The existence of a remanent magnetization (Mrem) on switching off the field of a field cooled (FC) sample of a highTc superconductor is often reported. It has recently been argued thatMrem should equal the difference in FC and zero field cooled (ZFC) magnetizations (MFC —MZFC) 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 whichMrem 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 toMrem+MZFC−MFC=0,Mrem<(MFC−MZFC) andMrem>(MFC−MZFC). At anyT, the equality holds forH<Hc1(T), and forH→Hc2 (MFC−MZFC) asymptotically vanishes and thereMrem>(MFC−MZFC). We show that there exists an intermediate region in all hard superconductors, whereMrem<(MFC−MZFC). 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 determineHc1(T) from such data in YBCO. We also propose a new criterion for putting limits onHc1(T) in hard superconductors.
Volume 94, 2020
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