Abstract
The importance of type-II superconductors with strong pinning comes from their ability to carry large electrical currents in the presence of a magnetic field. We report on the results of the bulk magnetization measurements in the superconducting state in high-quality single crystals of ${\mathrm{BaFe}}_{2\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{As}}_{2}$ at various doping levels ranging from the underdoped to the overdoped regimes. The zero-temperature superconducting critical current density ${J}_{c}$ at optimal composition $x$ = 0.10, where the superconducting transition temperature ${T}_{c}$ reaches a maximum of 19.9(0.4) K, displays a pronounced sharp peak in the doping dependence. Thus the observed doping dependence of the critical current implies that pinning becomes stronger upon initial doping. In addition, the best pinning conditions are realized in the presence of structural and magnetic domains. Our results strongly suggest that the high ${J}_{c}$ values are mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. The experimental results of the normalized ${J}_{c}$ present a remarkably good agreement with the $\ensuremath{\delta}l$ pinning theoretical curve, confirming that pinning in our samples originates from spatial variations of the charge carrier mean free path leading to small bundle vortex pinning by randomly distributed (weak) pinning centers for $H\ensuremath{\parallel}c$.
Highlights
Many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed [1,2,3,4], their potential for application is less clear
The importance of type-II superconductors with strong pinning comes from their ability to carry large electrical currents in the presence of a magnetic field
The zero-temperature superconducting critical current density Jc at optimal composition x = 0.10, where the superconducting transition temperature Tc reaches a maximum of 19.9(0.4) K, displays a pronounced sharp peak in the doping dependence
Summary
Many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed [1,2,3,4], their potential for application is less clear. High critical current density, high upper critical field, and flux jumps are important factors for practical applications of superconductors; though all of these issues are of great interest for fundamental research. The superconducting critical current density Jc is a measure of the strength of the pinning force density and can be very conveniently used to characterize the strength of disorder in the system. The Jc can be increased significantly by introducing artificial pinning centers It depends on a complex interplay of individual pinning centers, the interaction between vortices, and thermal fluctuations [5,6,7]. The possibility to achieve a strong pinning of vortices and high critical current densities in Fe-based superconductors is very high due to the short coherence lengths in these materials. This feature has been observed in the Bi-based and Tl-based cuprate superconductors and the SP is found to be temperature
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