Determination of irreversibility line in hard type-II superconductors: path dependence of field-cooled magentization

Abstract

Various procedures, based on dc and ac magnetization, have been used to determine the irreversibility line in hard type-II superconductors. In dc magnetization measurements these include (i) merger of zero-field-cooled (ZFC) and field-cooled (FC) magnetization and (ii) vanishing of the isothermal hysteresis. It has been generally believed that the FC magnetization is path independent but our present observations show that it may not be so in some specimens of hard type-II supreconductors like niobium powder (T_c = 9.3 K). Our measurements on Nb powder show that the merger of M_ZFC data with M_FC data occurs at temperature (_r^c) during the cooling cycle higher than that while it occurs at (T_r^ω) during the warming-up cycle. It is noted that T_r^c(H) > T_r^ω(H). Thus from this method, we can identify two irreversibility temperatures T_r^c(H) and T_r^ω;(H) at the same field value. The path dependent FC magnetization is a renewed effort to seek an understaneding of the flux distribution inside such hard superconductors which allows partial flux expulsion on cooling in a constant field. Thus, the FC magnetization in Nb powder has been found to depend on its thermomagnetic history. This fact makes quasi-irreversibility line determined via the merger of ZFC and FC magnetization to be path dependent. The family of quasi-irreversibility lines of determined, lie below the one determined via vanishing of the isothermal magnetization hysteresis.