Controlling the natural strong pinning sites in laser ablated YBa₂Cu₃O_7-δ thin films

Recently [1], we have shown that dislocations are the most important flux pinning centers in Pulsed Laser Deposited YBa₂Cu₃O_7-δ thin films. It appeared that the magnetic field upto which the critical current remains constant, is roughly equal to the matching field B_Φ=n_dislΦ₀, with n_disl the density of dislocations. Here, we investigate the formation mechanism of these dislocations. Using wet-chemical etching in combination with Atomic Force Microscopy, we find that dislocations are induced in the first stages of film growth and persist all the way up to the film surface parallel to the c-axis. Since the substrate temperature can be used to tune the defect density n_disl, the dislocation formation mechanism is closely related to the YBa₂Cu₃O_7-δ nucleation and growth mechanism. We propose that dislocations are induced as a result of merging of misaligned growth fronts due to the preferential formation of precipitates in the first stages of growth. Indeed, we find that we can increase the dislocation density by first depositing Y₂O₃ precipitates.