INFLUENCE OF OXYGEN STOICHIOMETRY ON THE IRREVERSIBLE MAGNETIZATION AND FLUX CREEP IN RBA2CU3O7-DELTA (R=Y,TM) SINGLE CRYSTALS

The influence of the oxygen stoichiometry on magnetization and relaxation of {ital R}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} ({ital R}=Y,Tm) single crystals was studied for 0{le}{delta}{le}0.55. The field dependences of the shielding currents {ital j}{sub {ital s}}({ital H}) and flux creep rates {ital S}({ital H}) were analyzed. Three different kinds of {ital j}{sub {ital s}}({ital H}) and {ital S}({ital H}) behavior can be seen in highly oxygenated samples ({delta}{lt}0.1). At high temperatures, {ital j}{sub {ital s}}({ital H}) shows a common fishtail peak, which is present for all oxygen contents. The field position and height of the {ital j}{sub {ital s}} peak increase rapidly with decreasing {delta}. Approaching the stoichiometric state {delta}=0, the value of the current starts to decrease with lower oxygen deficiency, in contrast to the still increasing irreversibility field {ital B}{sub irr}. This points to the importance of oxygen disorder for the pinning and stresses the melting nature of the irreversibility line. The peak position {ital B}{sub max} is found to correlate with {ital B}{sub irr}. In the intermediate-temperature region, some of the samples showed a new second peak with weak temperature and {delta} dependence. The probable origin of this feature is matching with twin structure. This peak disappears formore » {delta}{gt}0.1, when the fishtail position shifts below the matching field. The low-temperature region is characterized by a monotonic decrease of {ital j}{sub {ital s}}({ital B}). This region becomes narrower with larger {delta} and disappears for {delta}{gt}0.3. 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.« less