A comparative study of the upper critical field $H_{c2}$ and second magnetization peak $H_{\text{sp}}$ was performed using high-quality single crystals of hole-doped ${\text{Ba}}_{0.68}{\text{K}}_{0.32}{\text{Fe}}_2{\text{As}}_2$ and electron-doped ${\text{BaFe}}_{1.85}{\text{Co}}_{0.15}{\text{As}}_2$ and ${\text{BaFe}}_{1.91}{\text{Ni}}_{0.09}{\text{As}}_2$. The $H_{c2}$ was extracted from both resistivity and magnetization measurements using varying magnetic fields on $H\parallel c$ and $H\perp c$ orientations. The anisotropic ratio, $\gamma =H_{c2}^{\perp c}/H_{c2}^{\parallel c}$, was observed to decrease to $\sim 2.5$ for the hole-doped and $\sim 3.0$ 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 ${\text{Ba}}_{0.68}{\text{K}}_{0.32}{\text{Fe}}_2{\text{As}}_2$ the $H_{c2}$ and $H_{\text{sp}}$ shift toward the higher temperature and higher field regime in the temperature-normalized $\left(T/T_c\right)$ vortex phase diagram, suggesting a stronger vortex pinning by the comparison with the electron-doped ${\text{BaFe}}_{1.85}{\text{Co}}_{0.15}{\text{As}}_2$ and ${\text{BaFe}}_{1.91}{\text{Ni}}_{0.09}{\text{As}}_2$. In contrast to the As-deficiency or inhomogeneous doping distribution of K, Co, and Ni, the dense pinning centers in ${\text{Ba}}_{0.68}{\text{K}}_{0.32}{\text{Fe}}_2{\text{As}}_2$ may be attributed to the disordered structural domains and suggested to be responsible for the intrinsic disorder and anisotropy of iron arsenides.

• Received 29 July 2009

DOI:https://doi.org/10.1103/PhysRevB.80.144515