Enhancement of flux pinning and high-field critical current density in carbon-alloyed MgB2 thin films

We have studied flux pinning and critical current density in carbon-alloyed MgB2 thin films prepared by hybrid physical-chemical vapor deposition. We found that carbon alloying significantly enhances flux pinning. The thermal activation energy of vortices U(H) and critical current density J(c)(H) are much higher in carbon-alloyed films than in pure MgB2 films at high fields. From the scaling behavior of the reduced pinning force with reduced field, we found that the dominant pinning mechanism changes from the grain boundary pinning in pure MgB2 films to normal point pinning at low carbon content and back to grain boundary pinning at higher carbon contents for H perpendicular to ab. No dominant pinning mechanism exists when H parallel to ab.