Mixed-state hall effect and flux pinning in Ba(Fe_1-xCo _x)₂As₂ single crystals (x=0.08 and 0.10)

Longitudinal and Hall resistivities, scaling behavior, and magnetizations are examined to study the effect of flux pinning in Ba(Fe_1-xCo _x)₂As₂ (BFCA) single crystals with x=0.08 and 0.01. Larger values of activation energy, critical current density, and pinning force are obtained in BFCA with x=0.10, indicating relatively strong pinning. The sign reversal of Hall resistivities is clearly observed in BFCA with x=0.10. The correlation between longitudinal and Hall resistivities shows the scaling behavior of ρ_xy(ρ_xx)β with exponents β=3.0-3.4 and 2.0±0.2 for BFCA crystals with x=0.08 and 0.10, respectively. Furthermore, the normal-state Hall angle is also observed to follow cotθ_H=ΛT2+C in BFCA crystals, and is explained by the Anderson theory. The relatively large c/Λ value for BFCA with x=0.10 also implies a larger contribution of impurity scattering due to more Co atoms, which may cause stronger pinning of flux lines. The results are analyzed and coincide with theory, including the pinning-induced backflow effect and plastic flow mechanism in vortex dynamics.