FIELD AND OXYGEN DEPENDENCE OF THE MAGNETIC IRREVERSIBILITY LINE IN YBA2CU3O(7-DELTA)

We report on dc magnetic susceptibility measurements in polycrystallin e YBa2Cu3O7-delta as a function of the oxygen content in the limit of small delta. We focus mainly on the detailed behavior of the zero-fiel d-cooled-field-cooled irreversibility line at fields below 7 kOe. We f ind that this line shows a crossover from a H-2 dependence at higher f ields to the more commonly observed H-2/3 behavior at lower fields. Th is feature is analogous to the anisotropy induced crossover from the h igh-field Gabay-Toulouse line to the low-field de Almeida-Thouless beh avior, characteristic of the spin-glass transition. Our results are di scussed in the light of a dirty-limit vortex-glass model where the gra in charging energy plays a major role. Data concerning the behavior of the resistive transition as a function of the distribution of the oxy gen vacancies are also presented.