Small anisotropy, weak thermal fluctuations, and high field superconductivity in Co-doped iron pnictide Ba(Fe1-xCox)2As2.

We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe_1-xCo_x)₂As₂. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe_1-xCo_x)₂As₂ showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field H_c2 both parallel and perpendicular to the c-axis, the irreversibility field H_irr^c(T), and a rather unusual symmetric volume pinning force curve F_p(H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ=H_c2^ab/H_c2^c deduced from the slopes of dH_c2^ab/dT=4.9 T/K and dH_c2^c/dT=2.5 T/K decreases from ∼2 near T_c, to ∼1.5 at lower temperatures, much smaller than γ for 1111pnictides and high-T_c cuprates. [ABSTRACT FROM AUTHOR]