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| VOLUME 93 (2011) | ISSUE 11 | PAGE 746 |
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Upper critical magnetic field in a Ba0.68K0.32Fe2As2 and Ba(Fe0.93Co0.07)2As2
V. A. Gasparov+*, A. Audouard*, D. L. Sun**, C. T. Lin**, S. L. Bud'ko ***, P. C. Canfield***, F. Wolff-Fabris****, J. Wosnitza****
+Institute of Solid State Physics RAS, Chernogolovka, 142432 Russian Federation *Laboratoire National des Champs Magné **Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany ***Ames Laboratory, US DOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA ****Hochfeld-Magnetlabor Dresden (HLD), Helmholtz-Zentrum, 01314 Dresden, Germany
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 (  ) and in-plane
( ) 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 and a two-gap behavior for  .
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