Effect of oxygen-induced disorder on magnetic flux dynamics and critical current in a NdBa2Cu3O7-δ single crystal

The magnetic flux dynamics in a large (4.3x3.4x2.8 mm3) single crystal NdBa2Cu3O7-δ sample was investigated under fully-oxygenated and oxygen-deficient conditions using vibrating sample magnetometry (VSM) and ac flux profile measurements in the B parallel c-axis configuration. Using a split coil VSM the anisotropy (Γ) was determined. From the equilibrium magnetization above the irreversibility field (Birr), the London penetration depth (λ) was calculated. While a near total absence of the fishtail effect and more or less a constant J (15 000 A cm-2) in the field range of 1-10 T at 77 K was exhibited by the fully-oxygenated sample, a pronounced fishtail effect with an increase in current with a peak value of 32 000 A cm-2 was observed after oxygen reduction. However, the irreversibility field (Birr), was reduced from 13.1 to 7 T at 77 K after oxygen reduction. A vortex melting transition is indicated by the fully-oxygenated sample, with a marked deviation from this behaviour for temperatures below 86 K. A dynamic phase shift was observed through the superconducting transition which varies with field and exhibits a peak with a maximum around the irreversibility field. (author)