Effect of disorder on the irreversible magnetic properties of single crystalline MgB2: comparison of carbon doping and neutron irradiation

We report on the irreversible magnetic properties of MgB2 single crystals that were modified by neutron irradiation or carbon doping. Fast neutron irradiation introduces defects with a size comparable to the superconducting coherence length. Accordingly, the effects on the irreversible properties are large. The critical current density and the irreversibility field are strongly enhanced by neutron irradiation. Additionally, a second peak that is not found in unirradiated samples, emerges and gets more pronounced with increasing neutron fluence. Carbon doping was studied for different compositions of Mg(B1−xCx)2 with x=0.038, 0.066, and 0.095. Although disorder increases with carbon doping, the critical current density decreases. On the other hand, a small fishtail is observed, which becomes more pronounced with increasing x. The superconducting (reversible) parameters are modified in a similar way both by neutron irradiation and by carbon doping. The results indicate, that the irreversible properties are mainly influenced by changes of these reversible properties in the case of carbon doping (e.g., the decrease of the coherence length reduces the pinning energy), but rather by the new defect structure in the case of neutron irradiation.