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VOLUME 93 (2011) | ISSUE 11 | PAGE 746
Upper critical magnetic field in a Ba0.68K0.32Fe2As2 and Ba(Fe0.93Co0.07)2As2
Abstract
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\parallel {c}) and in-plane (H\parallel {ab}) directions for both compounds. For both field orientations in  Ba0.68K0.32Fe2As2 we find a concave curvature of the Hc2(T) lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism we can describe Hc2(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 Hc2(T) dependence that can be described by the Werthamer-Helfand-Hohenberg model for H\parallel {ab} and a two-gap behavior for H\parallel {c}.