Comparative study of upper critical field Hc2 and second magnetization peak Hsp in hole- and electron-doped BaFe2As2 superconductor

A comparative study of the upper critical field H-c2 and second magnetization peak H-sp was performed using high-quality single crystals of hole-doped Ba0.68K0.32Fe2As2 and electron-doped BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2. The Hc2 was extracted from both resistivity and magnetization measurements using varying magnetic fields on H parallel to c and H perpendicular to c orientations. The anisotropic ratio, gamma=H-c2(perpendicular to c)/H-c2(parallel to c), was observed to decrease to similar to 2.5 for the hole- doped and similar to 3.0 for both electron-doped samples as the magnetic fields were increased up to 9 T. It demonstrates that the anisotropic properties only show slight change by doping aliovalent ions either in or out-of the basal plane of FeAs. For the hole- doped Ba0.68K0.32Fe2As2 the H-c2 and H-sp shift toward the higher temperature and higher field regime in the temperature-normalized (T/T-c) vortex phase diagram, suggesting a stronger vortex pinning by the comparison with the electron-doped BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2. In contrast to the As-deficiency or inhomogeneous doping distribution of K, Co, and Ni, the dense pinning centers in Ba0.68K0.32Fe2As2 may be attributed to the disordered structural domains and suggested to be responsible for the intrinsic disorder and anisotropy of iron arsenides.