Flux jumps, cluster distribution model and vortex phase diagram of oxygenated YBa2Cu3−xAlxO6+δ single crystal for H || ab

Abstract Flux jumps were observed in the magnetisation hysteresis loops of a single crystal of an oxygenated high temperature superconductor YBa2Cu3−xAlxO6+δ at fields similar to which symmetry reorientation transitions have been observed in small angle neutron scattering measurements of YBa2Cu3O6+δ previously. Bean’s critical state model was found to be insufficient to explain the observation of the flux jumps at fields larger than the field of full penetration. From x-ray photoelectron spectroscopy measurements, a single peak was observed in the spectra of Y, Ba, Cu and O, unlike the previous observation of double peaks in as-grown and oxygenated crystals. An aluminium substituted, oxygen vacant clusters of the kind (Al-O-Cu-..) n has been schematically made to explain the observation of the single peak in the XPS spectra. Similar to the observation of a field direction dependence in the as-grown crystal for µ0H || c, the second magnetisation peak anomaly was observed to have a field direction dependence for µ0H || ab, but only at low temperatures. At temperatures larger than 25 K, the second magnetisation peak anomaly was found to be field direction independent. The onset and peak fields were found to comply to an order-disorder transition at such fields and temperatures. The flux jump line was also found to fit with an order-disorder transition suggesting an intimate link between symmetry reorientation transition and the state of spatial order in the vortex lattice of oxygenated YBa2Cu3−xAlxO6+δ single crystal µ0H || ab. Incorporating all features in the magnetisation hysteresis loop, a vortex phase diagram has been made for oxygenated YBa2Cu3−xAlxO6+δ for µ0H || ab depicting various phases.