Supercurrent transport in YBa2Cu3O7-δ epitaxial thin films in a dc magnetic field

Magnetic field and angle dependences of the critical current density J(c)(H,theta) in epitaxial c-oriented YBa2Cu3O7-delta thin films are measured by the four-probe transport current technique, low-frequency ac magnetic susceptibility, and superconducting quantum interference device magnetometry. The films under study are deposited by off-axis dc magnetron sputtering onto r-cut sapphire substrates buffered with a CeO2 layer. A consistent model of vortex pining and supercurrent limitation is developed and discussed. Rows of growth-induced out-of-plane edge dislocations forming low-angle boundaries (LAB's) are shown to play a key role in achievement of the highest critical current density J(c)>= 2x10(6) A/cm(2) at 77 K. The model takes into account the transparency of LAB's for supercurrent as well as the pinning of vortex lattice on a network of LAB's. Principal statistical parameters of the film defect structure, such as the domain size distribution and mean misorientation angle, are extracted from J(c)(H) curves measured in a magnetic field H applied parallel to the c axis and from x-ray diffraction data. An evolution of angle dependences J(c)(theta) with H is shown to be consistent with the model supposing dominant pinning on edge dislocations. Strongly pinned vortices parallel to the c axis appear to exist in tilted low magnetic fields up to a characteristic threshold field, below which the magnetic induction within the film obeys a simple relation B=H cos theta. This feature is shown to explain the absence of the expected maximum of J(c)(theta) at H parallel to c in a low applied field. A peak of J(c)(H) and an angular hysteresis of J(c)(theta), which have been observed in an intermediate-field range, are discussed in terms of film thickness, surface quality, and orientation of the applied field. The observed effects are found to be consistent with the developed model.