INFLUENCE OF OXYGEN DEFICIENCY ON THE SUPERCONDUCTIVE PROPERTIES OF GRAIN-ALIGNED YBA2CU3O7-DELTA

Magnetically aligned samples of sintered YBa2Cu3O7-delta were used to test the effects of oxygen-deficiency delta (with delta less-than-or-equal-to 0.2) on the superconductive magnetization M, critical current density J(c), irreversibility field B(irr), upper critical field H(c2), coherence length xi, condensation energy F(c), penetration depth lambda, and related properties as a function of temperature T and applied field H parallel-to c. In selected cases, studies were also made with H parallel-to ab. The O content was monitored in situ by thermogravimetric analysis. The open porosity and granularity of the material allowed rapid and homogeneous oxygenation. We found no significant enhancement of intragrain J(c) with chain-site O defects. With few exceptions, maximum J(c) occurred at full oxygenation. This implies that chain-site O defects are not strong or effective pinning centers over most of the field-temperature regime investigated. Except for T(c), which was practically independent of delta within the interval 0 less-than-or-equal-to delta less-than-or-equal-to 0.11 (so called "90-K T(c) plateau"), most properties such as J(c), F(c), H(c2), B(irr) (T), lambda, and xi were strongly and continuously influenced by the O deficiency. The observed abnormal magnetization with H parallel-to c was weak at low T but became more pronounced as T and delta increased. No abnormal magnetization was detected with H parallel-to ab. As oxygen was removed, B(irr)(T) and H(c2)(T) separated, and both lines shifted to lower T and lower B. Moreover, B(irr) was strongly correlated with J(c) at low temperature. Determination of the thermodynamic critical field H(c) yielded condensation energies F(c)(delta) that were well correlated with J(c)(delta). The results indicate that O defects weaken the energy barrier of the existing pinning sites and have a negative overall effect on the capacity of YBa2Cu3O7-delta to carry loss-free currents.