Incomplete flux expulsion is, however, a common occurrence in both type I and type II superconductors and such materials display a path-dependent or hysteretic magnetic response. Detailed studies in the high temperature superconductors (HTSC) have resulted in the observation1 that there exists a thermodynamic reversible region in (H,T) space just below the normal-superconductor phase boundary (T c (H) line). Here H is the applied field, T is the temperature of the sample. Hysteretic behaviour sets in only below the irreversibility line, (T r , H r ), and different techniques have been used to determine this (T r,Hr) line in the HTSC. We have searched for such an irreversibility line in specimens of conventional type I and type II superconductors by both dc and ac magnetization techniques. The following four different procedures were employed to determine the (H r , T r ) line in HTSC: (a) Field-cooled (FC) and zero field-cooled (ZFC) magnetization curves in a constant dc field H were measured as a function of temperature. The temperature at which these merge [T r (H)] gives a point on the irreversibility line. (b) Hysteretic isothermal magnetizaton, i.e, M vs H, curves were measured where M is the sample’s magnetization per unit volume and H is the applied field. The field, [H r (T)], at which the hysteresis vanishes gives a point on the (T r , H r ) line. (c) The complex ac magnetic susceptibility (χ = χ′ H + iχ″ H ) was measured in constant dc field H as a function of temperature. A thermoaynamic response requires that χ′ H (T) be positive just below T c and this is called the Differential Paramagnetic Effect (DPE).2 The temperature at which χ′ H (T) switches from positive to negative is identified with T r (H). (d) From the simultaneous measurements of χ″ H (T) as a function of temperature in a constant dc field, the temperature at which it peaks is identified as T p (H).