Decrease of Bulk Pinning Strength in Deoxygenated YBCO Single Crystals

Abstract

We measured the first and third harmonics of the complex ac susceptibility in YBCO single crystals with different oxygen contents (6.5 ≤ x ≤ 7). The amplitude of the ac field was varied in presence of an external dc field both applied parallel to the c‐axis of the crystals. We give evidence that deoxygenation leads to a reduction of the bulk pinning strength and consequently to a stronger contribution of geometrical barriers. These results support the recently reported investigations showing that deoxygenation makes the YBCO crystals more anisotropic reducing the effective bulk pinning for quasi‐two‐dimensional vortices. We also show measurements for the same crystals with the ac field applied perpendicular to the c‐axis.

Decoupling and tuning competing effects of different types of defects on flux creep in irradiated YBa2Cu3O7−δcoated conductors

“…At 1 T, the range of enhancement is limited to T<60 K, and at 0.3 T, the enhancement is minimal at low T, and irradiation is detrimental to J c for T>20 K. The inset of figure 4 shows the line in the H-T plane separating the regions in which oxygen irradiation increases and decreases J c . Clearly, irradiation is beneficial over the vast majority of the phase diagram, in particular in the region of interest for rotating machines [27,28] (H≈2-5 T and T≈20-40 K). For H=5 T, the enhancement in J c extends over the entire vortex solid phase, up to the irreversibility line, implying that oxygen irradiation would be beneficial for wires constituting very high field magnets.…”

“…This simple scenario is complicated by the fact that, at temperatures higher than 450 °C, the film will slightly deoxygenate [24]. Deoxygenation changes T c due to changes in the density of current carriers, reduces the effective bulk pinning [25,26] due to a decrease in x c (hence an increase in anisotropy g x x = ab c ), decreases the thermal activation energy U(T, H) for vortex motion [27,28], and increases the number of oxygen vacancies (point defects) [25]. To decouple these concomitant effects above 450 °C, we partially reoxygenate each sample by annealing in oxygen at 300 °C for 3-5 h after the last annealing step.…”

“…! ; decreases the thermal activation energy for vortex motion, 27,28 and increases the number of oxygen vacancies (point defects). 25 To decouple these concomitant effects above 450°C, we partially reoxygenate each sample by annealing in oxygen at 300°C for 3--5 hours after the last annealing step.…”