Enhanced critical current properties of MgB2 bulks by doping amorphous carbon containing magnetic impurity

The doping effect of amorphous carbon (C) containing magnetic impurity in MgB2 bulk has been studied. Structural characterization by means of X-ray diffraction and the superconducting transition temperature, T (c) , measurement indicate that little C effectively enters the MgB2 structure. This should be due to the lower sintering temperature. The upper critical field, H (c2), and irreversibility field, H (irr), of samples show no systematic evolution with C doping. However, critical current density J(c)(H) performance is greatly improved with C doping at 4, 15, and 28 K, respectively. Corresponding to this case, scanning electron microscope (SEM) image indicates that the grain size in samples becomes very small and grain boundary is developing roundness with the increasing of C content. This should be intimately related with the increase of magnetic impurity along with C doping. The result is discussed.