Thermomagnetic history effects in niobium and its implication for H
c1 in highT
c superconductors |
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Authors: | A K Grover P L Paulose P Chaddah G Ravikumar |
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Institution: | (1) Tata Institute of Fundamental Research, Homi Bhabha Road, 400 005 Bombay, India;(2) Nuclear Physics Division, Bhabha Atomic Research Centre, 400 085 Bombay, India |
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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. |
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Keywords: | Thermomagnetic history effects Conventional type II superconductors High temperature superconductors Magnetization curves Lower critical field |
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