We report the detailed study of the irreversibility field of single crystals of Mg(B_1-xC_x)_2 (x=0, 2, 3.5, 5 and 7 magnetic fields of up to 40 T. High sensitive torque measurements revealed that the is greatly enhanced by a factor of two (μ_0(0)≃33 T in x=5 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

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... mall amount of carbon substitution. We discuss these results focusing on the impact of carrier doping and impurity scattering on two-gap superconductor.