Thermomagnetic history effects in niobium and its implication for H c1 in highT c superconductors

Abstract

The existence of a remanent magnetization (M rem) on switching off the field of a field cooled (FC) sample of a highT c superconductor is often reported. It has recently been argued thatM 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 ofH values. We report the detailed magnetization measurements under various thermomagnetic histories (of whichM 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 inH, T space, corresponding toM rem+M ZFC−M FC=0,M rem<(M FC−M ZFC) andM rem>(M FC−M ZFC). At anyT, the equality holds forH<H c1(T), and forH→H c2 (M FC−M ZFC) asymptotically vanishes and thereM rem>(M FC−M ZFC). We show that there exists an intermediate region in all hard superconductors, whereM 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 determineH c1(T) from such data in YBCO. We also propose a new criterion for putting limits onH c1(T) in hard superconductors.