Supercurrent transport inYBa2Cu3O7−δepitaxial thin films in a dc magnetic field

Abstract

Magnetic field and angle dependences of the critical current density ${J}_{c}(H,\ensuremath{\theta})$ in epitaxial $c$-oriented $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ 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 $\mathrm{Ce}{\mathrm{O}}_{2}$ 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 ${J}_{c}\ensuremath{\geqslant}2\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕{\mathrm{cm}}^{2}$ at $77\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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 ${J}_{c}(\ensuremath{\theta})$ 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 $B=H\phantom{\rule{0.2em}{0ex}}\mathrm{cos}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}$. This feature is shown to explain the absence of the expected maximum of ${J}_{c}(\ensuremath{\theta})$ at $H\ensuremath{\parallel}c$ in a low applied field. A peak of ${J}_{c}(H)$ and an angular hysteresis of ${J}_{c}(\ensuremath{\theta})$, 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.