Magnetisation measurements on tubular samples of YBa2Cu3O7-y

Magnetisation hysteresis measurements have been performed at T=4.2 K and T=77 K on tubular samples of YBa2Cu3O7-y. There is clear evidence of two hysteresis loops. The first, at very low magnetic fields, represents a total-body superconducting behaviour with a shielding current circulating in the whole sample; the second one, at high magnetic fields, represents the hysteresis of single grains (or superconducting regions of smaller dimensions). This model emerges also from various bulk magnetisation and AC susceptibility measurements, as has been pointed out elsewhere. The particular geometry used assigns to the identified contributions opposite signs and it is potentially able to separate them. The critical currents related to the two hysteresis loops are very different in value (about four orders of magnitude at T=4.2 K and two-to-three orders of magnitude at 77 K). A preliminary evaluation of some critical fields related to both the hysteresis loops is given.