Enhancement of superconducting critical current density by Fe impurity substitution in NbSe single crystals and the vortex pinning mechanism.

Magnetization measurements have been used to determine the effect of magnetic impurities (Fe) on the Larkin-Ovchinnikov (LO) 3D collective pinning model in NbSe single crystals. Upon increasing the concentration of Fe impurities, the superconducting critical current density enhances appreciably compared to pure NbSe reflecting the fact that the addition of magnetic impurities assists in improving the practical applicability of NbSe. The random pinning potential that is introduced by the Fe impurities also shows a considerable change in the interaction between the vortices and the core region, resulting in a competitive nature of single vortex, small bundle and large bundle pinning regimes in the H-T phase diagram. The intrinsic disorder in pure NbSe single crystals shows δT flux pinning; however, the extrinsic disorder created by Fe atoms in NbSe shows δl flux pinning. Furthermore, the field dependence of the pinning force on both NbSe and Fe-incorporated NbSe represents the existence of point pinning and the surface pinning mechanism with a broadening of the f curves in the Fe-incorporated single crystals.