Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

► Substitution of MgB2 single crystals with carbon decreases critical current densities at low magnetic fields, at higher fields such substitution increases the critical current densities. ► Magnetic field dependence of the critical current density is stronger in the carbon substituted MgB2 crystals. ► AC magnetic losses were derived from the AC susceptibility data. At higher field amplitudes, the AC losses for the carbon substituted MgB2 are lower than those for the unsubstituted one.