Vortex-lattice transition in superconducting Nb/NbZr multilayers

Multilayers of Nb/NbZr are known to show two dimensional crossovers [three dimensional (3D) to two dimensional (2D) to 3D] in the behavior of the parallel critical field ${\mathit{H}}_{\mathit{c}2\mathrm{?}}$ as a function of temperature T. Here we report a systematic study of the behavior of the critical current ${\mathit{I}}_{\mathit{c}}$ as function of the magnetic field H in the different regimes. Varied were layer thicknesses as well as the angle between H and the sample surface, and the angle between H and the current I. Apart from earlier reported nonmonotonic behavior of ${\mathit{I}}_{\mathit{c}}$(H), which constitutes the continuation of the 2D behavior in the ${\mathit{H}}_{\mathrm{?}}$(T) phase diagram, we find strong maxima in ${\mathit{I}}_{\mathit{c}}$ at fields ${\mathit{H}}_{\mathit{p}}$ far below the 2D phase line. An analysis in terms of Lorentz forces on the flux lines shows that ${\mathit{H}}_{\mathit{p}}$ signifies a transition from a low-field region where straight vortices move freely in a direction perpendicular to the layers, to a high-field region where the vortices have developed kinks perpendicular to the layers. In this field regime ${\mathit{I}}_{\mathit{c}}$ is determined by motion of the kinks along the layers, while the portions of the vortex parallel to the layers are pinned by an intrinsic mechanism.