Enhanced flux pinning by refined structural/magnetic domains in nickel-doped BaFe2As2 single crystals

The critical current density at high magnetic fields of high-temperature superconducting single crystals and films is mainly determined by the flux pinning force arising from interactions between fluxons and crystalline defects or secondary phases. However, iron-based superconductors, which are acknowledged as promising candidates for high-field applications, suffer from a lack of strong flux pinning centers. It leads to a critical current density (Jc) inferior to that of iron-based superconducting films. In this work, we applied a simple annealing method to tailor flux pinning structures in slightly underdoped BaFe2-xNixAs2 single crystals without significantly compromising superconductivity. We propose that the probable strengthened orthorhombic distortion by post-annealing could generate twin domain boundaries or short-range antiferromagnetic clusters with sizes comparable to the in-plane coherence length. The inclusion of the abnormal structural/magnetic domains enhances electron scattering but also provides potent flux pinning centers. Ultimately, the Jc is enhanced by 2.5 times in the samples with optimized pinning landscapes. Our studies highlight the importance of further exploring underdoped iron-based superconductors, in which magnetic/orthorhombic phases that compete and coexist with superconductivity may also act as effective pinning centers.