Evidence of δ/ pinning induced by citric acid doping in MgB_2 superconductor

The critical current density ( Jc) and flux-pinning mechanism of MgB2 doped with citric acid ( C6H8O7 ) from 0 to 30wt% have been investigated in detail by magnetic and resistive measurements. The temperature dependence of critical current density,Jc(T),at particular field within the single vortex regime was analyzed. δTc pinning mechanism was found to be the only important pinning mechanism in pure MgB2 samples,while δl pinning is dominant with little effect of δTc pinning for citric acid doped samples. It was observed that the contribution of δl pinning enhances with the increasing doping level of critic acid,and it was also found that MgB2 doped with 15wt% critic acid shows the best performance in Jc-B behavior.