Anisotropic pancake vortex transport in the crossing lattices regime ofBi2Sr2CaCu2O8+δsingle crystals

Abstract

We have used micro-Hall probe magnetometry to investigate how the out-of-plane ``local'' magnetization of a superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystal depends on the strength and direction of an in-plane magnetic field, ${H}_{\ensuremath{\Vert}}$, in the crossing vortex lattices regime. The ${H}_{z}=0$ remanent magnetization exhibits a pronounced anisotropy, being largest with ${H}_{\ensuremath{\Vert}}$ parallel to the crystalline $a$-axis, and smallest when it is parallel to the orthogonal $b$-axis. We attribute this to the presence of strongly pinning linear defects (LDs), which are known to lie close to the $a$-axis in these crystals. With ${H}_{\ensuremath{\Vert}}$ parallel to the $a$-axis Josephson vortex (JV) stacks become indirectly pinned along LDs, and channel pancake vortices (PVs) into these regions of high disorder, increasing the measured irreversibility. With ${H}_{\ensuremath{\Vert}}$ along the $b$-axis PVs are efficiently channeled into the sample centre, readily cutting across LDs, and the irreversibility is low. At high temperatures the remanent magnetisation as a function of the in-plane field angle exhibits a pronounced shoulder which appears to be related to the critical accommodation angle for the indirect trapping of JVs by LDs, above which JVs snap free. At low temperatures random bulk pinning is increasingly in competition with LDs, and the measured anisotropy becomes much weaker.