Not very often we can see magnetic hysteresis loops of type-II superconductors in their entirety. The published figures mostly present only their upper and lower branch, but their "ends" are usually outside the figure. Yet, in situation when the exciting field amplitude falls between Hc1 and Hc2, the discontinuity (jump) is observed in sample magnetization, even its sign is changed. Anything like this never happens in ordinary magnetic materials: here the two branches are always coinciding at the return point, i.e. at the maximum of the external field. What could be the possible reason for such an amazing behavior?
In a type II superconductor, if the applied field is less than Hc2, then the material will still show diamagnetic magnetization. Now if the field is reversed (ie, when the applied field is less than Hc2), the superconducting shielding currents have to change the direction and hence the magnetization will shift to the positive values. This will result in a discontinuity in the hysteresis loops
Note that the field is *not* reversed, but only decreased infinitesimally, so the flux in the sample is practically unchanged. As the flux is a product of sample inductance and current (this is certainly true in type I superconductors), then I don't see why the current, and the magnetic moment as well, should change considerably, not mentioning the reversal of its direction. So - mystery remains.
When you start decreasing the magnetic field, you reverse the sign of its variation. This induces a screening current of opposite direction, first in the external surface layer, and results in a sign reversal of the overall moment of the sample.
According to me the best answer for you question is given in the two following papers:
- Magnetization of Hard Superconductors, C. P. Bean, Phys. Rev. Lett. 8, 250 (1962),
- Magnetization of High-Field Superconductors, C. P. Bean, Rev. Mod. Phys. 36, 31 (1962).
If you want to find analogy with magnetic materials you should take into consideration that in the H-T phase diagram of superconducting materials there is an additional line to the Hc1(T) and Hc2(T). It is so called irreversibility line, Hirr(T), located between Hc1(T) and Hc2(T). In the magnetic field Hc1, the vortices enter superconductor. However, in the magnetic field bellow Hirr, they are pinned – their arrangement and thus magnetization of superconducting material depends on the history. The hysteresis loops mentioned by you are recorded in magnetic field below Hirr, not above Hirr. To find analogy with magnetic materials you should refer to the minor hysteresis loop of magnetic material, where the irreversible domain wall motion takes place. For the hysteresis loop of type-II superconductors recorded in the field above Hirr the two branches are coinciding at the return point, i.e. at the maximum of the external field, as it is in the case of ordinary magnetic materials.
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