We report the detailed study of the irreversibility field {} of single crystals of Mg(B₁₋ₓCₓ)₂ (x=0, 2, 3.5, 5 and 7%) in strong magnetic fields of up to 40 T. High sensitive torque measurements revealed that the {} is greatly enhanced by a factor of two (μ₀{}(0)33 T in x=5%) by carbon substitution compared to the pristine {} owing to a reduction in the inplane coherence length. We also observed the temperature-dependent {} anisotropy in all the carbon contents. This fact strongly suggests that the two-gap superconductivity of {} is less influenced by a small amount of carbon substitution. We discuss these results focusing on the impact of carrier doping and impurity scattering on two-gap superconductor.
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Ohmichi et al. (2004) studied this question.