Upper critical field anisotropy in BaFe2-xCoxAs2 single crystals synthesized without flux

Abstract

The upper critical field was measured upto 12 T for three BaFe2-xCoxAs2 single crystals with estimated Co concentrations of x = 0.082, x = 0.117 and x = 0.143. HC2 versus temperature was measured from temperature dependent resistivity, for various applied magnetic fields, H || ab and H || c. The [dHC2/dT]T=Tc, normalized with the corresponding TC, decreases with increasing Co content, for both directions. The anisotropy γ defined as HC2 || ab / HC2 || c shows a distinct increase with Co content, and its temperature dependence shows a peak close to the TC. Magneto transport measurements, in the spin density wave regions, showed significant negative MR for H || ab and positive MR of H || c in the x = 0.082 sample. The implications of these results are discussed.