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Abstract:
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The magnetic behaviour (magnetization curves, Meissner effect, hysteresis losses, remanent flux and flux line cutting) for high $T\sb{c}$ samples of Nd and $YBa\sb{2}Cu\sb{3}O\sb{7-x}$ at 77 K, semi-reversible PbIn and hysteretic VTi at 4.2 K have been investigated and compared. The magnetic behaviour of the high $T\sb{c}$ samples and of the PbIn are remarkably similar but contrasts dramatically with the phenomena observed in the VTi. A hump structure appears in the low field region of the initial magnetization of the high $T\sb{c}$ samples. This phenomenon occurs because the sintered samples consist of a compact agglomeration of small irregularly shaped grains which are electrically coupled before the hump structure but are isolated after this feature. We show that the appropriate calibration should be based on the diamagnetic response of the uncoupled grains. A large ratio of $j\sb{c\Vert}$ to $j\sb{c\perp}$ (the critical current densities $\Vert$ and $\perp$ to the flux line density) accounts for the observations on the VTi in the flux cutting regime whereas $j\sb{c\Vert} \approx j\sb{c\perp}$ is indicated by the behaviour of the high $T\sb{c}$ and PbIn samples. |
