Upper Critical Magnetic Field in Ba_0.68K_0.32Fe₂As₂ and Ba(Fe_0.93Co_0.07)₂As₂

We report measurements of the temperature dependence of the radio frequency magnetic penetration depth in Ba_0.68K_0.32Fe₂As₂ and Ba(Fe_0.93Co_0.07)₂As₂ single crystals in pulsed magnetic fields up to 60 T. From our data, we construct an H–T-phase diagram for the interplane (H || c) and in-plane (H || ab) directions for both compounds. For both field orientations in Ba_0.68K_0.32Fe₂As₂ 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(Fe_0.93Co_0.07)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.