Interaction of vortices in anisotropic superconductors with isotropic defects

Abstract

To assess a superconductors suitability for high-current applications, it is imperative to know how its critical current density (Jc) depends on magnetic field and its orientation. We present a comprehensive study of the anisotropy of the in-plane Jc in iron-based superconducting single crystals carried out by an advanced magnetometry technique. As Jc is governed by material defects capable of vortex pinning, we investigated three samples with three essentially different defect landscapes representing weak, strong and intermediate pinning. In the latter case, a second maximum in the field dependence of the critical current (fishtail effect) occured, allowing us to investigate the influence of this effect on the angular dependence of Jc in an intermediate pinning regime for the first time in a quantitative way. For weak and strong pinning, the influence of the field orientation on the in-plane Jc can be described by the anisotropic scaling theory. For weak pinning, we find the predicted scaling for the field (ε-1(θ)) and for the in-plane current (unity). For strong pinning, we find the same scaling for the field, but show for the first time that an additional scaling factor (ε (θ)) emerges for the in-plane critical current density. We attribute this new scaling rule (double scaling) to the presence of defects which are larger than the vortex core.