Significant improvements of the high-field properties of carbon-doped MgB2 films by hot-filament-assisted hybrid physical-chemical vapor deposition using methane as the doping source

We report a significant enhancement in upper critical field H-c2 and irreversibility field H-irr in carbon-doped MgB2 films fabricated by hot-filament-assisted hybrid physical-chemical vapor deposition (HFA-HPCVD) using methane as the doping source. For the parallel field, a very large temperature derivative -dH(c2)(parallel to ab)/dT value of 3 T K-1 near T-c was achieved in a heavily doped film with T-c near 28 K. Carbon doping also enhanced flux pinning, resulting in a much higher critical current density in a magnetic field J(c)(H) than in undoped samples. The result suggests a more effective doping of carbon into the MgB2 structure and a better connectivity between the carbon-doped MgB2 grains than in previous reports. A clear correlation between the decrease in grain size and the enhancement of H-c2 was observed.