THE ORIGIN OF THE LOW FIELD PEAK IN THE HYSTERESIS CYCLE OF HIGH-TC OXIDES AND THE INFLUENCE OF MAGNETIC PINNINGS

We report on an extensive investigation of the hysteresis cycle of decoupled grains and single crystals of YBa2Cu3O7. At low temperature T and field H the cycle is characterized by a broad peak, the amplitude of which is almost independent of the radius R of the sample for R greater-than-or-equal-to 20-mu-m. At a much higher field the signal scales with R for R less-than-or-equal-to R(o) almost-equal-to 2 mm. At high temperature, close to the irreversibility line the cycle appears almost reversible near H = 0. It then exhibits a very sharp peak followed by a regime of low irreversibilities. All these features are reproduced by taking into account properly the Abrikosov's equilibrium magnetization M(eq)(H) and by assuming that the critical current density varies as J(c1)(H) + J(c2)(H) with J(c1) (0) almost-equal-to J(c2) (0)/2 almost-equal-to 2 to 4 x 10(6) A/cm3. J(c) is found to depend strongly on H (leading to a peak at low H) and is ascribed to long range interactions between vortices. J(c1) varies much more slowly with H and is associated with short range pinnings at and near the vortex cores. We also discuss the influence and the systematic errors induced by the irreversible magnetization on the equilibrium quantities H(c1) and M(eq)(H).