Anisotropic flux pinning in a network of planar defects

Abstract

We consider a strong flux pinning by a network of high-${\mathit{j}}_{\mathit{c}}$ planar defects which can result in high macroscopic critical current densities ${\mathit{J}}_{\mathit{c}}$ in bulk superconductors. Such a pinning potential gives rise to a qualitative change of the structure of normal cores which turn into highly anisotropic phase cores described by equations of nonlocal Josephson electrodynamics. We obtained a solution of these equations for a vortex parallel to the planar defect and calculated the magnetic-field distribution and the transversal pinning force ${\mathit{f}}_{\mathrm{\ensuremath{\perp}}}$ between the vortex and the defect. The longitudinal pinning force ${\mathit{f}}_{\mathrm{\ensuremath{\parallel}}}$ of vortices along the defect is determined by both their magnetic interaction with pinned intragrain fluxons and local inhomogeneities of the defect. The force f(H) is shown to be highly anisotropic with respect to the current direction, the value ${\mathit{f}}_{\mathrm{\ensuremath{\parallel}}}$ along the defect being much smaller than the perpendicular component ${\mathit{f}}_{\mathrm{\ensuremath{\perp}}}$. This can result in the preferential flux motion along the percolative paths formed by planar defects, giving rise to a nonmonotonic ${\mathit{J}}_{\mathit{c}}$(H) dependence due to the increase of ${\mathit{f}}_{\mathrm{\ensuremath{\parallel}}}$ with H caused by magnetic interaction of inter- and intragrain fluxons. The effect of topology of the pinning network on ${\mathit{J}}_{\mathit{c}}$ and flux creep is discussed. We also calculate the low-field dependences of ${\mathit{J}}_{\mathit{c}}$(H) and consider the regions of the T-H space, where a magnetic granularity transition can occur.