Upper critical magnetic field in Ba0.68K0.32Fe2As2 and Ba(Fe0.93Co0.07)2As2

We report measurements of the temperature dependence of the radio frequency magnetic penetration depth in Ba0.68K0.32Fe2As2 and Ba(Fe0.93Co0.07)2As2 single crystals in pulsed magnetic fields up to 60 T. From our data, we construct an H-T-phase diagram for the inter-plane (H ‖ c) and in-plane (H ‖ ab) directions for both compounds. For both field orientations in Ba0.68K0.32Fe2As2 we find a concave curvature of the H c2(T) lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism we can describe H c2(T) and its anisotropy. In contrast, we find that Pauli paramagnetic pair breaking is not essential for Ba(Fe0.93Co0.07)2As2. For this electron-doped compound, the data support a H c2(T) dependence that can be described by the Werthamer-Helfand-Hohenberg model for H ‖ ab and a two-gap behavior for H ‖ c.