Influence of oxygen stoichiometry on the irreversible magnetization and flux creep in RBa2Cu3O7- delta (R=Y,Tm) single crystals.

Abstract

The influence of the oxygen stoichiometry on magnetization and relaxation of R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (R=Y,Tm) single crystals was studied for 0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.55. The field dependences of the shielding currents ${\mathit{j}}_{\mathit{s}}$(H) and flux creep rates S(H) were analyzed. Three different kinds of ${\mathit{j}}_{\mathit{s}}$(H) and S(H) behavior can be seen in highly oxygenated samples (\ensuremath{\delta}0.1). (i) At high temperatures, ${\mathit{j}}_{\mathit{s}}$(H) shows a common fishtail peak, which is present for all oxygen contents. The field position and height of the ${\mathit{j}}_{\mathit{s}}$ peak increase rapidly with decreasing \ensuremath{\delta}. Approaching the stoichiometric state \ensuremath{\delta}=0, the value of the current starts to decrease with lower oxygen deficiency, in contrast to the still increasing irreversibility field ${\mathit{B}}_{\mathrm{irr}}$. This points to the importance of oxygen disorder for the pinning and stresses the melting nature of the irreversibility line. The peak position ${\mathit{B}}_{\mathrm{max}}$ is found to correlate with ${\mathit{B}}_{\mathrm{irr}}$. A possible relation of the fishtail with a synchronization effect and with the vanishing of ${\mathrm{C}}_{66}$ at the melting transition is proposed. (ii) In the intermediate-temperature region, some of the samples showed a new second peak with weak temperature and \ensuremath{\delta} dependence. The probable origin of this feature is matching with twin structure. This peak disappears for \ensuremath{\delta}g0.1, when the fishtail position shifts below the matching field. (iii) The low-temperature region is characterized by a monotonic decrease of ${\mathit{j}}_{\mathit{s}}$(B). This region becomes narrower with larger \ensuremath{\delta} and disappears for \ensuremath{\delta}g0.3. Thus in highly deoxygenated samples, the fishtail feature is observed for practically all temperatures. The possible connection of the low-temperature region with the pinning of the small vortex bundles or with the slipping of vortices along the twin planes is considered.