Magnetic measurements of the lower critical fields and irreversible properties of oxygen deficient YBa2Cu 3O(7-delta) single crystals

摘要

We present results for lower critical field, irreversibility field and hysteresis on an oxygen deficient single crystal of YBa2Cu3O( 7-delta) annealed to a (Tc) of 28 and 85 K. We compare results for off-plateau ((Tc) = 28 and (Tc) = 85 K) stoichiometries to earlier results on the 60 K (Tc) plateau crystals and the fully oxygenated 92 K (Tc) crystals. We find a systematic decrease in the lower critical field with decreasing (Tc), and a decrease in the remanent magnetization by a factor of 10, at T = 5 K, for the (Tc) = 28 K phase compared to the (Tc) = 85 K phase. Our irreversibility field versus reduced temperature data indicate much stronger curvature and hence a larger reversible region for the reduced oxygen stoichiometries. For crystals with (Tc) = 62.2, 56, 50, and 28 K, a crossover occurs in the slope of log(H(sub irr)) vs log(l-T/(Tc)). The field at which this crossover occurs is lower for the more oxygen deficient phases and may be linked to their lower H(sub c1) values and corresponding longer penetration depths.