Field and oxygen dependence of the magnetic irreversibility line in YBa2Cu3O7- delta.

Abstract

We report on dc magnetic susceptibility measurements in polycrystalline YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] as a function of the oxygen content in the limit of small [delta]. We focus mainly on the detailed behavior of the zero-field-cooled--field-cooled irreversibility line at fields below 7 kOe. We find that this line shows a crossover from a [ital H][sup 2] dependence at higher fields to the more commonly observed [ital H][sup 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 concerning the behavior of the resistive transition as a function of the distribution of the oxygen vacancies are also presented.