Thermomagnetic history effects in niobium and its implication for H_c1 in high T_c superconductors

Abstract

The existence of a remanent magnetization (M_rem) on switching off the field of a field cooled (FC) sample of a high T_c superconductor is often reported. It has recently been argued that M_rem should equal the difference in FC and zero field cooled (ZFC) magnetizations (M_FC-M_ZFC) in hard superconductors and this has been demonstrated to hold in single crystals of YBCO at 4.2K over a limited range of H values. We report the detailed magnetization measurements under various thermomagnetic histories (of which M_rem is one special case) on two specimens of Nb, which show different extents of flux trapping. We find that there are in general three regions in H, T space, corresponding to M_rem+M_ZFC-M_FC=0, M_rem<(M_FC-M_ZFC) and M_rem>(M_FC-M_ZFC). At any T, the equality holds for H<H_c1(T), and for H→H_c2 (M_FC-M_ZFC) asymptotically vanishes and there M_rem>(M_FC-M_ZFC). We show that there exists an intermediate region in all hard superconductors, where M_rem<(M_FC-M_ZFC). The range over which this situation persists, however, depends on the degree of irreversibility in a sample. We can explain qualitatively all the history dependent magnetization data in terms of the critical state model. We point out an inconsistency in an earlier analysis to determine H_c1(T) from such data in YBCO. We also propose a new criterion for putting limits on H_c1(T) in hard superconductors.