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(Fe1-xCox)(2)As-2. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe1-xCox)(2)As-2 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 gamma=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 similar to 2 near T-c, to similar to 1.5 at lower temperatures, much smaller than gamma for 1111pnictides and high-T-c cuprates.