Vortex dynamics as a function of field orientation in BaFe1.9Ni0.1As2

Vortex dynamics in a multiband anisotropic superconductor, such as the Fe based superconductors, is interesting and potentially important for applications. In this study we examine flux-creep data for fields along the second magnetization peak observed in M(H) curves of BaFe1.9Ni0.1As2 for H ‖ c-axis, H ‖ ab-planes and H forming a 45° angle with ab-planes. We find that the M–H loops taken from the different field directions can be collapsed onto a single universal curve at all temperatures with a simple scaling factor equivalent to the superconducting anisotropy, showing that the vortex pinning is not only isotropic and three-dimensional but most likely related to point-like defects. The resulting critical currents, however, taken from the Bean model appear to show enhanced low field pinning for H ‖ c. The features in the vortex dynamics also differ in different field orientations and show no direct correlation with the second magnetization peak Hp as is the case with a direct crossover in pinning regimes. Isofield plots of the scaled activation energy obtained from flux-creep data are found to be a smooth function of temperature as the Hp(T) line is crossed, consistent with a single type of pinning regime operating at this field, independent of field orientation. The functional form of the Hp(T) lines in the resulting phase diagrams also support this view.