THE REVERSIBLE MAGNETIZATION OF OXYGEN DEFICIENT YBA2CU3O7-DELTA

The reversible magnetization M(rev) of polycrystalline YBa2Cu3O7-delta (0 < delta < 0.5) WaS measured. Due to a small grain size and low critical current densities in oxygen deficient specimens M(rev) could be measured over most of the superconducting regime from 10 K to T(c) up to a magnetic field of 5.5 T. The magnetization data is in excellent agreement with calculated values obtained from Ginzburg-Landau (GL) theory. Values for the thermodynamic critical field H(c) and the GL parameter-kappa are obtained. High values of kappa almost-equal-to 170-300 (for H parallel-to c) are necessary to fit the magnetization data. H(c)(T = 0) decreases strongly for increasing oxygen deficiencies up to delta almost-equal-to 0.15, and then remains nearly constant. Specific heat jumps DELTA(c) at T(c) were calculated from the data and the f it. Quantitative agreement with data in the literature is obtained. Just like H(c), the specific heat jump is strongly dependent on the oxygen deficiency in the near stoichiometric regime.