Field and oxygen dependence of the magnetic irreversibility line in yba(2)cu(3)o(7-g)

Abstract

We report on de magnetic susceptibility measurements in polycrystalline YBa2Cu30 7- δ as a function of the oxygen content in the limit of small δ. We focus mainly on the detailed behavior of the zero-fie1dcooled- field-cooled irreversibility line at fields below 7 kOe. We find that this line shows a crossover from a H 2 dependence at higher fields to the more commonly observed H 2/3 behavior at lower fields. This feature is analogous to the anisotropy induced crossover from the high-field Gabay-Toulouse line to the low-field de Almeida-Thouless behavior, characteristic of the spin-glass transition. Our results are discussed in the light of a dirty-limit vortex-glass model where the grain charging energy plays a major role. Data conceming the behavior of the resistive transition as a function of the distribution of the oxygen vacancies are also presented.